linux-kernel - Re: [PATCH v10 3/6] arm64: add uaccess to machine check safe

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Message-ID: <ZbfjvD1_yKK6IVVY@FVFF77S0Q05N>
Date: Mon, 29 Jan 2024 17:43:24 +0000
From: Mark Rutland <mark.rutland@....com>
To: Tong Tiangen <tongtiangen@...wei.com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, James Morse <james.morse@....com>,
	Robin Murphy <robin.murphy@....com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>,
	Alexander Potapenko <glider@...gle.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrey Konovalov <andreyknvl@...il.com>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Vincenzo Frascino <vincenzo.frascino@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Nicholas Piggin <npiggin@...il.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	"Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
	"Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	kasan-dev@...glegroups.com, wangkefeng.wang@...wei.com,
	Guohanjun <guohanjun@...wei.com>
Subject: Re: [PATCH v10 3/6] arm64: add uaccess to machine check safe

On Mon, Jan 29, 2024 at 09:46:49PM +0800, Tong Tiangen wrote:
> If user process access memory fails due to hardware memory error, only the
> relevant processes are affected, so it is more reasonable to kill the user
> process and isolate the corrupt page than to panic the kernel.
> 
> Signed-off-by: Tong Tiangen <tongtiangen@...wei.com>
> ---
>  arch/arm64/lib/copy_from_user.S | 10 +++++-----
>  arch/arm64/lib/copy_to_user.S   | 10 +++++-----
>  arch/arm64/mm/extable.c         |  8 ++++----
>  3 files changed, 14 insertions(+), 14 deletions(-)
> 
> diff --git a/arch/arm64/lib/copy_from_user.S b/arch/arm64/lib/copy_from_user.S
> index 34e317907524..1bf676e9201d 100644
> --- a/arch/arm64/lib/copy_from_user.S
> +++ b/arch/arm64/lib/copy_from_user.S
> @@ -25,7 +25,7 @@
>  	.endm
>  
>  	.macro strb1 reg, ptr, val
> -	strb \reg, [\ptr], \val
> +	USER(9998f, strb \reg, [\ptr], \val)
>  	.endm

This is a store to *kernel* memory, not user memory. It should not be marked
with USER().

I understand that you *might* want to handle memory errors on these stores, but
the commit message doesn't describe that and the associated trade-off. For
example, consider that when a copy_form_user fails we'll try to zero the
remaining buffer via memset(); so if a STR* instruction in copy_to_user
faulted, upon handling the fault we'll immediately try to fix that up with some
more stores which will also fault, but won't get fixed up, leading to a panic()
anyway...

Further, this change will also silently fixup unexpected kernel faults if we
pass bad kernel pointers to copy_{to,from}_user, which will hide real bugs.

So NAK to this change as-is; likewise for the addition of USER() to other ldr*
macros in copy_from_user.S and the addition of USER() str* macros in
copy_to_user.S.

If we want to handle memory errors on some kaccesses, we need a new EX_TYPE_*
separate from the usual EX_TYPE_KACESS_ERR_ZERO that means "handle memory
errors, but treat other faults as fatal". That should come with a rationale and
explanation of why it's actually useful.

[...]

> diff --git a/arch/arm64/mm/extable.c b/arch/arm64/mm/extable.c
> index 478e639f8680..28ec35e3d210 100644
> --- a/arch/arm64/mm/extable.c
> +++ b/arch/arm64/mm/extable.c
> @@ -85,10 +85,10 @@ bool fixup_exception_mc(struct pt_regs *regs)
>  	if (!ex)
>  		return false;
>  
> -	/*
> -	 * This is not complete, More Machine check safe extable type can
> -	 * be processed here.
> -	 */
> +	switch (ex->type) {
> +	case EX_TYPE_UACCESS_ERR_ZERO:
> +		return ex_handler_uaccess_err_zero(ex, regs);
> +	}

Please fold this part into the prior patch, and start ogf with *only* handling
errors on accesses already marked with EX_TYPE_UACCESS_ERR_ZERO. I think that
change would be relatively uncontroversial, and it would be much easier to
build atop that.

Thanks,
Mark.