linux-kernel - Re: [syzbot] [mm?] kernel BUG in sg_init

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Date: Mon, 18 Mar 2024 11:00:10 -0700
From: Nhat Pham <nphamcs@...il.com>
To: syzbot <syzbot+adbc983a1588b7805de3@...kaller.appspotmail.com>
Cc: akpm@...ux-foundation.org, chengming.zhou@...ux.dev, hannes@...xchg.org, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	syzkaller-bugs@...glegroups.com, yosryahmed@...gle.com, 
	Barry Song <v-songbaohua@...o.com>
Subject: Re: [syzbot] [mm?] kernel BUG in sg_init_one

On Mon, Mar 18, 2024 at 9:58 AM syzbot
<syzbot+adbc983a1588b7805de3@...kaller.appspotmail.com> wrote:
>
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit:    e5eb28f6d1af Merge tag 'mm-nonmm-stable-2024-03-14-09-36' ..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=13043abe180000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=19bb57c23dffc38e
> dashboard link: https://syzkaller.appspot.com/bug?extid=adbc983a1588b7805de3
> compiler:       arm-linux-gnueabi-gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> userspace arch: arm
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1706d231180000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13ba7959180000
>
> Downloadable assets:
> disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/8ead8862021c/non_bootable_disk-e5eb28f6.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/0a7371c63ff2/vmlinux-e5eb28f6.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/7539441b4add/zImage-e5eb28f6.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+adbc983a1588b7805de3@...kaller.appspotmail.com
>
> ------------[ cut here ]------------
> kernel BUG at include/linux/scatterlist.h:187!

Looks like the provided buffer is invalid:

#ifdef CONFIG_DEBUG_SG
BUG_ON(!virt_addr_valid(buf));
#endif

which is "src" from:

sg_init_one(&input, src, entry->length);

Looking at the surrounding code and recent history, there's this
commit that stands out:

mm/zswap: remove the memcpy if acomp is not sleepable
(sha: 270700dd06ca41a4779c19eb46608f076bb7d40e)

which has the effect of, IIUC, using the zpool mapped memory directly
as src, instead of acomp_ctx->buffer (which was previously the case,
as zsmalloc was not sleepable).

This might not necessarily be a bug with that commit itself, but might
have revealed another bug elsewhere.

Anyway, cc-ing the author, Barry Song, to fact check me :) Will take a
closer look later.