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

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Date:   Tue, 9 Mar 2021 16:51:15 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     Ravi Bangoria <ravi.bangoria@...ux.ibm.com>, 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
Subject: Re: [PATCH v4] powerpc/uprobes: Validation for prefixed instruction

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")

> 
> > ---
> > v3: https://lore.kernel.org/r/20210304050529.59391-1-ravi.bangoria@linux.ibm.com
> > v3->v4:
> >   - CONFIG_PPC64 check was not required, remove it.
> >   - Use SZ_ macros instead of hardcoded numbers.
> >
> >  arch/powerpc/kernel/uprobes.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > 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 ;)

- Naveen