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| author | Mike Rapoport (Microsoft) <rppt@kernel.org> | 2026-05-27 21:47:49 +0300 |
|---|---|---|
| committer | Andrew Morton <akpm@linux-foundation.org> | 2026-05-28 21:30:15 -0700 |
| commit | 9568102be2b678151d18b790469e5a7883c4e412 (patch) | |
| tree | 255a91ed29dfb0240bcca5e5365094cc377c64ea /mm | |
| parent | 5cda5711176be900eb5c1c5f541c00eb90071a06 (diff) | |
| download | linux-next-history-9568102be2b678151d18b790469e5a7883c4e412.tar.gz | |
userfaultfd: verify VMA state across UFFDIO_COPY retry
Patch series "userfaultfd: verify VMA state across UFFDIO_COPY retry", v2.
... and two more small fixes.
This patch (of 3):
mfill_copy_folio_retry() drops the VMA lock for copy_from_user() and
reacquires it afterwards. The destination VMA can be replaced during that
window.
The existing check compares vma_uffd_ops() before and after the retry, but
if a shmem VMA with MAP_SHARED is replaced with a shmem VMA with
MAP_PRIVATE (or vice versa) the replacement goes undetected.
The change from MAP_PRIVATE to MAP_SHARED will treat the folio allocated
with shmem_alloc_folio() as anonymous and this will cause BUG() when
mfill_atomic_install_pte() will try to folio_add_new_anon_rmap().
The change from MAP_SHARED to MAP_PRIVATE allows injection of folios into
the page cache of the original VMA.
There is no need to change for hugetlb because it never uses
mfill_copy_folio_retry().
Introduce helpers for more comprehensive comparison of VMA state:
- mfill_retry_state_save() to save the relevant VMA state into a struct
mfill_retry_state (original uffd_ops, relevant VMA flags, vm_file and
pgoff) before dropping the lock
- mfill_retry_state_changed() to compare the saved state with the state
of the VMA acquired after retaking the locks
- mfill_retry_state_put() to release vm_file pinning.
Use DEFINE_FREE() cleanup to wrap mfill_retry_state_put() to avoid
complicating error handling paths in mfill_copy_folio_retry().
Link: https://lore.kernel.org/20260527184751.4147364-1-rppt@kernel.org
Link: https://lore.kernel.org/20260527184751.4147364-2-rppt@kernel.org
Fixes: 292411fda25b ("mm/userfaultfd: detect VMA type change after copy retry in mfill_copy_folio_retry()")
Fixes: 6ab703034f14 ("userfaultfd: mfill_atomic(): remove retry logic")
Co-developed-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Suggested-by: Peter Xu <peterx@redhat.com>
Co-developed-by: David Carlier <devnexen@gmail.com>
Signed-off-by: David Carlier <devnexen@gmail.com>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'mm')
| -rw-r--r-- | mm/userfaultfd.c | 85 |
1 files changed, 73 insertions, 12 deletions
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 180bad42fc79d..e5d2fb3ce2c1a 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -14,6 +14,8 @@ #include <linux/userfaultfd_k.h> #include <linux/mmu_notifier.h> #include <linux/hugetlb.h> +#include <linux/file.h> +#include <linux/cleanup.h> #include <asm/tlbflush.h> #include <asm/tlb.h> #include "internal.h" @@ -443,16 +445,80 @@ static int mfill_copy_folio_locked(struct folio *folio, unsigned long src_addr) return ret; } -static int mfill_copy_folio_retry(struct mfill_state *state, +#define MFILL_RETRY_STATE_VMA_FLAGS \ + append_vma_flags(__VMA_UFFD_FLAGS, VMA_SHARED_BIT) + +/* + * VMA state saved before dropping the locks in mfill_copy_folio_retry(). + * Used to detect VMA replacement or incompatible changes after reacquiring the + * locks. + */ +struct mfill_retry_state { + const struct vm_uffd_ops *ops; + struct file *file; + vma_flags_t flags; + pgoff_t pgoff; +}; + +static void mfill_retry_state_save(struct mfill_retry_state *s, + struct vm_area_struct *vma) +{ + s->flags = vma_flags_and_mask(&vma->flags, MFILL_RETRY_STATE_VMA_FLAGS); + s->ops = vma_uffd_ops(vma); + s->pgoff = vma->vm_pgoff; + + if (vma->vm_file) + s->file = get_file(vma->vm_file); +} + +static bool mfill_retry_state_changed(struct mfill_retry_state *state, + struct vm_area_struct *vma) +{ + vma_flags_t flags = vma_flags_and_mask(&vma->flags, + MFILL_RETRY_STATE_VMA_FLAGS); + + /* Have any UFFD flags (missing, WP, minor) changed? */ + if (!vma_flags_same_pair(&state->flags, &flags)) + return true; + + /* VMA type or effective uffd_ops changed while the lock was dropped */ + if (state->ops != vma_uffd_ops(vma)) + return true; + + /* VMA was anonymous before; changed only if it no longer is */ + if (!state->file) + return !vma_is_anonymous(vma); + + /* VMA was file backed, but file, inode or offset has changed */ + if (!vma->vm_file || vma->vm_file->f_inode != state->file->f_inode || + state->file != vma->vm_file || vma->vm_pgoff != state->pgoff) + return true; + + return false; +} + +static void mfill_retry_state_put(struct mfill_retry_state *s) +{ + if (s->file) + fput(s->file); +} + +DEFINE_FREE(retry_put, struct mfill_retry_state *, + if (_T) mfill_retry_state_put(_T)); + +static int mfill_copy_folio_retry(struct mfill_state *mfill_state, struct folio *folio) { - const struct vm_uffd_ops *orig_ops = vma_uffd_ops(state->vma); - unsigned long src_addr = state->src_addr; + struct mfill_retry_state retry_state = { 0 }; + struct mfill_retry_state *for_free __free(retry_put) = &retry_state; + unsigned long src_addr = mfill_state->src_addr; void *kaddr; int err; + mfill_retry_state_save(&retry_state, mfill_state->vma); + /* retry copying with mm_lock dropped */ - mfill_put_vma(state); + mfill_put_vma(mfill_state); kaddr = kmap_local_folio(folio, 0); err = copy_from_user(kaddr, (const void __user *) src_addr, PAGE_SIZE); @@ -463,19 +529,14 @@ static int mfill_copy_folio_retry(struct mfill_state *state, flush_dcache_folio(folio); /* reget VMA and PMD, they could change underneath us */ - err = mfill_get_vma(state); + err = mfill_get_vma(mfill_state); if (err) return err; - /* - * The VMA type may have changed while the lock was dropped - * (e.g. replaced with a hugetlb mapping), making the caller's - * ops pointer stale. - */ - if (vma_uffd_ops(state->vma) != orig_ops) + if (mfill_retry_state_changed(&retry_state, mfill_state->vma)) return -EAGAIN; - err = mfill_establish_pmd(state); + err = mfill_establish_pmd(mfill_state); if (err) return err; |