linux-kernel - Re: kernel BUG at fs/inode.c:LINE!

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Date:   Tue, 18 Dec 2018 13:01:45 +0200
From:   Amir Goldstein <amir73il@...il.com>
To:     Ian Kent <raven@...maw.net>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        syzbot+5399ed0832693e29f392@...kaller.appspotmail.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        syzkaller-bugs@...glegroups.com
Subject: Re: kernel BUG at fs/inode.c:LINE!

On Tue, Dec 18, 2018 at 12:43 PM Ian Kent <raven@...maw.net> wrote:
>
> On Mon, 2018-12-17 at 07:21 +0000, Al Viro wrote:
> > On Sun, Dec 16, 2018 at 10:11:04PM -0800, syzbot wrote:
> > > Hello,
> > >
> > > syzbot found the following crash on:
> > >
> > > HEAD commit:    d14b746c6c1c Add linux-next specific files for 20181214
> > > git tree:       linux-next
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=13706347400000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=1da6d2d18f803140
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=5399ed0832693e29f392
> > > compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=101032b3400000
> > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16534063400000
> > >
> > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > > Reported-by: syzbot+5399ed0832693e29f392@...kaller.appspotmail.com
> > >
> > >  slab_pre_alloc_hook mm/slab.h:423 [inline]
> > >  slab_alloc mm/slab.c:3365 [inline]
> > >  kmem_cache_alloc+0x2c4/0x730 mm/slab.c:3539
> > >  __d_alloc+0xc8/0xb90 fs/dcache.c:1599
> > > ------------[ cut here ]------------
> > > kernel BUG at fs/inode.c:1566!
> > >  d_alloc_anon fs/dcache.c:1698 [inline]
> > >  d_make_root+0x43/0xc0 fs/dcache.c:1885
> > >  autofs_fill_super+0x6f1/0x1c30 fs/autofs/inode.c:273
> >
> > Huh?  BUG is in iput(), AFAICS, so the stack trace is rather misreported.
> > iput() can be called by d_make_root(), provided that dentry allocation
> > fails.  So the most straightforward interpretation would be that we
> > had an allocation failure (injected?), followed by iput() of the inode
> > passed to d_make_root().  Which happened to find I_CLEAR in ->i_state
> > of that inode somehow, which should be impossible short of seriously
> > buggered inode refcounting somewhere - the inode has just been returned
> > by new_inode(), which clears i_state, and it would have to have passed
> > clear_inode() (i.e. has been through inode eviction) since then...
>
> Sorry Al, that's my bad.
>
> See https://www.ozlabs.org/~akpm/mmotm/broken-out/autofs-fix-possible-inode-leak-in-autofs_fill_super.patch
>
> I think this will fix it, I'll forward it to Andrew if you agree:
>
> autofs - fix handling of d_make_root() return in autofs_fill_super()

You realize you can just revert that patch.
d_make_root() can take NULL inode as argument.
At the very least, please mention the offending commit with Fixes tag.

Thanks,
Amir.

>
> From: Ian Kent <raven@...maw.net>
>
> A previous change to handle a possible inode leak in autofs_fill_super()
> added an iput() on d_make_root() failure but d_make_root() already puts
> the passed in inode on failure.
>
> Reported-by: syzbot+5399ed0832693e29f392@...kaller.appspotmail.com
> Signed-off-by: Ian Kent <raven@...maw.net>
> ---
>  fs/autofs/inode.c |    4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/fs/autofs/inode.c b/fs/autofs/inode.c
> index 501833cc49a8..953f76b95172 100644
> --- a/fs/autofs/inode.c
> +++ b/fs/autofs/inode.c
> @@ -271,7 +271,7 @@ int autofs_fill_super(struct super_block *s, void *data, int silent)
>         }
>         root = d_make_root(root_inode);
>         if (!root)
> -               goto fail_iput;
> +               goto fail_ino;
>         pipe = NULL;
>
>         root->d_fsdata = ino;
> @@ -347,8 +347,6 @@ int autofs_fill_super(struct super_block *s, void *data, int silent)
>  fail_dput:
>         dput(root);
>         goto fail_free;
> -fail_iput:
> -       iput(root_inode);
>  fail_ino:
>         autofs_free_ino(ino);
>  fail_free:
>