Re: [PATCH v3] powerpc/uprobes: Validation for prefixed instruction

[Ravi Bangoria <ravi.bangoria@linux.ibm.com>]

On 3/4/21 1:02 PM, Christophe Leroy wrote:
> 
> 
> Le 04/03/2021 à 06:05, Ravi Bangoria a écrit :
>> As per ISA 3.1, prefixed instruction should not cross 64-byte
>> boundary. So don't allow Uprobe on such prefixed instruction.
>>
>> There are two ways probed instruction is changed in mapped pages.
>> First, when Uprobe is activated, it searches for all the relevant
>> pages and replace instruction in them. In this case, if that probe
>> is on the 64-byte unaligned prefixed instruction, error out
>> directly. Second, when Uprobe is already active and user maps a
>> relevant page via mmap(), instruction is replaced via mmap() code
>> path. But because Uprobe is invalid, entire mmap() operation can
>> not be stopped. In this case just print an error and continue.
>>
>> Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
>> ---
>> v2: https://lore.kernel.org/r/20210204104703.273429-1-ravi.bangoria@linux.ibm.com
>> v2->v3:
>>    - Drop restriction for Uprobe on suffix of prefixed instruction.
>>      It needs lot of code change including generic code but what
>>      we get in return is not worth it.
>>
>>   arch/powerpc/kernel/uprobes.c | 8 ++++++++
>>   1 file changed, 8 insertions(+)
>>
>> diff --git a/arch/powerpc/kernel/uprobes.c b/arch/powerpc/kernel/uprobes.c
>> index e8a63713e655..c400971ebe70 100644
>> --- a/arch/powerpc/kernel/uprobes.c
>> +++ b/arch/powerpc/kernel/uprobes.c
>> @@ -41,6 +41,14 @@ int arch_uprobe_analyze_insn(struct arch_uprobe *auprobe,
>>       if (addr & 0x03)
>>           return -EINVAL;
>> +    if (!IS_ENABLED(CONFIG_PPC64) || !cpu_has_feature(CPU_FTR_ARCH_31))
> 
> cpu_has_feature(CPU_FTR_ARCH_31) should return 'false' when CONFIG_PPC64 is not enabled, no need to double check.

Ok.

I'm going to drop CONFIG_PPC64 check because it's not really
required as I replied to Naveen. So, I'll keep CPU_FTR_ARCH_31
check as is.

> 
>> +        return 0;
>> +
>> +    if (ppc_inst_prefixed(auprobe->insn) && (addr & 0x3F) == 0x3C) {
> 
> Maybe 3C instead of 4F ? : (addr & 0x3C) == 0x3C

Didn't follow. It's not (addr & 0x3C), it's (addr & 0x3F).

> 
> What about
> 
> (addr & (SZ_64 - 4)) == SZ_64 - 4 to make it more explicit ?

Yes this is bit better. Though, it should be:

     (addr & (SZ_64 - 1)) == SZ_64 - 4

Ravi