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

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Date:   Wed, 3 Mar 2021 14:08:36 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Ravi Bangoria <ravi.bangoria@...ux.ibm.com>
Cc:     mpe@...erman.id.au, 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
Subject: Re: [PATCH v3] powerpc/uprobes: Validation for prefixed instruction

On 2021/03/04 10:35AM, Ravi Bangoria wrote:
> 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>
> ---
> 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))
> +		return 0;

Sorry, I missed this last time, but I think we can drop the above 
checks. ppc_inst_prefixed() already factors in the dependency on 
CONFIG_PPC64, and I don't think we need to confirm if we're running on a 
ISA V3.1 for the below check.

With that:
Acked-by: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>

> +
> +	if (ppc_inst_prefixed(auprobe->insn) && (addr & 0x3F) == 0x3C) {
> +		pr_info_ratelimited("Cannot register a uprobe on 64 byte unaligned prefixed instruction\n");
> +		return -EINVAL;
> +	}
> +

- Naveen