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

Ravi Bangoria [off-list ref] writes:

On 3/9/21 4:51 PM, Naveen N. Rao wrote:

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On 2021/03/09 08:54PM, Michael Ellerman wrote:

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Ravi Bangoria [off-list ref] writes:

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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 <redacted>
Acked-by: Naveen N. Rao <redacted>

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.

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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