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Howlett" , Vlastimil Babka , Harry Yoo , linux-mm@kvack.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: E0B20100009 X-Stat-Signature: sf3k6z1kn1y3izktoid4s6dumbi9frbt X-Rspam-User: X-HE-Tag: 1753456591-843689 X-HE-Meta: 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 EcIm4dLP 7dPsmGk8qSpKbCMeTb0tv/v4LIq9L4UzrC1oqvlx8kue+DWvypWwnlYbvR9KCzeg5avpFWvBqN/NcUknN5d1fA2fqKQv0QFCv0CtpQPr4oCkZPwpUBmTj2hAxcUDUfmy8U5tZvkqPcDN9X29ZTRdeJCWg5VemG09IPO4HKPHeq+/LZglMx7FnRKS6dJ5MK9uLrHPf7+owMPN5vvgXh3rLKmG1STyDjih1dmLb X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Jul 25, 2025 at 5:07=E2=80=AFPM Lorenzo Stoakes wrote: > On Fri, Jul 25, 2025 at 04:48:09PM +0200, Jann Horn wrote: > > On Fri, Jul 25, 2025 at 3:49=E2=80=AFPM Lorenzo Stoakes > > wrote: > > > On Fri, Jul 25, 2025 at 02:00:18PM +0200, Jann Horn wrote: > > > > > We're sort of relying on this > > > > > > > > > > a. being a UAF > > > > > > > > > > b. the thing we're UAF-ing not either corrupting this field or (i= f that > > > > > memory is actually reused as an anon_vma - I'm not familiar w= ith slab > > > > > caches - so maybe it's quite likely - getting its refcount in= cremented. > > > > > > > > KASAN sees the memory read I'm doing with this atomic_read(), so in > > > > KASAN builds, if this is a UAF, it should trigger a KASAN splat > > > > (modulo KASAN limitations around when UAF can be detected). Basical= ly, > > > > in KASAN builds, the actual explicit check I'm doing here is only > > > > relevant if the object has not yet been freed. That's why I wrote t= he > > > > comment "Part of the purpose of the atomic_read() is to make KASAN > > > > check that the anon_vma is still alive.". > > > > > > Hm, I'm confused, how can you detect a UAF if the object cannot yet b= e > > > freed? :P > > > > > > or would that be the case were it not an atomic_read()? > > > > > > I guess this permits this to be detected in a timely manner. > > > > If the anon_vma hasn't yet been freed, but its refcount is 0, then > > that's still a bug because we rely on the anon_vma to have a nonzero > > refcount as long as there are folios with a nonzero mapcount that are > > tied to it, and it is likely to allow UAF at a later point. > > But how is this happening? > > The only places where we might explicitly manipulate anon_vma->refcount > are: > > - anon_vma_ctor() -> set to 0 on construction used by slab. > - folio_lock_anon_vma_read() / put_anon_vma() - both cases call > __put_anon_vma() when 0 to free the anon_vma. > > So how could we get to a refcount of 0 but the anon_vma still be hanging > around, except if it's freshly allocated? Due to SLAB_TYPESAFE_BY_RCU, the anon_vma is guaranteed to still be accessible (possibly post-recycling) for an RCU grace period after its refcount drops to zero. Under CONFIG_SLUB_RCU_DEBUG (which you need for KASAN to catch UAF in such slabs semi-reliably), from KASAN's perspective, the anon_vma is effectively freed after an RCU grace period. Basically CONFIG_SLUB_RCU_DEBUG turns kmem_cache_free() on SLAB_TYPESAFE_BY_RCU slabs into something like kfree_rcu(), and this allows KASAN to catch UAF access. > It's surely only UAF? I mean, "UAF" is kind of vague when talking about SLAB_TYPESAFE_BY_RCU slabs. I am only using the term "UAF" when talking about a situation where accessing the anon_vma object is entirely forbidden because an RCU grace period has passed after it was "freed" with kmem_cache_free().