Re: [RFC PATCH for 4.18 1/2] rseq: use __u64 for rseq_cs fields, validate abort_ip < TASK_SIZE

----- On Jul 2, 2018, at 4:52 PM, Linus Torvalds torvalds@linux-foundation.org wrote:

On Mon, Jul 2, 2018 at 1:41 PM Mathieu Desnoyers
[off-list ref] wrote:

quoted

-       if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)))
+       if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)) ||
+           rseq_cs->abort_ip >= TASK_SIZE)
                return -EFAULT;

I think the abort_ip check should have the same error value as the
other sanity checks, ie just be of this format:

quoted

        if (rseq_cs->version > 0)
                return -EINVAL;

OK, so I'll go for -EINVAL.

also, I think you should check start_ip to be consistent. You kind of
accidentally do it with the check for

       if (rseq_cs->abort_ip - rseq_cs->start_ip - rseq_cs->post_commit_offset)

The check is actually:

        /* Ensure that abort_ip is not in the critical section. */
        if (rseq_cs->abort_ip - rseq_cs->start_ip < rseq_cs->post_commit_offset)
                return -EINVAL;

but honestly, that has underflow issues already, so I think you want
to basically make the check be

       if (rseq_cs->abort_ip >= TASK_SIZE)
               return -EINVAL;

that works.

       if (rseq_cs->start_ip >= rseq_cs->abort_ip)
               return -EINVAL;

this one does not work. We need to ensure that abort_ip
is not between [ start_ip, start_ip + post_commit_offset ]. The
check you propose validates that start_ip is below abort_ip,
which is bogus. For instance, abort_ip can very well be in a
different section of the binary, at an address either below
or above start_ip.

which takes care of checkint  start_ip, and also the underflow for the
post_commit_offset check.

What underflow issues are you concerned with ?

If somebody is depending on negative offsets, then that
post_commit_offset logic is already wrong.

quoted

+       usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32));
        ret = get_user(sig, usig);

That can underflow too, but I guess we can just rely on get_user()
getting it right.

Yes, get_user() should handle that one properly.

Thanks,

Mathieu

               Linus

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com