linux-kernel - Re: [PATCH resend*3] VFS: simplify seq

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Date:   Sat, 07 Jul 2018 13:23:08 +1000
From:   NeilBrown <neilb@...e.com>
To:     Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-doc@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH resend*3] VFS: simplify seq_file iteration code and interface

On Sat, Jul 07 2018, Jann Horn wrote:

>> @@ -287,11 +278,7 @@ ssize_t seq_read(struct file *file, char __user *buf, size_t size, loff_t *ppos)
>>                 goto Efault;
>>         copied += n;
>>         m->count -= n;
>> -       if (m->count)
>> -               m->from = n;
>> -       else
>> -               pos++;
>> -       m->index = pos;
>> +       m->from = n;
>
> This patch introduces a kernel memory disclosure bug when something
> like the following sequence of events happens (starting from a freshly
> opened seq file):
>
> 1. read(seq_fd, buf, 2000): sets m->from=2000, m->count=100
> 2. create a buffer broken_buf which consists of 1000 bytes writable
> memory followed by unmapped memory
> 3. read(seq_fd, broken_buf, 3100):
>         - flushes buffered data to userspace, result: m->from=2100, m->count=0
>         - accumulates new data, result: m->from=2100, m->count=3050
>         - tries to copy new data to userspace, but fails ("goto Efault")
> 4. read(seq_fd, buf, 4096): does copy_to_user(buf, m->buf + m->from, n)

Thanks for testing and for the report.
I think I see where I went wrong in the patch.
As I said in the description:

  - don't clear ->from when ->count is zero, as ->from is dead when
     ->count is zero.

It is true that ->from is dead when ->count is zero, but as soon as
count becomes non-zero, ->from becomes important again.
So we either need to clear ->from whenever ->count is changed from zero
(which would be clumsy and error prone) we we need to clear
->from somewhere else.

->count is only increased in ->show() calls and there are three ->show()
calls.
- in traverse() ->from is set to zero early, and set once more shortly
  before the function exits, so it is always correct.
- in "we need at least one record in buffer" ->count starts at zero
  so ->from needs to be set to zero as well.
- in "Fill:" ->from is still correct from previous setting.

So I think we just need
	m->from = 0;
at "we need at least one record in buffer".  I'm fairly sure that
will fix the problem you found.  I would appreciate it if you
would test and confirm.
I'll send a patch separately.

Thanks again,
NeilBrown

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