RE: WARNING in __kernel_read (2)

[David Laight]

From: Christoph Hellwig
> Sent: 29 September 2020 07:56
> 
> On Mon, Sep 28, 2020 at 11:46:48PM -0700, Eric Biggers wrote:
> > > 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@xxxxxxxxxx
> 
> Passing a fs without read permissions definitively looks bogus for
> the finit_module syscall.  So I think all we need is an extra check
> to validate the fd.

The sysbot test looked like it didn't even have a regular file.
I thought I saw a test for that - but it might be in a different path.

You do need to ensure that 'exec' doesn't need read access.

	David

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