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linux/fs/internal.h
Christian Brauner ed5a7047d2
attr: use consistent sgid stripping checks 
Currently setgid stripping in file_remove_privs()'s should_remove_suid()
helper is inconsistent with other parts of the vfs. Specifically, it only
raises ATTR_KILL_SGID if the inode is S_ISGID and S_IXGRP but not if the
inode isn't in the caller's groups and the caller isn't privileged over the
inode although we require this already in setattr_prepare() and
setattr_copy() and so all filesystem implement this requirement implicitly
because they have to use setattr_{prepare,copy}() anyway.

But the inconsistency shows up in setgid stripping bugs for overlayfs in
xfstests (e.g., generic/673, generic/683, generic/685, generic/686,
generic/687). For example, we test whether suid and setgid stripping works
correctly when performing various write-like operations as an unprivileged
user (fallocate, reflink, write, etc.):

echo "Test 1 - qa_user, non-exec file $verb"
setup_testfile
chmod a+rws $junk_file
commit_and_check "$qa_user" "$verb" 64k 64k

The test basically creates a file with 6666 permissions. While the file has
the S_ISUID and S_ISGID bits set it does not have the S_IXGRP set. On a
regular filesystem like xfs what will happen is:

sys_fallocate()
-> vfs_fallocate()
   -> xfs_file_fallocate()
      -> file_modified()
         -> __file_remove_privs()
            -> dentry_needs_remove_privs()
               -> should_remove_suid()
            -> __remove_privs()
               newattrs.ia_valid = ATTR_FORCE | kill;
               -> notify_change()
                  -> setattr_copy()

In should_remove_suid() we can see that ATTR_KILL_SUID is raised
unconditionally because the file in the test has S_ISUID set.

But we also see that ATTR_KILL_SGID won't be set because while the file
is S_ISGID it is not S_IXGRP (see above) which is a condition for
ATTR_KILL_SGID being raised.

So by the time we call notify_change() we have attr->ia_valid set to
ATTR_KILL_SUID | ATTR_FORCE. Now notify_change() sees that
ATTR_KILL_SUID is set and does:

ia_valid = attr->ia_valid |= ATTR_MODE
attr->ia_mode = (inode->i_mode & ~S_ISUID);

which means that when we call setattr_copy() later we will definitely
update inode->i_mode. Note that attr->ia_mode still contains S_ISGID.

Now we call into the filesystem's ->setattr() inode operation which will
end up calling setattr_copy(). Since ATTR_MODE is set we will hit:

if (ia_valid & ATTR_MODE) {
        umode_t mode = attr->ia_mode;
        vfsgid_t vfsgid = i_gid_into_vfsgid(mnt_userns, inode);
        if (!vfsgid_in_group_p(vfsgid) &&
            !capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FSETID))
                mode &= ~S_ISGID;
        inode->i_mode = mode;
}

and since the caller in the test is neither capable nor in the group of the
inode the S_ISGID bit is stripped.

But assume the file isn't suid then ATTR_KILL_SUID won't be raised which
has the consequence that neither the setgid nor the suid bits are stripped
even though it should be stripped because the inode isn't in the caller's
groups and the caller isn't privileged over the inode.

If overlayfs is in the mix things become a bit more complicated and the bug
shows up more clearly. When e.g., ovl_setattr() is hit from
ovl_fallocate()'s call to file_remove_privs() then ATTR_KILL_SUID and
ATTR_KILL_SGID might be raised but because the check in notify_change() is
questioning the ATTR_KILL_SGID flag again by requiring S_IXGRP for it to be
stripped the S_ISGID bit isn't removed even though it should be stripped:

sys_fallocate()
-> vfs_fallocate()
   -> ovl_fallocate()
      -> file_remove_privs()
         -> dentry_needs_remove_privs()
            -> should_remove_suid()
         -> __remove_privs()
            newattrs.ia_valid = ATTR_FORCE | kill;
            -> notify_change()
               -> ovl_setattr()
                  // TAKE ON MOUNTER'S CREDS
                  -> ovl_do_notify_change()
                     -> notify_change()
                  // GIVE UP MOUNTER'S CREDS
     // TAKE ON MOUNTER'S CREDS
     -> vfs_fallocate()
        -> xfs_file_fallocate()
           -> file_modified()
              -> __file_remove_privs()
                 -> dentry_needs_remove_privs()
                    -> should_remove_suid()
                 -> __remove_privs()
                    newattrs.ia_valid = attr_force | kill;
                    -> notify_change()

The fix for all of this is to make file_remove_privs()'s
should_remove_suid() helper to perform the same checks as we already
require in setattr_prepare() and setattr_copy() and have notify_change()
not pointlessly requiring S_IXGRP again. It doesn't make any sense in the
first place because the caller must calculate the flags via
should_remove_suid() anyway which would raise ATTR_KILL_SGID.

While we're at it we move should_remove_suid() from inode.c to attr.c
where it belongs with the rest of the iattr helpers. Especially since it
returns ATTR_KILL_S{G,U}ID flags. We also rename it to
setattr_should_drop_suidgid() to better reflect that it indicates both
setuid and setgid bit removal and also that it returns attr flags.

Running xfstests with this doesn't report any regressions. We should really
try and use consistent checks.

Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
2022-10-18 10:09:47 +02:00

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/* SPDX-License-Identifier: GPL-2.0-or-later */
/* fs/ internal definitions
*
* Copyright (C) 2006 Red Hat, Inc. All Rights Reserved.
* Written by David Howells (dhowells@redhat.com)
*/
struct super_block;
struct file_system_type;
struct iomap;
struct iomap_ops;
struct linux_binprm;
struct path;
struct mount;
struct shrink_control;
struct fs_context;
struct user_namespace;
struct pipe_inode_info;
struct iov_iter;
/*
* block/bdev.c
*/
#ifdef CONFIG_BLOCK
extern void __init bdev_cache_init(void);
void emergency_thaw_bdev(struct super_block *sb);
#else
static inline void bdev_cache_init(void)
{
}
static inline int emergency_thaw_bdev(struct super_block *sb)
{
return 0;
}
#endif /* CONFIG_BLOCK */
/*
* buffer.c
*/
int __block_write_begin_int(struct folio *folio, loff_t pos, unsigned len,
get_block_t *get_block, const struct iomap *iomap);
/*
* char_dev.c
*/
extern void __init chrdev_init(void);
/*
* fs_context.c
*/
extern const struct fs_context_operations legacy_fs_context_ops;
extern int parse_monolithic_mount_data(struct fs_context *, void *);
extern void vfs_clean_context(struct fs_context *fc);
extern int finish_clean_context(struct fs_context *fc);
/*
* namei.c
*/
extern int filename_lookup(int dfd, struct filename *name, unsigned flags,
struct path *path, struct path *root);
extern int vfs_path_lookup(struct dentry *, struct vfsmount *,
const char *, unsigned int, struct path *);
int do_rmdir(int dfd, struct filename *name);
int do_unlinkat(int dfd, struct filename *name);
int may_linkat(struct user_namespace *mnt_userns, const struct path *link);
int do_renameat2(int olddfd, struct filename *oldname, int newdfd,
struct filename *newname, unsigned int flags);
int do_mkdirat(int dfd, struct filename *name, umode_t mode);
int do_symlinkat(struct filename *from, int newdfd, struct filename *to);
int do_linkat(int olddfd, struct filename *old, int newdfd,
struct filename *new, int flags);
/*
* namespace.c
*/
extern struct vfsmount *lookup_mnt(const struct path *);
extern int finish_automount(struct vfsmount *, const struct path *);
extern int sb_prepare_remount_readonly(struct super_block *);
extern void __init mnt_init(void);
extern int __mnt_want_write_file(struct file *);
extern void __mnt_drop_write_file(struct file *);
extern void dissolve_on_fput(struct vfsmount *);
extern bool may_mount(void);
int path_mount(const char *dev_name, struct path *path,
const char *type_page, unsigned long flags, void *data_page);
int path_umount(struct path *path, int flags);
/*
* fs_struct.c
*/
extern void chroot_fs_refs(const struct path *, const struct path *);
/*
* file_table.c
*/
extern struct file *alloc_empty_file(int, const struct cred *);
extern struct file *alloc_empty_file_noaccount(int, const struct cred *);
static inline void put_file_access(struct file *file)
{
if ((file->f_mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ) {
i_readcount_dec(file->f_inode);
} else if (file->f_mode & FMODE_WRITER) {
put_write_access(file->f_inode);
__mnt_drop_write(file->f_path.mnt);
}
}
/*
* super.c
*/
extern int reconfigure_super(struct fs_context *);
extern bool trylock_super(struct super_block *sb);
struct super_block *user_get_super(dev_t, bool excl);
void put_super(struct super_block *sb);
extern bool mount_capable(struct fs_context *);
/*
* open.c
*/
struct open_flags {
int open_flag;
umode_t mode;
int acc_mode;
int intent;
int lookup_flags;
};
extern struct file *do_filp_open(int dfd, struct filename *pathname,
const struct open_flags *op);
extern struct file *do_file_open_root(const struct path *,
const char *, const struct open_flags *);
extern struct open_how build_open_how(int flags, umode_t mode);
extern int build_open_flags(const struct open_how *how, struct open_flags *op);
extern struct file *__close_fd_get_file(unsigned int fd);
long do_sys_ftruncate(unsigned int fd, loff_t length, int small);
int chmod_common(const struct path *path, umode_t mode);
int do_fchownat(int dfd, const char __user *filename, uid_t user, gid_t group,
int flag);
int chown_common(const struct path *path, uid_t user, gid_t group);
extern int vfs_open(const struct path *, struct file *);
/*
* inode.c
*/
extern long prune_icache_sb(struct super_block *sb, struct shrink_control *sc);
int dentry_needs_remove_privs(struct user_namespace *, struct dentry *dentry);
bool in_group_or_capable(struct user_namespace *mnt_userns,
const struct inode *inode, vfsgid_t vfsgid);
/*
* fs-writeback.c
*/
extern long get_nr_dirty_inodes(void);
extern int invalidate_inodes(struct super_block *, bool);
/*
* dcache.c
*/
extern int d_set_mounted(struct dentry *dentry);
extern long prune_dcache_sb(struct super_block *sb, struct shrink_control *sc);
extern struct dentry *d_alloc_cursor(struct dentry *);
extern struct dentry * d_alloc_pseudo(struct super_block *, const struct qstr *);
extern char *simple_dname(struct dentry *, char *, int);
extern void dput_to_list(struct dentry *, struct list_head *);
extern void shrink_dentry_list(struct list_head *);
/*
* pipe.c
*/
extern const struct file_operations pipefifo_fops;
/*
* fs_pin.c
*/
extern void group_pin_kill(struct hlist_head *p);
extern void mnt_pin_kill(struct mount *m);
/*
* fs/nsfs.c
*/
extern const struct dentry_operations ns_dentry_operations;
/* direct-io.c: */
int sb_init_dio_done_wq(struct super_block *sb);
/*
* fs/stat.c:
*/
int getname_statx_lookup_flags(int flags);
int do_statx(int dfd, struct filename *filename, unsigned int flags,
unsigned int mask, struct statx __user *buffer);
/*
* fs/splice.c:
*/
long splice_file_to_pipe(struct file *in,
struct pipe_inode_info *opipe,
loff_t *offset,
size_t len, unsigned int flags);
/*
* fs/xattr.c:
*/
struct xattr_name {
char name[XATTR_NAME_MAX + 1];
};
struct xattr_ctx {
/* Value of attribute */
union {
const void __user *cvalue;
void __user *value;
};
void *kvalue;
size_t size;
/* Attribute name */
struct xattr_name *kname;
unsigned int flags;
};
ssize_t do_getxattr(struct user_namespace *mnt_userns,
struct dentry *d,
struct xattr_ctx *ctx);
int setxattr_copy(const char __user *name, struct xattr_ctx *ctx);
int do_setxattr(struct user_namespace *mnt_userns, struct dentry *dentry,
struct xattr_ctx *ctx);
ssize_t __kernel_write_iter(struct file *file, struct iov_iter *from, loff_t *pos);
/*
* fs/attr.c
*/
int setattr_should_drop_sgid(struct user_namespace *mnt_userns,
const struct inode *inode);
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