linux-kernel - Re: WARNING in __kernel

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Date:   Tue, 29 Sep 2020 08:56:01 +0200
From:   Christoph Hellwig <hch@....de>
To:     Eric Biggers <ebiggers@...nel.org>
Cc:     Christoph Hellwig <hch@....de>,
        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 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@google.com

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.