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authorJann Horn <jannh@google.com>2023-07-21 05:46:43 +0200
committerAndrew Morton <akpm@linux-foundation.org>2023-07-27 13:07:04 -0700
commitd8ab9f7b644a2c9b64de405c1953c905ff219dc9 (patch)
tree049617f4231849695118ea609f609b6d60ea8189 /.mailmap
parentb1f02b95758d05b799731d939e76a0bd6da312db (diff)
mm: lock VMA in dup_anon_vma() before setting ->anon_vma
When VMAs are merged, dup_anon_vma() is called with `dst` pointing to the VMA that is being expanded to cover the area previously occupied by another VMA. This currently happens while `dst` is not write-locked. This means that, in the `src->anon_vma && !dst->anon_vma` case, as soon as the assignment `dst->anon_vma = src->anon_vma` has happened, concurrent page faults can happen on `dst` under the per-VMA lock. This is already icky in itself, since such page faults can now install pages into `dst` that are attached to an `anon_vma` that is not yet tied back to the `anon_vma` with an `anon_vma_chain`. But if `anon_vma_clone()` fails due to an out-of-memory error, things get much worse: `anon_vma_clone()` then reverts `dst->anon_vma` back to NULL, and `dst` remains completely unconnected to the `anon_vma`, even though we can have pages in the area covered by `dst` that point to the `anon_vma`. This means the `anon_vma` of such pages can be freed while the pages are still mapped into userspace, which leads to UAF when a helper like folio_lock_anon_vma_read() tries to look up the anon_vma of such a page. This theoretically is a security bug, but I believe it is really hard to actually trigger as an unprivileged user because it requires that you can make an order-0 GFP_KERNEL allocation fail, and the page allocator tries pretty hard to prevent that. I think doing the vma_start_write() call inside dup_anon_vma() is the most straightforward fix for now. For a kernel-assisted reproducer, see the notes section of the patch mail. Link: https://lkml.kernel.org/r/20230721034643.616851-1-jannh@google.com Fixes: 5e31275cc997 ("mm: add per-VMA lock and helper functions to control it") Signed-off-by: Jann Horn <jannh@google.com> Reviewed-by: Suren Baghdasaryan <surenb@google.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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