linux-kernel - Re: [syzbot] [mm?] KMSAN: uninit-value in swap

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Message-ID: <7ng6tntadu62ls32r54aetyevgbghta4oufyzxtq5ym6bprjai@hc2ozb2mbcyb>
Date: Tue, 23 Dec 2025 23:43:32 +0000
From: Pedro Falcato <pfalcato@...e.de>
To: Barry Song <21cnbao@...il.com>
Cc: syzbot <syzbot+178fff6149127421c2cc@...kaller.appspotmail.com>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, Hugh Dickins <hughd@...gle.com>, akpm@...ux-foundation.org, 
	bhe@...hat.com, chrisl@...nel.org, kasong@...cent.com, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, nphamcs@...il.com, shikemeng@...weicloud.com, 
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [mm?] KMSAN: uninit-value in swap_writeout

On Wed, Dec 24, 2025 at 11:46:44AM +1300, Barry Song wrote:
> >
> > Uninit was created at:
> >  __alloc_frozen_pages_noprof+0x421/0xab0 mm/page_alloc.c:5233
> >  alloc_pages_mpol+0x328/0x860 mm/mempolicy.c:2486
> >  folio_alloc_mpol_noprof+0x56/0x1d0 mm/mempolicy.c:2505
> >  shmem_alloc_folio mm/shmem.c:1890 [inline]
> >  shmem_alloc_and_add_folio+0xc56/0x1bd0 mm/shmem.c:1932
> >  shmem_get_folio_gfp+0xad3/0x1fc0 mm/shmem.c:2556
> >  shmem_get_folio mm/shmem.c:2662 [inline]
> >  shmem_symlink+0x562/0xad0 mm/shmem.c:4129
> >  vfs_symlink+0x42f/0x4c0 fs/namei.c:5514
> >  do_symlinkat+0x2ae/0xbb0 fs/namei.c:5541
> 
> +Hugh and Baolin.
> 
> This happens in the shmem symlink path, where newly allocated
> folios are not cleared for some reason. As a result,
> is_folio_zero_filled() ends up reading uninitialized data.
> 

I'm not Hugh nor Baolin, but I would guess that letting
is_folio_zero_filled() skip/disable KMSAN would also work. Since all we want
is to skip writeout if the folio is zero, whether it is incidentally zero, or not,
does not really matter, I think.

-- 
Pedro