linux-kernel - Re: WARNING in __kernel

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Date:   Mon, 28 Sep 2020 23:46:48 -0700
From:   Eric Biggers <ebiggers@...nel.org>
To:     Christoph Hellwig <hch@....de>
Cc:     David Laight <David.Laight@...lab.com>,
        syzbot+51177e4144d764827c45@...kaller.appspotmail.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk
Subject: Re: WARNING in __kernel_read (2)

On Tue, Sep 29, 2020 at 08:38:15AM +0200, Christoph Hellwig wrote:
> On Mon, Sep 28, 2020 at 03:14:41PM -0700, Eric Biggers wrote:
> > On Sat, Sep 26, 2020 at 01:17:04PM +0000, David Laight wrote:
> > > From: David Laight
> > > > Sent: 26 September 2020 12:16
> > > > To: 'syzbot' <syzbot+51177e4144d764827c45@...kaller.appspotmail.com>; linux-fsdevel@...r.kernel.org;
> > > > linux-kernel@...r.kernel.org; syzkaller-bugs@...glegroups.com; viro@...iv.linux.org.uk
> > > > Subject: RE: WARNING in __kernel_read (2)
> > > > 
> > > > > From: syzbot <syzbot+51177e4144d764827c45@...kaller.appspotmail.com>
> > > > > Sent: 26 September 2020 03:58
> > > > > To: linux-fsdevel@...r.kernel.org; linux-kernel@...r.kernel.org; syzkaller-bugs@...glegroups.com;
> > > > > viro@...iv.linux.org.uk
> > > > > Subject: WARNING in __kernel_read (2)
> > > > 
> > > > I suspect this is calling finit_module() on an fd
> > > > that doesn't have read permissions.
> > > 
> > > Code inspection also seems to imply that the check means
> > > the exec() also requires read permissions on the file.
> > > 
> > > This isn't traditionally true.
> > > suid #! scripts are particularly odd without 'owner read'
> > > (everyone except the owner can run them!).
> > 
> > Christoph, any thoughts here?  You added this WARN_ON_ONCE in:
> > 
> > 	commit 61a707c543e2afe3aa7e88f87267c5dafa4b5afa
> > 	Author: Christoph Hellwig <hch@....de>
> > 	Date:   Fri May 8 08:54:16 2020 +0200
> > 
> > 	    fs: add a __kernel_read helper
> 
> Linus asked for it.  What is the call chain that we hit it with?

Call Trace:
 kernel_read+0x52/0x70 fs/read_write.c:471
 kernel_read_file fs/exec.c:989 [inline]
 kernel_read_file+0x2e5/0x620 fs/exec.c:952
 kernel_read_file_from_fd+0x56/0xa0 fs/exec.c:1076
 __do_sys_finit_module+0xe6/0x190 kernel/module.c:4066
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

See the email from syzbot for the full details:
https://lkml.kernel.org/linux-fsdevel/000000000000da992305b02e9a51@google.com