On Thu, Jan 31, 2019 at 11:51 AM Greg KH <gregkh@linuxfoundation.org> wrote:
>
> On Thu, Jan 31, 2019 at 10:54:18PM +1300, Kees Cook wrote:
> > On Thu, Jan 31, 2019 at 7:53 AM syzbot
> > <syzbot+16c3a70e1e9b29346c43@syzkaller.appspotmail.com> wrote:
> > >
> > > Hello,
> > >
> > > syzbot found the following crash on:
> > >
> > > HEAD commit:    02495e76ded5 Add linux-next specific files for 20190130
> > > git tree:       linux-next
> > > console output: https://syzkaller.appspot.com/x/log.txt?x=12cf10df400000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=a2b2e9c0bc43c14d
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=16c3a70e1e9b29346c43
> > > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13266698c00000
> > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1715bb64c00000
> > >
> > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > > Reported-by: syzbot+16c3a70e1e9b29346c43@syzkaller.appspotmail.com
> > >
> > > kasan: CONFIG_KASAN_INLINE enabled
> > > kasan: GPF could be caused by NULL-ptr deref or user memory access
> > > general protection fault: 0000 [#1] PREEMPT SMP KASAN
> > > CPU: 0 PID: 8092 Comm: syz-executor405 Not tainted 5.0.0-rc4-next-20190130
> > > #22
> > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > > Google 01/01/2011
> > > RIP: 0010:relay_set_buf_dentry kernel/relay.c:412 [inline]
> >
> > static inline void relay_set_buf_dentry(struct rchan_buf *buf,
> >                                         struct dentry *dentry)
> > {
> >         buf->dentry = dentry;
> >         d_inode(buf->dentry)->i_size = buf->early_bytes; <--
> > }
> >
> > Doing a bisect landed on this:
> >
> > ff9fb72bc07705c00795ca48631f7fffe24d2c6b ("debugfs: return error
> > values, not NULL")
> >
> > If I revert this patch, I can't reproduce any more. I don't see a
> > relationship, though...
> >
> > My crash appears as:
> > [  121.934378] BUG: unable to handle kernel NULL pointer dereference
> > at 0000000000000047
> > [  121.937187] #PF error: [normal kernel read fault]
> > [  121.938824] PGD 800000041f699067 P4D 800000041f699067 PUD 42d08f067 PMD 0
> > [  121.941166] Oops: 0000 [#1] SMP PTI
> > [  121.942381] CPU: 2 PID: 3134 Comm: relay Not tainted
> > 5.0.0-rc4-next-20190130 #1020
> > [  121.943873] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
> > BIOS 1.10.2-1ubuntu1 04/01/2014
> > [  121.945395] RIP: 0010:relay_open_buf.part.10+0x2b8/0x330
> > ...
> > [  121.960021] Call Trace:
> > [  121.960453]  relay_open+0x18e/0x2c0
> > [  121.961070]  __blk_trace_setup+0x1af/0x350
> > [  121.961777]  blk_trace_ioctl+0x93/0x100
> >
> >
> > $ ./scripts/faddr2line vmlinux relay_open_buf.part.10+0x2b8/0x330
> > relay_open_buf.part.10+0x2b8/0x330:
> > relay_set_buf_dentry at kernel/relay.c:412
> > (inlined by) relay_open_buf at kernel/relay.c:458
> >
> > So it's the same location, but not sure about 0x47 offset. d_inode is
> > 0x58 from dentry. And i_size is 0x50 from inode. If this isn't NULL,
> > but rather an ERR_PTR, the errno is either:
> >
> > EBADF 9 Bad file descriptor
> > EEXIST 17 File exists
> >
> > Neither are used in the debugfs patch, but debugfs is clearly used in
> > do_blk_trace_setup():
> >
> >         if (!blk_debugfs_root)
> >                 return -ENOENT;
> > ...
> >         dir = debugfs_lookup(buts->name, blk_debugfs_root);
> >         if (!dir)
> >                 bt->dir = dir = debugfs_create_dir(buts->name,
> > blk_debugfs_root);
> >         if (!dir)
> >                 goto err;
> > ...
> >         bt->rchan = relay_open("trace", dir, buts->buf_size,
> >                                 buts->buf_nr, &blk_relay_callbacks, bt);
> >
> > Which is confirmed by the next line in my traceback:
> >
> > $ ./scripts/faddr2line vmlinux __blk_trace_setup+0x1af/0x350
> > __blk_trace_setup+0x1af/0x350:
> > do_blk_trace_setup at kernel/trace/blktrace.c:534
> > (inlined by) __blk_trace_setup at kernel/trace/blktrace.c:577
>
> Can you test the patch below?


This can be done as self-service by saying:

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
master

(is it the right tree/base commit for your change? a patch can
generally be applied only to the tree/base commit that you used to
obtain the diff)

See https://github.com/google/syzkaller/blob/master/docs/syzbot.md#testing-patches
for details.

> thanks,
>
> greg k-h
>
> --------------
>
> diff --git a/kernel/relay.c b/kernel/relay.c
> index 04f248644e06..9e0f52375487 100644
> --- a/kernel/relay.c
> +++ b/kernel/relay.c
> @@ -428,6 +428,8 @@ static struct dentry *relay_create_buf_file(struct rchan *chan,
>         dentry = chan->cb->create_buf_file(tmpname, chan->parent,
>                                            S_IRUSR, buf,
>                                            &chan->is_global);
> +       if (IS_ERR(dentry))
> +               dentry = NULL;
>
>         kfree(tmpname);
>
> @@ -461,7 +463,7 @@ static struct rchan_buf *relay_open_buf(struct rchan *chan, unsigned int cpu)
>                 dentry = chan->cb->create_buf_file(NULL, NULL,
>                                                    S_IRUSR, buf,
>                                                    &chan->is_global);
> -               if (WARN_ON(dentry))
> +               if (IS_ERR_OR_NULL(dentry))
>                         goto free_buf;
>         }