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[188.141.5.72]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4887e822078sm46433505e9.9.2026.03.31.06.42.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 31 Mar 2026 06:42:00 -0700 (PDT) From: David Carlier To: Peter Xu , Andrew Morton Cc: Mike Rapoport , linux-mm@kvack.org, linux-kernel@vger.kernel.org, David Carlier Subject: [PATCH v4] mm/userfaultfd: detect VMA replacement after copy retry in mfill_copy_folio_retry() Date: Tue, 31 Mar 2026 14:41:58 +0100 Message-ID: <20260331134158.622084-1-devnexen@gmail.com> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" In mfill_copy_folio_retry(), all locks are dropped to retry copy_from_user() with page faults enabled. During this window, the VMA can be replaced entirely (e.g. munmap + mmap + UFFDIO_REGISTER by another thread), but the caller proceeds with a folio allocated from the original VMA's backing store. Checking ops alone is insufficient: the replacement VMA could be the same type (e.g. shmem -> shmem) with identical flags but a different backing inode. Take a snapshot of the VMA's file and flags before dropping locks, and compare after re-acquiring them. If anything changed, bail out with -EINVAL. Use get_file()/fput() rather than ihold()/iput() to hold the file reference across the lock-dropped window, avoiding potential deadlocks from filesystem eviction under mmap_lock. Fixes: 56a3706fd7f9 ("shmem, userfaultfd: implement shmem uffd operations u= sing vm_uffd_ops") Suggested-by: Peter Xu Signed-off-by: David Carlier --- mm/userfaultfd.c | 63 ++++++++++++++++++++++++++++++++++++++++++------ 1 file changed, 56 insertions(+), 7 deletions(-) diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 481ec7eb4442..93d6a954e659 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -443,33 +443,82 @@ static int mfill_copy_folio_locked(struct folio *foli= o, unsigned long src_addr) return ret; } =20 +struct vma_snapshot { + struct file *file; + vma_flags_t flags; +}; + +static void vma_snapshot_take(struct vm_area_struct *vma, + struct vma_snapshot *s) +{ + memcpy(&s->flags, &vma->flags, sizeof(s->flags)); + if (vma->vm_file) + s->file =3D get_file(vma->vm_file); + else + s->file =3D NULL; +} + +static bool vma_snapshot_changed(struct vm_area_struct *vma, + struct vma_snapshot *s) +{ + if (memcmp(&s->flags, &vma->flags, sizeof(s->flags))) + return true; + + if (s->file && (!vma->vm_file || + vma->vm_file->f_inode !=3D s->file->f_inode)) + return true; + + if (!s->file && !vma_is_anonymous(vma)) + return true; + + return false; +} + +static void vma_snapshot_release(struct vma_snapshot *s) +{ + if (s->file) { + fput(s->file); + s->file =3D NULL; + } +} + static int mfill_copy_folio_retry(struct mfill_state *state, struct folio = *folio) { unsigned long src_addr =3D state->src_addr; + struct vma_snapshot s; void *kaddr; int err; =20 + /* Take a quick snapshot of the current vma */ + vma_snapshot_take(state->vma, &s); + /* retry copying with mm_lock dropped */ mfill_put_vma(state); =20 kaddr =3D kmap_local_folio(folio, 0); err =3D copy_from_user(kaddr, (const void __user *) src_addr, PAGE_SIZE); kunmap_local(kaddr); - if (unlikely(err)) - return -EFAULT; + if (unlikely(err)) { + err =3D -EFAULT; + goto out; + } =20 flush_dcache_folio(folio); =20 /* reget VMA and PMD, they could change underneath us */ err =3D mfill_get_vma(state); if (err) - return err; + goto out; =20 - err =3D mfill_establish_pmd(state); - if (err) - return err; + if (vma_snapshot_changed(state->vma, &s)) { + err =3D -EINVAL; + goto out; + } =20 - return 0; + err =3D mfill_establish_pmd(state); +out: + vma_snapshot_release(&s); + return err; } =20 static int __mfill_atomic_pte(struct mfill_state *state, --=20 2.53.0