Re: [PATCH v4 7/7] ARM: implement support for vmap'ed stacks

From: Krzysztof Kozlowski <krzk@kernel.org>
To: Marek Szyprowski <m.szyprowski@samsung.com>,
	Ard Biesheuvel <ardb@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Cc: Russell King <linux@armlinux.org.uk>,
	Nicolas Pitre <nico@fluxnic.net>, Arnd Bergmann <arnd@arndb.de>,
	Kees Cook <keescook@chromium.org>,
	Keith Packard <keithpac@amazon.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Tony Lindgren <tony@atomide.com>,
	'Linux Samsung SOC' <linux-samsung-soc@vger.kernel.org>
Subject: Re: [PATCH v4 7/7] ARM: implement support for vmap'ed stacks
Date: Tue, 21 Dec 2021 11:42:18 +0100	[thread overview]
Message-ID: <b484dea4-da88-3e09-64c5-e1b5bb6cd639@kernel.org> (raw)
In-Reply-To: <0ffc858f-27e7-6789-4be1-c4c5ad61eb9d@samsung.com>

On 21/12/2021 11:38, Marek Szyprowski wrote:
> Hi,
> 
> On 22.11.2021 10:28, Ard Biesheuvel wrote:
>> Wire up the generic support for managing task stack allocations via vmalloc,
>> and implement the entry code that detects whether we faulted because of a
>> stack overrun (or future stack overrun caused by pushing the pt_regs array)
>>
>> While this adds a fair amount of tricky entry asm code, it should be
>> noted that it only adds a TST + branch to the svc_entry path. The code
>> implementing the non-trivial handling of the overflow stack is emitted
>> out-of-line into the .text section.
>>
>> Since on ARM, we rely on do_translation_fault() to keep PMD level page
>> table entries that cover the vmalloc region up to date, we need to
>> ensure that we don't hit such a stale PMD entry when accessing the
>> stack. So we do a dummy read from the new stack while still running from
>> the old one on the context switch path, and bump the vmalloc_seq counter
>> when PMD level entries in the vmalloc range are modified, so that the MM
>> switch fetches the latest version of the entries.
>>
>> Note that we need to increase the per-mode stack by 1 word, to gain some
>> space to stash a GPR until we know it is safe to touch the stack.
>> However, due to the cacheline alignment of the struct, this does not
>> actually increase the memory footprint of the struct stack array at all.
>>
>> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
>> Tested-by: Keith Packard <keithpac@amazon.com>
> 
> 
> This patch landed recently in linux-next 20211220 as commit a1c510d0adc6 
> ("ARM: implement support for vmap'ed stacks"). Sadly it breaks 
> suspend/resume operation on all ARM 32bit Exynos SoCs. Probably the 
> suspend/resume related code must be updated somehow (it partially works 
> on physical addresses and disabled MMU), but I didn't analyze it yet. If 
> you have any hints, let me know.
> 
> 

Maybe this one would help?
https://lore.kernel.org/lkml/20211218085843.212497-2-cuigaosheng1@huawei.com/

Best regards,
Krzysztof

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