Re: WARNING in __kernel_read (2)

[Christoph Hellwig]

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.