Re: [PATCH v2 3/4] arm64: hyp-stub: Forbid kprobing of the hyp-stub

From: Christoffer Dall <christoffer.dall@arm.com>
To: James Morse <james.morse@arm.com>
Cc: Marc Zyngier <marc.zyngier@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2 3/4] arm64: hyp-stub: Forbid kprobing of the hyp-stub
Date: Thu, 31 Jan 2019 09:04:21 +0100	[thread overview]
Message-ID: <20190131080421.GJ13482@e113682-lin.lund.arm.com> (raw)
In-Reply-To: <20190124163257.233929-4-james.morse@arm.com>

On Thu, Jan 24, 2019 at 04:32:56PM +0000, James Morse wrote:
> The hyp-stub is loaded by the kernel's early startup code at EL2
> during boot, before KVM takes ownership later. The hyp-stub's
> text is part of the regular kernel text, meaning it can be kprobed.
> 
> A breakpoint in the hyp-stub causes the CPU to spin in el2_sync_invalid.
> 
> Add it to the __hyp_text.
> 
> Signed-off-by: James Morse <james.morse@arm.com>
> Cc: stable@vger.kernel.org
> ---
> 
> This has been a problem since kprobes was merged, it should
> probably have been covered in 888b3c8720e0.
> 
> I'm not sure __hyp_text is the right place. Its not idmaped,
> and as it contains a set of vectors, adding it to the host/hyp
> idmap sections could grow them beyond a page... but it does
> run with the MMU off, so does need to be cleaned to PoC when
> anything wacky, like hibernate happens. With this patch,
> hibernate should clean the __hyp_text to PoC too.

How did this code get cleaned before?

Is there a problem you can identify with putting it in __hyp_text?
Seems to me we should just stick it there if it has no negative
side-effects and otherwise we have to make up a separate section with a
specialized meaning.

Thanks,

    Christoffer

> ---
>  arch/arm64/kernel/hyp-stub.S | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/arm64/kernel/hyp-stub.S b/arch/arm64/kernel/hyp-stub.S
> index e1261fbaa374..17f325ba831e 100644
> --- a/arch/arm64/kernel/hyp-stub.S
> +++ b/arch/arm64/kernel/hyp-stub.S
> @@ -28,6 +28,8 @@
>  #include <asm/virt.h>
>  
>  	.text
> +	.pushsection	.hyp.text, "ax"
> +
>  	.align 11
>  
>  ENTRY(__hyp_stub_vectors)
> -- 
> 2.20.1
> 