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[188.141.5.72]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-43d6fc9d525sm8979201f8f.36.2026.04.12.09.48.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 12 Apr 2026 09:48:52 -0700 (PDT) From: David Carlier To: Andrew Morton , Mike Rapoport Cc: Peter Xu , Usama Arif , linux-mm@kvack.org, linux-kernel@vger.kernel.org, David Carlier Subject: [PATCH v6] mm/userfaultfd: detect VMA type change after copy retry in mfill_copy_folio_retry() Date: Sun, 12 Apr 2026 17:48:49 +0100 Message-ID: <20260412164849.106307-1-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" mfill_copy_folio_retry() drops mmap_lock for the copy_from_user() call. During this window, the VMA can be replaced with a different type (e.g. hugetlb), making the caller's ops pointer stale. Subsequent use of the stale ops can lead to incorrect folio handling or a kernel crash. Capture the VMA's ops via vma_uffd_ops() before dropping the lock and compare against the current vma_uffd_ops() after re-acquiring it. Return -EAGAIN if they differ so the operation can be retried. This avoids comparing against the caller's ops which may have been overridden to anon_uffd_ops for MAP_PRIVATE file-backed mappings. Fixes: 59da5c32ffa3 ("userfaultfd: mfill_atomic(): remove retry logic") Reported-by: Usama Arif Signed-off-by: David Carlier --- mm/userfaultfd.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-) diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 481ec7eb4442..7fbe8eca3668 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -443,8 +443,10 @@ static int mfill_copy_folio_locked(struct folio *folio= , unsigned long src_addr) return ret; } =20 -static int mfill_copy_folio_retry(struct mfill_state *state, struct folio = *folio) +static int mfill_copy_folio_retry(struct mfill_state *state, + struct folio *folio) { + const struct vm_uffd_ops *orig_ops =3D vma_uffd_ops(state->vma); unsigned long src_addr =3D state->src_addr; void *kaddr; int err; @@ -465,6 +467,14 @@ static int mfill_copy_folio_retry(struct mfill_state *= state, struct folio *folio if (err) return err; =20 + /* + * 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) !=3D orig_ops) + return -EAGAIN; + err =3D mfill_establish_pmd(state); if (err) return err; --=20 2.53.0