linux-kernel - Re: [syzbot] [fs?] WARNING in stable_page

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Z5TzIj7A_DzT5688@casper.infradead.org>
Date: Sat, 25 Jan 2025 14:20:18 +0000
From: Matthew Wilcox <willy@...radead.org>
To: syzbot <syzbot+069bb8b6fd64a600ab7b@...kaller.appspotmail.com>
Cc: linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [fs?] WARNING in stable_page_flags

On Fri, Jan 24, 2025 at 06:34:23PM -0800, syzbot wrote:
> ------------[ cut here ]------------
> WARNING: CPU: 0 PID: 6789 at ./include/linux/mm.h:1228 folio_large_mapcount include/linux/mm.h:1228 [inline]
> WARNING: CPU: 0 PID: 6789 at ./include/linux/mm.h:1228 folio_mapcount include/linux/mm.h:1262 [inline]
> WARNING: CPU: 0 PID: 6789 at ./include/linux/mm.h:1228 folio_mapped include/linux/mm.h:1273 [inline]
> WARNING: CPU: 0 PID: 6789 at ./include/linux/mm.h:1228 page_mapped include/linux/mm.h:1283 [inline]
> WARNING: CPU: 0 PID: 6789 at ./include/linux/mm.h:1228 stable_page_flags+0xab5/0xbc0 fs/proc/page.c:132

I'm shocked we haven't seen this before.

kpageflags_read() iterates over every PFN in a range, calling
pfn_to_page() and then page_folio() on each of them.  Since it makes
no attempt to establish a refcount on the folio (nor should it), the
page/folio can be freed under it.  And that's what's happened; when it
first looked, this page was part of a slab, and now it's free.

What we need to do is memcpy() the page, just like we do in dump_page().
I'll get to it next week.