On Sat, Jul 07 2018, Jann Horn wrote:

quoted

@@ -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