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

[Christophe Leroy]

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.

> 
> 
>>
>> Signed-off-by: Michael Ellerman <mpe at ellerman.id.au>
>> ---
>>   arch/powerpc/kernel/signal_64.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
>> index dca66481d0c2..f58e7a98d0df 100644
>> --- a/arch/powerpc/kernel/signal_64.c
>> +++ b/arch/powerpc/kernel/signal_64.c
>> @@ -948,8 +948,8 @@ int handle_rt_signal64(struct ksignal *ksig, sigset_t *set,
>>   	regs->gpr[3] = ksig->sig;
>>   	regs->result = 0;
>>   	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);
>> +		regs->gpr[4] = (unsigned long)&frame->info;
>> +		regs->gpr[5] = (unsigned long)&frame->uc;
>>   		regs->gpr[6] = (unsigned long) frame;
>>   	} else {
>>   		regs->gpr[4] = (unsigned long)&frame->uc.uc_mcontext;
>> -- 
>> 2.25.1
>>
>>