From: Nadav Amit <namit@xxxxxxxxxx>
mmap_changing is currently a boolean variable, which is set and cleared
without any lock that protects against concurrent modifications.
mmap_chanign 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.
Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
Fixes: df2cc96e77011 ("userfaultfd: prevent non-cooperative events vs mcopy_atomic races")
Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
---
fs/userfaultfd.c | 25 +++++++++++++------------
include/linux/userfaultfd_k.h | 8 ++++----
mm/userfaultfd.c | 15 ++++++++-------
3 files changed, 25 insertions(+), 23 deletions(-)
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index 5c2d806e6ae5..29a3016f16c9 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -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);
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index 331d2ccf0bcc..33cea484d1ad 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -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,
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 0e2132834bc7..7a9008415534 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -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;
--
2.25.1
