[PATCH] powerpc/signal64: Don't read sigaction arguments back from user memory

[Nicholas Piggin]

Excerpts from Christophe Leroy's message of June 14, 2021 3:30 pm:
> 
> 
> Le 14/06/2021 à 03:32, Nicholas Piggin a écrit :
>> Excerpts from Michael Ellerman's message of June 10, 2021 5:29 pm:
>>> When delivering a signal to a sigaction style handler (SA_SIGINFO), we
>>> pass pointers to the siginfo and ucontext via r4 and r5.
>>>
>>> Currently we populate the values in those registers by reading the
>>> pointers out of the sigframe in user memory, even though the values in
>>> user memory were written by the kernel just prior:
>>>
>>>    unsafe_put_user(&frame->info, &frame->pinfo, badframe_block);
>>>    unsafe_put_user(&frame->uc, &frame->puc, badframe_block);
>>>    ...
>>>    if (ksig->ka.sa.sa_flags & SA_SIGINFO) {
>>>    	err |= get_user(regs->gpr[4], (unsigned long __user *)&frame->pinfo);
>>>    	err |= get_user(regs->gpr[5], (unsigned long __user *)&frame->puc);
>>>
>>> ie. we write &frame->info into frame->pinfo, and then read frame->pinfo
>>> back into r4, and similarly for &frame->uc.
>>>
>>> The code has always been like this, since linux-fullhistory commit
>>> d4f2d95eca2c ("Forward port of 2.4 ppc64 signal changes.").
>>>
>>> There's no reason for us to read the values back from user memory,
>>> rather than just setting the value in the gpr[4/5] directly. In fact
>>> reading the value back from user memory opens up the possibility of
>>> another user thread changing the values before we read them back.
>>> Although any process doing that would be racing against the kernel
>>> delivering the signal, and would risk corrupting the stack, so that
>>> would be a userspace bug.
>>>
>>> Note that this is 64-bit only code, so there's no subtlety with the size
>>> of pointers differing between kernel and user. Also the frame variable
>>> is not modified to point elsewhere during the function.
>>>
>>> In the past reading the values back from user memory was not costly, but
>>> now that we have KUAP on some CPUs it is, so we'd rather avoid it for
>>> that reason too.
>>>
>>> So change the code to just set the values directly, using the same
>>> values we have written to the sigframe previously in the function.
>>>
>>> Note also that this matches what our 32-bit signal code does.
>>>
>>> Using a version of will-it-scale's signal1_threads that sets SA_SIGINFO,
>>> this results in a ~4% increase in signals per second on a Power9, from
>>> 229,777 to 239,766.
>> 
>> Good find, nice improvement. Will make it possible to make the error
>> handling much nicer too I think.
>> 
>> Reviewed-by: Nicholas Piggin <npiggin at gmail.com>
>> 
>> You've moved copy_siginfo_to_user right up to the user access unlock,
>> could save 2 more KUAP lock/unlocks if we had an unsafe_clear_user. If
>> we can move the other user access stuff up as well, the stack frame
>> put_user could use unsafe_put_user as well, saving 1 more. Another few
>> percent?
> 
> I'm looking at making an 'unsafe' version of copy_siginfo_to_user().
> That's straight forward for 'native' signals, but for compat signals that's more tricky.

Ah nice. Native is most important at the moment.

Thanks,
Nick