Re: [syzbot] [mm?] kernel BUG in sg_init_one

[Yosry Ahmed]

On Mon, Mar 18, 2024 at 1:25 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Tue, Mar 19, 2024 at 7:00 AM Nhat Pham <nphamcs@xxxxxxxxx> wrote:
> >
> > On Mon, Mar 18, 2024 at 9:58 AM syzbot
> > <syzbot+adbc983a1588b7805de3@xxxxxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > >
> > > ------------[ 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.
>
> I guess that is because on arm32 , we have highmem but
> sg_init_one supports lowmem only. the below should be
> able to fix?
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 9dec853647c8..47c0386caba2 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1086,7 +1086,8 @@ static void zswap_decompress(struct zswap_entry
> *entry, struct page *page)
>                 zpool_unmap_handle(zpool, entry->handle);
>         }
>
> -       sg_init_one(&input, src, entry->length);
> +       sg_init_table(&input, 1);
> +       sg_set_page(&input, kmap_to_page(src), entry->length,
> offset_in_page(src));

Is this working around the debug check in sg_init_one()? IIUC, only
lowmem pages are supported. We may be passing in a highmem page to
sg_set_page() now, right?

Also, it seems like if src is a lowmem address kmap_to_page() will be
doing unnecessary checks (assuming it's working correctly)?

Would it be more robust to just use the temporary buffer if src is a
kmap address?

Also FWIW, I think you can use "#sys test" to check if a diff fixes the problem.

>         sg_init_table(&output, 1);
>         sg_set_page(&output, page, PAGE_SIZE, 0);
>         acomp_request_set_params(acomp_ctx->req, &input, &output,
> entry->length, PAGE_SIZE);
>
>
>
> >