Re: general protection fault in relay_open_buf

[Greg KH]

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@xxxxxxxxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > 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?

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;
 	}