linux-kernel - Re: [PATCH] arm64: kprobes: check the return value of set_memory

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Message-ID: <b5b978f3-bb29-4cbc-b006-fb9c4402b067@arm.com>
Date: Tue, 4 Nov 2025 13:14:15 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Yang Shi <yang@...amperecomputing.com>, catalin.marinas@....com,
 will@...nel.org
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: kprobes: check the return value of
 set_memory_rox()

On 03/11/2025 19:45, Yang Shi wrote:
> Since commit a166563e7ec3 ("arm64: mm: support large block mapping when
> rodata=full"), __change_memory_common has more chance to fail due to
> memory allocation fialure when splitting page table. So check the return
> value of set_memory_rox(), then bail out if it fails otherwise we may have
> RW memory mapping for kprobes insn page.
> 
> Fixes: 195a1b7d8388 ("arm64: kprobes: call set_memory_rox() for kprobe page")
> Signed-off-by: Yang Shi <yang@...amperecomputing.com>

This patch looks correct so:

Reviewed-by: Ryan Roberts <ryan.roberts@....com>

but, I think I see an separate issue below...

> ---
> I actually epxected 195a1b7d8388 ("arm64: kprobes: call set_memory_rox()
> for kprobe page") can be merged in 6.17-rcX, so I just restored it to
> before commit 10d5e97c1bf8 ("arm64: use PAGE_KERNEL_ROX directly in
> alloc_insn_page"), however it turned out to be merged in 6.18-rc1 and it
> is after commit a166563e7ec3 ("arm64: mm: support large block mapping when
> rodata=full"). So I made the fix tag point to it.
> And I don't think we need to backport this patch to pre-6.18.
> 
>  arch/arm64/kernel/probes/kprobes.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
> index 8ab6104a4883..43a0361a8bf0 100644
> --- a/arch/arm64/kernel/probes/kprobes.c
> +++ b/arch/arm64/kernel/probes/kprobes.c
> @@ -49,7 +49,10 @@ void *alloc_insn_page(void)
>  	addr = execmem_alloc(EXECMEM_KPROBES, PAGE_SIZE);
>  	if (!addr)
>  		return NULL;
> -	set_memory_rox((unsigned long)addr, 1);
> +	if (set_memory_rox((unsigned long)addr, 1)) {

How does x get cleared when freeing this memory? arm64's set_memory_x() sets
PTE_MAYBE_GP and clears PTE_PXN. The only function that will revert that is
set_memory_nx(). But that only gets called from module_enable_data_nx() (which I
don't think is applicable here) and execmem_force_rw() - but only if
CONFIG_ARCH_HAS_EXECMEM_ROX is enabled, which I don't think it is for arm64?

So I think once we flip a page executable, it will be executable forever?

Do we need to modify set_direct_map_default_noflush() to make the memory nx?
Then vm_reset_perms() will fix it up at vfree time?

Thanks,
Ryan

> +		execmem_free(addr);
> +		return NULL;
> +	}
>  	return addr;
>  }
>