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

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Date:   Thu, 4 Mar 2021 08:32:59 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Ravi Bangoria <ravi.bangoria@...ux.ibm.com>, mpe@...erman.id.au
Cc:     jniethe5@...il.com, oleg@...hat.com, rostedt@...dmis.org,
        linux-kernel@...r.kernel.org, paulus@...ba.org,
        sandipan@...ux.ibm.com, naveen.n.rao@...ux.ibm.com,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v3] powerpc/uprobes: Validation for prefixed instruction



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@...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))

cpu_has_feature(CPU_FTR_ARCH_31) should return 'false' when CONFIG_PPC64 is not enabled, no need to 
double check.

> +		return 0;
> +
> +	if (ppc_inst_prefixed(auprobe->insn) && (addr & 0x3F) == 0x3C) {

Maybe 3C instead of 4F ? : (addr & 0x3C) == 0x3C

What about

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

Or ALIGN(addr, SZ_64) != ALIGN(addr + 8, SZ_64)

> +		pr_info_ratelimited("Cannot register a uprobe on 64 byte unaligned prefixed instruction\n");
> +		return -EINVAL;
> +	}
> +
>   	return 0;
>   }
>   
> 

Christophe