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userfaultfd: change mmap_changing to atomic

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Patch series "userfaultfd: minor bug fixes".

Three unrelated bug fixes. The first two addresses possible issues (not
too theoretical ones), but I did not encounter them in practice.

The third patch addresses a test bug that causes the test to fail on my
system. It has been sent before as part of a bigger RFC.

This patch (of 3):

mmap_changing is currently a boolean variable, which is set and cleared
without any lock that protects against concurrent modifications.

mmap_changing is supposed to mark whether userfaultfd page-faults handling
should be retried since mappings are undergoing a change.  However,
concurrent calls, for instance to madvise(MADV_DONTNEED), might cause
mmap_changing to be false, although the remove event was still not read
(hence acknowledged) by the user.

Change mmap_changing to atomic_t and increase/decrease appropriately.  Add
a debug assertion to see whether mmap_changing is negative.

Link: https://lkml.kernel.org/r/20210808020724.1022515-1-namit@vmware.com
Link: https://lkml.kernel.org/r/20210808020724.1022515-2-namit@vmware.com
Fixes: df2cc96e77 ("userfaultfd: prevent non-cooperative events vs mcopy_atomic races")
Signed-off-by: Nadav Amit <namit@vmware.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Nadav Amit 2021-09-02 14:58:56 -07:00 committed by Linus Torvalds
parent 09a26e8327
commit a759a909d4
3 changed files with 25 additions and 23 deletions
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25
fs/userfaultfd.c
View file

@ -74,7 +74,7 @@ struct userfaultfd_ctx {
/* released */
bool released;
/* memory mappings are changing because of non-cooperative event */
bool mmap_changing;
atomic_t mmap_changing;
/* mm with one ore more vmas attached to this userfaultfd_ctx */
struct mm_struct *mm;
};
@ -623,7 +623,8 @@ static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
* already released.
*/
out:
WRITE_ONCE(ctx->mmap_changing, false);
atomic_dec(&ctx->mmap_changing);
VM_BUG_ON(atomic_read(&ctx->mmap_changing) < 0);
userfaultfd_ctx_put(ctx);
}
@ -669,12 +670,12 @@ int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
ctx->state = UFFD_STATE_RUNNING;
ctx->features = octx->features;
ctx->released = false;
ctx->mmap_changing = false;
atomic_set(&ctx->mmap_changing, 0);
ctx->mm = vma->vm_mm;
mmgrab(ctx->mm);
userfaultfd_ctx_get(octx);
WRITE_ONCE(octx->mmap_changing, true);
atomic_inc(&octx->mmap_changing);
fctx->orig = octx;
fctx->new = ctx;
list_add_tail(&fctx->list, fcs);
@ -721,7 +722,7 @@ void mremap_userfaultfd_prep(struct vm_area_struct *vma,
if (ctx->features & UFFD_FEATURE_EVENT_REMAP) {
vm_ctx->ctx = ctx;
userfaultfd_ctx_get(ctx);
WRITE_ONCE(ctx->mmap_changing, true);
atomic_inc(&ctx->mmap_changing);
} else {
/* Drop uffd context if remap feature not enabled */
vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
@ -766,7 +767,7 @@ bool userfaultfd_remove(struct vm_area_struct *vma,
return true;
userfaultfd_ctx_get(ctx);
WRITE_ONCE(ctx->mmap_changing, true);
atomic_inc(&ctx->mmap_changing);
mmap_read_unlock(mm);
msg_init(&ewq.msg);
@ -810,7 +811,7 @@ int userfaultfd_unmap_prep(struct vm_area_struct *vma,
return -ENOMEM;
userfaultfd_ctx_get(ctx);
WRITE_ONCE(ctx->mmap_changing, true);
atomic_inc(&ctx->mmap_changing);
unmap_ctx->ctx = ctx;
unmap_ctx->start = start;
unmap_ctx->end = end;
@ -1700,7 +1701,7 @@ static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
user_uffdio_copy = (struct uffdio_copy __user *) arg;
ret = -EAGAIN;
if (READ_ONCE(ctx->mmap_changing))
if (atomic_read(&ctx->mmap_changing))
goto out;
ret = -EFAULT;
@ -1757,7 +1758,7 @@ static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
user_uffdio_zeropage = (struct uffdio_zeropage __user *) arg;
ret = -EAGAIN;
if (READ_ONCE(ctx->mmap_changing))
if (atomic_read(&ctx->mmap_changing))
goto out;
ret = -EFAULT;
@ -1807,7 +1808,7 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx,
struct userfaultfd_wake_range range;
bool mode_wp, mode_dontwake;
if (READ_ONCE(ctx->mmap_changing))
if (atomic_read(&ctx->mmap_changing))
return -EAGAIN;
user_uffdio_wp = (struct uffdio_writeprotect __user *) arg;
@ -1855,7 +1856,7 @@ static int userfaultfd_continue(struct userfaultfd_ctx *ctx, unsigned long arg)
user_uffdio_continue = (struct uffdio_continue __user *)arg;
ret = -EAGAIN;
if (READ_ONCE(ctx->mmap_changing))
if (atomic_read(&ctx->mmap_changing))
goto out;
ret = -EFAULT;
@ -2087,7 +2088,7 @@ SYSCALL_DEFINE1(userfaultfd, int, flags)
ctx->features = 0;
ctx->state = UFFD_STATE_WAIT_API;
ctx->released = false;
ctx->mmap_changing = false;
atomic_set(&ctx->mmap_changing, 0);
ctx->mm = current->mm;
/* prevent the mm struct to be freed */
mmgrab(ctx->mm);

8
include/linux/userfaultfd_k.h
View file

@ -60,16 +60,16 @@ extern int mfill_atomic_install_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
extern ssize_t mcopy_atomic(struct mm_struct *dst_mm, unsigned long dst_start,
unsigned long src_start, unsigned long len,
bool *mmap_changing, __u64 mode);
atomic_t *mmap_changing, __u64 mode);
extern ssize_t mfill_zeropage(struct mm_struct *dst_mm,
unsigned long dst_start,
unsigned long len,
bool *mmap_changing);
atomic_t *mmap_changing);
extern ssize_t mcopy_continue(struct mm_struct *dst_mm, unsigned long dst_start,
unsigned long len, bool *mmap_changing);
unsigned long len, atomic_t *mmap_changing);
extern int mwriteprotect_range(struct mm_struct *dst_mm,
unsigned long start, unsigned long len,
bool enable_wp, bool *mmap_changing);
bool enable_wp, atomic_t *mmap_changing);
/* mm helpers */
static inline bool is_mergeable_vm_userfaultfd_ctx(struct vm_area_struct *vma,

15
mm/userfaultfd.c
View file

@ -483,7 +483,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
unsigned long src_start,
unsigned long len,
enum mcopy_atomic_mode mcopy_mode,
bool *mmap_changing,
atomic_t *mmap_changing,
__u64 mode)
{
struct vm_area_struct *dst_vma;
@ -517,7 +517,7 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
* request the user to retry later
*/
err = -EAGAIN;
if (mmap_changing && READ_ONCE(*mmap_changing))
if (mmap_changing && atomic_read(mmap_changing))
goto out_unlock;
/*
@ -650,28 +650,29 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm,
ssize_t mcopy_atomic(struct mm_struct *dst_mm, unsigned long dst_start,
unsigned long src_start, unsigned long len,
bool *mmap_changing, __u64 mode)
atomic_t *mmap_changing, __u64 mode)
{
return __mcopy_atomic(dst_mm, dst_start, src_start, len,
MCOPY_ATOMIC_NORMAL, mmap_changing, mode);
}
ssize_t mfill_zeropage(struct mm_struct *dst_mm, unsigned long start,
unsigned long len, bool *mmap_changing)
unsigned long len, atomic_t *mmap_changing)
{
return __mcopy_atomic(dst_mm, start, 0, len, MCOPY_ATOMIC_ZEROPAGE,
mmap_changing, 0);
}
ssize_t mcopy_continue(struct mm_struct *dst_mm, unsigned long start,
unsigned long len, bool *mmap_changing)
unsigned long len, atomic_t *mmap_changing)
{
return __mcopy_atomic(dst_mm, start, 0, len, MCOPY_ATOMIC_CONTINUE,
mmap_changing, 0);
}
int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
unsigned long len, bool enable_wp, bool *mmap_changing)
unsigned long len, bool enable_wp,
atomic_t *mmap_changing)
{
struct vm_area_struct *dst_vma;
pgprot_t newprot;
@ -694,7 +695,7 @@ int mwriteprotect_range(struct mm_struct *dst_mm, unsigned long start,
* request the user to retry later
*/
err = -EAGAIN;
if (mmap_changing && READ_ONCE(*mmap_changing))
if (mmap_changing && atomic_read(mmap_changing))
goto out_unlock;
err = -ENOENT;