linux-ext4 - Re: [BUG mm-unstable] BUG: KASAN: use-after-free in shrink_folio

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Date:   Fri, 7 Jul 2023 19:33:15 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     David Wysochanski <dwysocha@...hat.com>
Cc:     Hyeonggon Yoo <42.hyeyoo@...il.com>,
        David Howells <dhowells@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jeff Layton <jlayton@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        linux-afs@...ts.infradead.org, linux-nfs@...r.kernel.org,
        linux-cifs@...r.kernel.org, ceph-devel@...r.kernel.org,
        v9fs-developer@...ts.sourceforge.net, linux-erofs@...ts.ozlabs.org,
        linux-ext4@...r.kernel.org, linux-cachefs@...hat.com,
        linux-fsdevel@...r.kernel.org,
        Rohith Surabattula <rohiths.msft@...il.com>,
        Steve French <sfrench@...ba.org>,
        Shyam Prasad N <nspmangalore@...il.com>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Ilya Dryomov <idryomov@...il.com>, linux-mm@...ck.org,
        Daire Byrne <daire.byrne@...il.com>
Subject: Re: [BUG mm-unstable] BUG: KASAN: use-after-free in
 shrink_folio_list+0x9f4/0x1ae0

On Fri, Jul 07, 2023 at 02:12:06PM -0400, David Wysochanski wrote:
> I think myself / Daire Byrne may have already tracked this down and I
> found a 1-liner that fixed a similar crash in his environment.
> 
> Can you try this patch on top and let me know if it still crashes?
> https://github.com/DaveWysochanskiRH/kernel/commit/902c990e311120179fa5de99d68364b2947b79ec

Said one-liner:
-	struct address_space *mapping = folio->mapping;
+	struct address_space *mapping = folio_mapping(folio);

This will definitely fix the problem.  shrink_folio_list() sees
anonymous folios as well as file folios.

I wonder if we want to go a step further and introduce ...

+static inline bool __folio_needs_release(struct address_space *mapping,
+               struct folio *folio)
+{
+       return folio_has_private(folio) ||
+               (mapping && mapping_release_always(mapping));
+}
+
 /*
  * Return true if a folio needs ->release_folio() calling upon it.
  */
 static inline bool folio_needs_release(struct folio *folio)
 {
-       struct address_space *mapping = folio->mapping;
-
-       return folio_has_private(folio) ||
-               (mapping && mapping_release_always(mapping));
+       return __folio_needs_release(folio_mapping(folio), folio);
 }

since two of the three callers already have done the necessary dance to
get the mapping (and they're the two which happen regularly; the third
is an unusual situation).