----- On Jul 2, 2018, at 5:20 PM, Linus Torvalds torvalds@xxxxxxxxxxxxxxxxxxxx wrote:
> On Mon, Jul 2, 2018 at 2:03 PM Mathieu Desnoyers
> <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>>
>> /* 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;
>> ...
>> What underflow issues are you concerned with ?
>
> That.
>
> Looking closer, it looks like what you want to do is
>
> if (rseq_cs->abort_ip >= rseq_cs->start_ip && rseq_cs->abort_ip <
> rseq_cs->start_ip + rseq_cs->post_commit_offset)
>
> but you're not actually verifying that the range you're testing is
> even vlid, because "rseq_cs->start_ip + rseq_cs->post_commit_offset"
> could be something invalid that overflowed (or, put another way, the
> subtraction you did on both sides to get the simplified version
> underflowed).
>
> So to actually get the range check you want, you should check the
> overflow/underflow condition. Maybe it ends up being
>
> if (rseq_cs->start_ip + rseq_cs->post_commit_offset < rseq_cs->start_ip)
> return -EINVAL;
>
> after which your simplified conditional looks fine.
>
> But I think you should also do
>
> if (rseq_cs->start_ip + rseq_cs->post_commit_offset > TASK_SIZE)
> return -EINVAL;
>
> to make sure the range is valid in the first place.
Taking into account your comments, and adding also an extra check for
rseq_cs->start_ip >= TASK_SIZE, and restricting the end of range
rseq_cs->start_ip + rseq_cs->post_commit_offset to exclude TASK_SIZE
(>= rather than >), the resulting function now looks like this:
static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
{
struct rseq_cs __user *urseq_cs;
unsigned long ptr;
u32 __user *usig;
u32 sig;
if (__get_user(ptr, &t->rseq->rseq_cs))
return -EINVAL;
if (check_rseq_cs_padding(t))
return -EINVAL;
if (!ptr) {
memset(rseq_cs, 0, sizeof(*rseq_cs));
return 0;
}
urseq_cs = (struct rseq_cs __user *)ptr;
if (copy_from_user(rseq_cs, urseq_cs, sizeof(*rseq_cs)) ||
rseq_cs->start_ip >= TASK_SIZE ||
rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE ||
rseq_cs->abort_ip >= TASK_SIZE ||
rseq_cs->version > 0)
return -EINVAL;
/* Check for overflow. */
if (rseq_cs->start_ip + rseq_cs->post_commit_offset < rseq_cs->start_ip)
return -EINVAL;
/* 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;
usig = (u32 __user *)(unsigned long)(rseq_cs->abort_ip - sizeof(u32));
if (get_user(sig, usig))
return -EINVAL;
if (current->rseq_sig != sig) {
printk_ratelimited(KERN_WARNING
"Possible attack attempt. Unexpected rseq signature 0x%x, expecting 0x%x (pid=%d, addr=%p).\n",
sig, current->rseq_sig, current->pid, usig);
return -EINVAL;
}
return 0;
}
The end of range exclusion with (rseq_cs->start_ip + rseq_cs->post_commit_offset >= TASK_SIZE)
stems from the reasoning that we need a valid user-space instruction _after_ the range, so
having the range end exactly at the very last byte of TASK_SIZE would require to have a
user-space instruction at TASK_SIZE, which is not valid.
Does it capture your intent ?
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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