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Date:   Sat, 28 Apr 2018 08:14:01 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Jeffrey Hugo <jhugo@codeaurora.org>
Cc:     linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        Mark Rutland <mark.rutland@arm.com>,
        Jan Glauber <jan.glauber@caviumnetworks.com>,
        Kees Cook <keescook@chromium.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Laura Abbott <labbott@redhat.com>,
        Timur Tabi <timur@codeaurora.org>,
        Stephen Smalley <sds@tycho.nsa.gov>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH v2] init: Fix false positives in W+X checking
Message-ID: <20180428061401.oj4tytn6yy277f7y@gmail.com>
References: <1524866145-20337-1-git-send-email-jhugo@codeaurora.org>
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* Jeffrey Hugo <jhugo@codeaurora.org> wrote:

> load_module() creates W+X mappings via __vmalloc_node_range() (from
> layout_and_allocate()->move_module()->module_alloc()) by using
> PAGE_KERNEL_EXEC.  These mappings are later cleaned up via
> "call_rcu_sched(&freeinit->rcu, do_free_init)" from do_init_module().
> 
> This is a problem because call_rcu_sched() queues work, which can be run
> after debug_checkwx() is run, resulting in a race condition.  If hit, the
> race results in a nasty splat about insecure W+X mappings, which results
> in a poor user experience as these are not the mappings that
> debug_checkwx() is intended to catch.
> 
> This issue is observed on multiple arm64 platforms, and has been
> artificially triggered on an x86 platform.
> 
> Address the race by flushing the queued work before running the
> arch-defined mark_rodata_ro() which then calls debug_checkwx().
> 
> Reported-by: Timur Tabi <timur@codeaurora.org>
> Reported-by: Jan Glauber <jan.glauber@caviumnetworks.com>
> Fixes: e1a58320a38d ("x86/mm: Warn on W^X mappings")
> Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>
> ---
> 
> v1:
> -was "arm64: mm: Fix false positives in W+X checking" (see [1])
> -moved to common code based on review and confirmation of issue on x86
> 
> [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2018-April/573776.html
> 
>  init/main.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/init/main.c b/init/main.c
> index b795aa3..499d957 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1034,6 +1034,13 @@ static int __init set_debug_rodata(char *str)
>  static void mark_readonly(void)
>  {
>  	if (rodata_enabled) {
> +		/*
> +		 * load_module() results in W+X mappings, which are cleaned up
> +		 * with call_rcu_sched().  Let's make sure that queued work is
> +		 * flushed so that we don't hit false positives looking for
> +		 * insecure pages which are W+X.
> +		 */
> +		rcu_barrier_sched();

I'd suggest adding a matching comment to the module loading portion as well,
to make sure this connection does not get lost. With that:

Acked-by: Ingo Molnar <mingo@kernel.org>

Thanks,

	Ingo