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charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20240812-uffd-thp-flip-fix-v1-1-4fc1db7ccdd0@google.com> References: <20240812-uffd-thp-flip-fix-v1-0-4fc1db7ccdd0@google.com> In-Reply-To: <20240812-uffd-thp-flip-fix-v1-0-4fc1db7ccdd0@google.com> To: Andrew Morton , Pavel Emelyanov , Andrea Arcangeli , Hugh Dickins Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jann Horn , stable@vger.kernel.org X-Mailer: b4 0.15-dev X-Developer-Signature: v=1; a=ed25519-sha256; t=1723480955; l=1791; i=jannh@google.com; s=20240730; h=from:subject:message-id; bh=cyEqwtJdd73Z0XZVi41RqSUaVwXAJZY8AuIAhW9WDHE=; b=aJA7M7sFrH6fD8l5+lxzjeojnHNOZEjoqfdSfrX7+15r0/hEZsfJhoS9jBzIQjwlC/09kjeLt aliIZZ7VCkXCOia7BI5iwegqhOGJUdxnMWHy2QCvS7PsVHzM0h5yHQr X-Developer-Key: i=jannh@google.com; a=ed25519; pk=AljNtGOzXeF6khBXDJVVvwSEkVDGnnZZYqfWhP1V+C8= The following race can occur: mfill_atomic other thread =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D pmdp_get_lockless() [reads none pmd] __pte_alloc [no-op] BUG_ON(pmd_none(*dst_pmd)) I have experimentally verified this in a kernel with extra mdelay() calls; the BUG_ON(pmd_none(*dst_pmd)) triggers. On kernels newer than commit 0d940a9b270b ("mm/pgtable: allow pte_offset_map[_lock]() to fail"), this can't lead to anything worse than a BUG_ON(), since the page table access helpers are actually designed to deal with page tables concurrently disappearing; but on older kernels (<=3D6.4), I think we could probably theoretically race past the two BUG_ON= () checks and end up treating a hugepage as a page table. Cc: stable@vger.kernel.org Fixes: c1a4de99fada ("userfaultfd: mcopy_atomic|mfill_zeropage: UFFDIO_COPY= |UFFDIO_ZEROPAGE preparation") Signed-off-by: Jann Horn Acked-by: David Hildenbrand --- mm/userfaultfd.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index e54e5c8907fa..ec3750467aa5 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -801,7 +801,8 @@ static __always_inline ssize_t mfill_atomic(struct user= faultfd_ctx *ctx, break; } /* If an huge pmd materialized from under us fail */ - if (unlikely(pmd_trans_huge(*dst_pmd))) { + dst_pmdval =3D pmdp_get_lockless(dst_pmd); + if (unlikely(pmd_none(dst_pmdval) || pmd_trans_huge(dst_pmdval))) { err =3D -EFAULT; break; } --=20 2.46.0.76.ge559c4bf1a-goog