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

[Ian Kent <raven@themaw.net>]

On Tue, 2018-12-18 at 13:01 +0200, Amir Goldstein wrote:
> On Tue, Dec 18, 2018 at 12:43 PM Ian Kent <raven@themaw.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@syzkaller.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, I did get that after looking again once the panic
of my stupid mistake passed, ;)

> 
> Thanks,
> Amir.
> 
> > 
> > From: Ian Kent <raven@themaw.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@syzkaller.appspotmail.com
> > Signed-off-by: Ian Kent <raven@themaw.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:
> >