Le 23/02/2022 à 21:48, Gabriel Paubert a écrit :
> On Wed, Feb 23, 2022 at 06:11:36PM +0100, Christophe Leroy wrote:
>> Commit a82adfd5c7cb ("hardening: Introduce CONFIG_ZERO_CALL_USED_REGS")
>> added zeroing of used registers at function exit.
>>
>> At the time being, PPC64 clears volatile registers on syscall exit but
>> PPC32 doesn't do it for performance reason.
>>
>> Add that clearing in PPC32 syscall exit as well, but only when
>> CONFIG_ZERO_CALL_USED_REGS is selected.
>>
>> On an 8xx, the null_syscall selftest gives:
>> - Without CONFIG_ZERO_CALL_USED_REGS : 288 cycles
>> - With CONFIG_ZERO_CALL_USED_REGS : 305 cycles
>> - With CONFIG_ZERO_CALL_USED_REGS + this patch : 319 cycles
>>
>> Note that (independent of this patch), with pmac32_defconfig,
>> vmlinux size is as follows with/without CONFIG_ZERO_CALL_USED_REGS:
>>
>> text data bss dec hex filename
>> 9578869 2525210 194400 12298479 bba8ef vmlinux.without
>> 10318045 2525210 194400 13037655 c6f057 vmlinux.with
>>
>> That is a 7.7% increase on text size, 6.0% on overall size.
>>
>> Signed-off-by: Christophe Leroy <christophe.le...@csgroup.eu>
>> ---
>> arch/powerpc/kernel/entry_32.S | 15 +++++++++++++++
>> 1 file changed, 15 insertions(+)
>>
>> diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
>> index 7748c278d13c..199f23092c02 100644
>> --- a/arch/powerpc/kernel/entry_32.S
>> +++ b/arch/powerpc/kernel/entry_32.S
>> @@ -151,6 +151,21 @@ syscall_exit_finish:
>> bne 3f
>> mtcr r5
>>
>> +#ifdef CONFIG_ZERO_CALL_USED_REGS
>> + /* Zero volatile regs that may contain sensitive kernel data */
>> + li r0,0
>> + li r4,0
>> + li r5,0
>> + li r6,0
>> + li r7,0
>> + li r8,0
>> + li r9,0
>> + li r10,0
>> + li r11,0
>> + li r12,0
>> + mtctr r0
>> + mtxer r0
>
> Here, I'm almost sure that on some processors, it would be better to
> separate mtctr form mtxer. mtxer is typically very expensive (pipeline
> flush) but I don't know what's the best ordering for the average core.
In the 8xx, CTR and LR are handled by the BPU as any other reg (Latency
1 blocage 1).
AFAIU, XER is serialize + 1
>
> And what about lr? Should it also be cleared?
LR is restored from stack.
Christophe
