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![Taylor Jackson](/assets/img/avatar_default.png)
Currently, it is possible to create an idmapped mount using a user namespace without any mappings. However, this yields an idmapped mount that doesn't actually map the ids. With the following change, it will no longer be possible to create an idmapped mount when using a user namespace with no mappings, and will instead return EINVAL, an “invalid argument” error code. Reviewed-by: Christian Brauner <brauner@kernel.org> Signed-off-by: Taylor Jackson <taylor.a.jackson@me.com> Link: https://lore.kernel.org/r/20240208-mnt-idmap-inval-v2-1-58ef26d194e0@me.com Signed-off-by: Christian Brauner <brauner@kernel.org>
319 lines
8.7 KiB
C
319 lines
8.7 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/* Copyright (c) 2022 Christian Brauner <brauner@kernel.org> */
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#include <linux/cred.h>
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#include <linux/fs.h>
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#include <linux/mnt_idmapping.h>
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#include <linux/slab.h>
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#include <linux/user_namespace.h>
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#include "internal.h"
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/*
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* Outside of this file vfs{g,u}id_t are always created from k{g,u}id_t,
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* never from raw values. These are just internal helpers.
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*/
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#define VFSUIDT_INIT_RAW(val) (vfsuid_t){ val }
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#define VFSGIDT_INIT_RAW(val) (vfsgid_t){ val }
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struct mnt_idmap {
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struct uid_gid_map uid_map;
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struct uid_gid_map gid_map;
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refcount_t count;
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};
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/*
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* Carries the initial idmapping of 0:0:4294967295 which is an identity
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* mapping. This means that {g,u}id 0 is mapped to {g,u}id 0, {g,u}id 1 is
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* mapped to {g,u}id 1, [...], {g,u}id 1000 to {g,u}id 1000, [...].
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*/
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struct mnt_idmap nop_mnt_idmap = {
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.count = REFCOUNT_INIT(1),
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};
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EXPORT_SYMBOL_GPL(nop_mnt_idmap);
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/**
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* initial_idmapping - check whether this is the initial mapping
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* @ns: idmapping to check
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*
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* Check whether this is the initial mapping, mapping 0 to 0, 1 to 1,
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* [...], 1000 to 1000 [...].
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*
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* Return: true if this is the initial mapping, false if not.
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*/
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static inline bool initial_idmapping(const struct user_namespace *ns)
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{
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return ns == &init_user_ns;
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}
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/**
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* make_vfsuid - map a filesystem kuid according to an idmapping
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* @idmap: the mount's idmapping
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* @fs_userns: the filesystem's idmapping
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* @kuid : kuid to be mapped
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*
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* Take a @kuid and remap it from @fs_userns into @idmap. Use this
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* function when preparing a @kuid to be reported to userspace.
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*
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* If initial_idmapping() determines that this is not an idmapped mount
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* we can simply return @kuid unchanged.
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* If initial_idmapping() tells us that the filesystem is not mounted with an
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* idmapping we know the value of @kuid won't change when calling
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* from_kuid() so we can simply retrieve the value via __kuid_val()
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* directly.
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*
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* Return: @kuid mapped according to @idmap.
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* If @kuid has no mapping in either @idmap or @fs_userns INVALID_UID is
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* returned.
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*/
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vfsuid_t make_vfsuid(struct mnt_idmap *idmap,
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struct user_namespace *fs_userns,
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kuid_t kuid)
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{
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uid_t uid;
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if (idmap == &nop_mnt_idmap)
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return VFSUIDT_INIT(kuid);
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if (initial_idmapping(fs_userns))
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uid = __kuid_val(kuid);
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else
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uid = from_kuid(fs_userns, kuid);
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if (uid == (uid_t)-1)
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return INVALID_VFSUID;
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return VFSUIDT_INIT_RAW(map_id_down(&idmap->uid_map, uid));
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}
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EXPORT_SYMBOL_GPL(make_vfsuid);
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/**
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* make_vfsgid - map a filesystem kgid according to an idmapping
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* @idmap: the mount's idmapping
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* @fs_userns: the filesystem's idmapping
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* @kgid : kgid to be mapped
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*
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* Take a @kgid and remap it from @fs_userns into @idmap. Use this
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* function when preparing a @kgid to be reported to userspace.
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*
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* If initial_idmapping() determines that this is not an idmapped mount
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* we can simply return @kgid unchanged.
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* If initial_idmapping() tells us that the filesystem is not mounted with an
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* idmapping we know the value of @kgid won't change when calling
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* from_kgid() so we can simply retrieve the value via __kgid_val()
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* directly.
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*
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* Return: @kgid mapped according to @idmap.
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* If @kgid has no mapping in either @idmap or @fs_userns INVALID_GID is
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* returned.
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*/
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vfsgid_t make_vfsgid(struct mnt_idmap *idmap,
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struct user_namespace *fs_userns, kgid_t kgid)
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{
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gid_t gid;
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if (idmap == &nop_mnt_idmap)
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return VFSGIDT_INIT(kgid);
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if (initial_idmapping(fs_userns))
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gid = __kgid_val(kgid);
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else
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gid = from_kgid(fs_userns, kgid);
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if (gid == (gid_t)-1)
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return INVALID_VFSGID;
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return VFSGIDT_INIT_RAW(map_id_down(&idmap->gid_map, gid));
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}
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EXPORT_SYMBOL_GPL(make_vfsgid);
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/**
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* from_vfsuid - map a vfsuid into the filesystem idmapping
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* @idmap: the mount's idmapping
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* @fs_userns: the filesystem's idmapping
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* @vfsuid : vfsuid to be mapped
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*
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* Map @vfsuid into the filesystem idmapping. This function has to be used in
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* order to e.g. write @vfsuid to inode->i_uid.
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*
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* Return: @vfsuid mapped into the filesystem idmapping
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*/
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kuid_t from_vfsuid(struct mnt_idmap *idmap,
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struct user_namespace *fs_userns, vfsuid_t vfsuid)
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{
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uid_t uid;
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if (idmap == &nop_mnt_idmap)
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return AS_KUIDT(vfsuid);
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uid = map_id_up(&idmap->uid_map, __vfsuid_val(vfsuid));
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if (uid == (uid_t)-1)
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return INVALID_UID;
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if (initial_idmapping(fs_userns))
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return KUIDT_INIT(uid);
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return make_kuid(fs_userns, uid);
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}
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EXPORT_SYMBOL_GPL(from_vfsuid);
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/**
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* from_vfsgid - map a vfsgid into the filesystem idmapping
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* @idmap: the mount's idmapping
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* @fs_userns: the filesystem's idmapping
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* @vfsgid : vfsgid to be mapped
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*
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* Map @vfsgid into the filesystem idmapping. This function has to be used in
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* order to e.g. write @vfsgid to inode->i_gid.
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*
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* Return: @vfsgid mapped into the filesystem idmapping
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*/
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kgid_t from_vfsgid(struct mnt_idmap *idmap,
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struct user_namespace *fs_userns, vfsgid_t vfsgid)
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{
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gid_t gid;
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if (idmap == &nop_mnt_idmap)
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return AS_KGIDT(vfsgid);
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gid = map_id_up(&idmap->gid_map, __vfsgid_val(vfsgid));
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if (gid == (gid_t)-1)
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return INVALID_GID;
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if (initial_idmapping(fs_userns))
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return KGIDT_INIT(gid);
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return make_kgid(fs_userns, gid);
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}
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EXPORT_SYMBOL_GPL(from_vfsgid);
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#ifdef CONFIG_MULTIUSER
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/**
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* vfsgid_in_group_p() - check whether a vfsuid matches the caller's groups
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* @vfsgid: the mnt gid to match
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*
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* This function can be used to determine whether @vfsuid matches any of the
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* caller's groups.
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*
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* Return: 1 if vfsuid matches caller's groups, 0 if not.
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*/
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int vfsgid_in_group_p(vfsgid_t vfsgid)
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{
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return in_group_p(AS_KGIDT(vfsgid));
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}
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#else
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int vfsgid_in_group_p(vfsgid_t vfsgid)
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{
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return 1;
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}
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#endif
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EXPORT_SYMBOL_GPL(vfsgid_in_group_p);
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static int copy_mnt_idmap(struct uid_gid_map *map_from,
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struct uid_gid_map *map_to)
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{
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struct uid_gid_extent *forward, *reverse;
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u32 nr_extents = READ_ONCE(map_from->nr_extents);
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/* Pairs with smp_wmb() when writing the idmapping. */
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smp_rmb();
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/*
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* Don't blindly copy @map_to into @map_from if nr_extents is
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* smaller or equal to UID_GID_MAP_MAX_BASE_EXTENTS. Since we
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* read @nr_extents someone could have written an idmapping and
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* then we might end up with inconsistent data. So just don't do
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* anything at all.
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*/
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if (nr_extents == 0)
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return -EINVAL;
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/*
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* Here we know that nr_extents is greater than zero which means
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* a map has been written. Since idmappings can't be changed
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* once they have been written we know that we can safely copy
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* from @map_to into @map_from.
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*/
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if (nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS) {
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*map_to = *map_from;
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return 0;
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}
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forward = kmemdup(map_from->forward,
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nr_extents * sizeof(struct uid_gid_extent),
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GFP_KERNEL_ACCOUNT);
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if (!forward)
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return -ENOMEM;
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reverse = kmemdup(map_from->reverse,
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nr_extents * sizeof(struct uid_gid_extent),
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GFP_KERNEL_ACCOUNT);
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if (!reverse) {
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kfree(forward);
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return -ENOMEM;
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}
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/*
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* The idmapping isn't exposed anywhere so we don't need to care
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* about ordering between extent pointers and @nr_extents
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* initialization.
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*/
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map_to->forward = forward;
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map_to->reverse = reverse;
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map_to->nr_extents = nr_extents;
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return 0;
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}
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static void free_mnt_idmap(struct mnt_idmap *idmap)
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{
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if (idmap->uid_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
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kfree(idmap->uid_map.forward);
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kfree(idmap->uid_map.reverse);
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}
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if (idmap->gid_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) {
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kfree(idmap->gid_map.forward);
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kfree(idmap->gid_map.reverse);
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}
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kfree(idmap);
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}
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struct mnt_idmap *alloc_mnt_idmap(struct user_namespace *mnt_userns)
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{
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struct mnt_idmap *idmap;
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int ret;
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idmap = kzalloc(sizeof(struct mnt_idmap), GFP_KERNEL_ACCOUNT);
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if (!idmap)
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return ERR_PTR(-ENOMEM);
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refcount_set(&idmap->count, 1);
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ret = copy_mnt_idmap(&mnt_userns->uid_map, &idmap->uid_map);
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if (!ret)
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ret = copy_mnt_idmap(&mnt_userns->gid_map, &idmap->gid_map);
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if (ret) {
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free_mnt_idmap(idmap);
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idmap = ERR_PTR(ret);
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}
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return idmap;
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}
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/**
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* mnt_idmap_get - get a reference to an idmapping
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* @idmap: the idmap to bump the reference on
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*
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* If @idmap is not the @nop_mnt_idmap bump the reference count.
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*
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* Return: @idmap with reference count bumped if @not_mnt_idmap isn't passed.
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*/
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struct mnt_idmap *mnt_idmap_get(struct mnt_idmap *idmap)
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{
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if (idmap != &nop_mnt_idmap)
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refcount_inc(&idmap->count);
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return idmap;
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}
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EXPORT_SYMBOL_GPL(mnt_idmap_get);
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/**
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* mnt_idmap_put - put a reference to an idmapping
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* @idmap: the idmap to put the reference on
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*
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* If this is a non-initial idmapping, put the reference count when a mount is
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* released and free it if we're the last user.
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*/
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void mnt_idmap_put(struct mnt_idmap *idmap)
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{
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if (idmap != &nop_mnt_idmap && refcount_dec_and_test(&idmap->count))
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free_mnt_idmap(idmap);
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}
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EXPORT_SYMBOL_GPL(mnt_idmap_put);
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