linux-kernel - Re: [syzbot] [mm?] [udf?] KASAN: null-ptr-deref Read in filemap

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Message-ID: <20230503122337.fxyfchh4ximvnuwn@quack3>
Date:   Wed, 3 May 2023 14:23:37 +0200
From:   Jan Kara <jack@...e.cz>
To:     syzbot <syzbot+48011b86c8ea329af1b9@...kaller.appspotmail.com>
Cc:     akpm@...ux-foundation.org, hch@....de, jack@...e.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, netdev@...r.kernel.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [mm?] [udf?] KASAN: null-ptr-deref Read in filemap_fault

On Wed 03-05-23 02:23:19, syzbot wrote:
> syzbot has bisected this issue to:
> 
> commit 66dabbb65d673aef40dd17bf62c042be8f6d4a4b
> Author: Christoph Hellwig <hch@....de>
> Date:   Tue Mar 7 14:34:10 2023 +0000
> 
>     mm: return an ERR_PTR from __filemap_get_folio
> 
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=15220608280000
> start commit:   865fdb08197e Merge tag 'input-for-v6.4-rc0' of git://git.k..
> git tree:       upstream
> final oops:     https://syzkaller.appspot.com/x/report.txt?x=17220608280000
> console output: https://syzkaller.appspot.com/x/log.txt?x=13220608280000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=d1c8518c09009bad
> dashboard link: https://syzkaller.appspot.com/bug?extid=48011b86c8ea329af1b9
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=137594c4280000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=10cfd602280000
> 
> Reported-by: syzbot+48011b86c8ea329af1b9@...kaller.appspotmail.com
> Fixes: 66dabbb65d67 ("mm: return an ERR_PTR from __filemap_get_folio")
> 
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection

Actually looking into commit 66dabbb65d67 it seems it has two problems -
one in filemap_fault() where in:

               if (IS_ERR(folio)) {
                        if (fpin)
                                goto out_retry;

we need to clear folio so that we don't try to folio_put() the error
pointer. And another in afs_dir_get_folio() where the changes effectively
make that function return ERR_PTR instead of NULL as well which will then
confuse afs_edit_dir_add() and other callers (which were not updated).

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR