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McKenney" , Akira Yokosawa , Daniel Lustig , Joel Fernandes Subject: [PATCH 1/2] mm: lock_vma_under_rcu() must check vma->anon_vma under vma lock Date: Wed, 26 Jul 2023 23:41:03 +0200 Message-ID: <20230726214103.3261108-3-jannh@google.com> X-Mailer: git-send-email 2.41.0.487.g6d72f3e995-goog In-Reply-To: <20230726214103.3261108-1-jannh@google.com> References: <20230726214103.3261108-1-jannh@google.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" lock_vma_under_rcu() tries to guarantee that __anon_vma_prepare() can't be called in the VMA-locked page fault path by ensuring that vma->anon_vma is set. However, this check happens before the VMA is locked, which means a concurrent move_vma() can concurrently call unlink_anon_vmas(), which disassociates the VMA's anon_vma. This means we can get UAF in the following scenario: THREAD 1 THREAD 2 =3D=3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D lock_vma_under_rcu() rcu_read_lock() mas_walk() check vma->anon_vma mremap() syscall move_vma() vma_start_write() unlink_anon_vmas() handle_mm_fault() __handle_mm_fault() handle_pte_fault() do_pte_missing() do_anonymous_page() anon_vma_prepare() __anon_vma_prepare() find_mergeable_anon_vma() mas_walk() [looks up VMA X] munmap() syscall (deletes VMA X) reusable_anon_vma() [called on freed VMA X] This is a security bug if you can hit it, although an attacker would have to win two races at once where the first race window is only a few instructions wide. This patch is based on some previous discussion with Linus Torvalds on the security list. Cc: stable@vger.kernel.org Fixes: 5e31275cc997 ("mm: add per-VMA lock and helper functions to control = it") Signed-off-by: Jann Horn Reviewed-by: Suren Baghdasaryan --- mm/memory.c | 28 ++++++++++++++++------------ 1 file changed, 16 insertions(+), 12 deletions(-) diff --git a/mm/memory.c b/mm/memory.c index 01f39e8144ef..603b2f419948 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -5393,27 +5393,28 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm= _struct *mm, if (!vma_is_anonymous(vma) && !vma_is_tcp(vma)) goto inval; =20 - /* find_mergeable_anon_vma uses adjacent vmas which are not locked */ - if (!vma->anon_vma && !vma_is_tcp(vma)) - goto inval; - if (!vma_start_read(vma)) goto inval; =20 + /* + * find_mergeable_anon_vma uses adjacent vmas which are not locked. + * This check must happen after vma_start_read(); otherwise, a + * concurrent mremap() with MREMAP_DONTUNMAP could dissociate the VMA + * from its anon_vma. + */ + if (unlikely(!vma->anon_vma && !vma_is_tcp(vma))) + goto inval_end_read; + /* * Due to the possibility of userfault handler dropping mmap_lock, avoid * it for now and fall back to page fault handling under mmap_lock. */ - if (userfaultfd_armed(vma)) { - vma_end_read(vma); - goto inval; - } + if (userfaultfd_armed(vma)) + goto inval_end_read; =20 /* Check since vm_start/vm_end might change before we lock the VMA */ - if (unlikely(address < vma->vm_start || address >=3D vma->vm_end)) { - vma_end_read(vma); - goto inval; - } + if (unlikely(address < vma->vm_start || address >=3D vma->vm_end)) + goto inval_end_read; =20 /* Check if the VMA got isolated after we found it */ if (vma->detached) { @@ -5425,6 +5426,9 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_s= truct *mm, =20 rcu_read_unlock(); return vma; + +inval_end_read: + vma_end_read(vma); inval: rcu_read_unlock(); count_vm_vma_lock_event(VMA_LOCK_ABORT); --=20 2.41.0.487.g6d72f3e995-goog