linux-hardening - Re: [PATCH v7 3/6] stack: Optionally randomize kernel stack offset each syscall

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Date:   Sun, 28 Mar 2021 16:42:03 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Kees Cook <keescook@...omium.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Elena Reshetova <elena.reshetova@...el.com>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Potapenko <glider@...gle.com>,
        Alexander Popov <alex.popov@...ux.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Jann Horn <jannh@...gle.com>, Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        Randy Dunlap <rdunlap@...radead.org>,
        kernel-hardening@...ts.openwall.com,
        linux-hardening@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 3/6] stack: Optionally randomize kernel stack offset each syscall

Kees,

On Fri, Mar 19 2021 at 14:28, Kees Cook wrote:
> +/*
> + * Do not use this anywhere else in the kernel. This is used here because
> + * it provides an arch-agnostic way to grow the stack with correct
> + * alignment. Also, since this use is being explicitly masked to a max of
> + * 10 bits, stack-clash style attacks are unlikely. For more details see
> + * "VLAs" in Documentation/process/deprecated.rst

VLAs are bad, VLAs to the rescue! :)

> + * The asm statement is designed to convince the compiler to keep the
> + * allocation around even after "ptr" goes out of scope.
> + */
> +void *__builtin_alloca(size_t size);
> +
> +#define add_random_kstack_offset() do {					\
> +	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
> +				&randomize_kstack_offset)) {		\
> +		u32 offset = this_cpu_read(kstack_offset);              \

Not that it matters on x86, but as this has to be called in the
interrupt disabled region of the syscall entry, shouldn't this be a
raw_cpu_read(). The asm-generic version has a preempt_disable/enable
pair around the raw read for native wordsize reads, otherwise a
irqsave/restore pair.

__this_cpu_read() is fine as well, but that has an sanity check before
the raw read when CONFIG_DEBUG_PREEMPT is on, which is harmless but
also pointless in this case.

Probably the same for the counterpart this_cpu_write().

Thanks,

        tglx