linux-kernel - Re: [PATCH v4] powerpc/uprobes: Validation for prefixed instruction

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Date:   Wed, 10 Mar 2021 16:13:51 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Ravi Bangoria <ravi.bangoria@...ux.ibm.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:     oleg@...hat.com, rostedt@...dmis.org, paulus@...ba.org,
        jniethe5@...il.com, naveen.n.rao@...ux.ibm.com,
        sandipan@...ux.ibm.com, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, christophe.leroy@...roup.eu,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Subject: Re: [PATCH v4] powerpc/uprobes: Validation for prefixed instruction

Ravi Bangoria <ravi.bangoria@...ux.ibm.com> writes:
> On 3/9/21 4:51 PM, Naveen N. Rao wrote:
>> On 2021/03/09 08:54PM, Michael Ellerman wrote:
>>> Ravi Bangoria <ravi.bangoria@...ux.ibm.com> writes:
>>>> 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@...ux.ibm.com>
>>>> Acked-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
>>>
>>> Do we have a Fixes: tag for this?
>> 
>> Since this is an additional check we are adding, I don't think we should
>> add a Fixes: tag. Nothing is broken per-se -- we're just adding more
>> checks to catch simple mistakes. Also, like Oleg pointed out, there are
>> still many other ways for users to shoot themselves in the foot with
>> uprobes and prefixed instructions, if they so desire.
>> 
>> However, if you still think we should add a Fixes: tag, we can perhaps
>> use the below commit since I didn't see any specific commit adding
>> support for prefixed instructions for uprobes:
>> 
>> Fixes: 650b55b707fdfa ("powerpc: Add prefixed instructions to
>> instruction data type")
>
> True. IMO, It doesn't really need any Fixes tag.

Yep OK, I'm happy without a Fixes tag based on that explanation.

>>>> diff --git a/arch/powerpc/kernel/uprobes.c b/arch/powerpc/kernel/uprobes.c
>>>> index e8a63713e655..4cbfff6e94a3 100644
>>>> --- a/arch/powerpc/kernel/uprobes.c
>>>> +++ b/arch/powerpc/kernel/uprobes.c
>>>> @@ -41,6 +41,13 @@ int arch_uprobe_analyze_insn(struct arch_uprobe *auprobe,
>>>>   	if (addr & 0x03)
>>>>   		return -EINVAL;
>>>>   
>>>> +	if (cpu_has_feature(CPU_FTR_ARCH_31) &&
>>>> +	    ppc_inst_prefixed(auprobe->insn) &&
>>>> +	    (addr & (SZ_64 - 4)) == SZ_64 - 4) {
>>>> +		pr_info_ratelimited("Cannot register a uprobe on 64 byte unaligned prefixed instruction\n");
>>>> +		return -EINVAL;
>>>
>>> I realise we already did the 0x03 check above, but I still think this
>>> would be clearer simply as:
>>>
>>> 	    (addr & 0x3f == 60)
>> 
>> Indeed, I like the use of `60' there -- hex is overrated ;)
>
> Sure. Will resend.

Thanks.

cheers