From: Jann Horn <jannh@xxxxxxxxxx> commit d8ab9f7b644a2c9b64de405c1953c905ff219dc9 upstream. 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@xxxxxxxxxx Fixes: 5e31275cc997 ("mm: add per-VMA lock and helper functions to control it") Signed-off-by: Jann Horn <jannh@xxxxxxxxxx> Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- mm/mmap.c | 1 + 1 file changed, 1 insertion(+) --- a/mm/mmap.c +++ b/mm/mmap.c @@ -647,6 +647,7 @@ static inline int dup_anon_vma(struct vm * anon pages imported. */ if (src->anon_vma && !dst->anon_vma) { + vma_start_write(dst); dst->anon_vma = src->anon_vma; return anon_vma_clone(dst, src); }
