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Message-ID: <20180428061401.oj4tytn6yy277f7y@gmail.com>
Date: Sat, 28 Apr 2018 08:14:01 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Jeffrey Hugo <jhugo@...eaurora.org>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
Mark Rutland <mark.rutland@....com>,
Jan Glauber <jan.glauber@...iumnetworks.com>,
Kees Cook <keescook@...omium.org>,
Ard Biesheuvel <ard.biesheuvel@...aro.org>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will.deacon@....com>,
Laura Abbott <labbott@...hat.com>,
Timur Tabi <timur@...eaurora.org>,
Stephen Smalley <sds@...ho.nsa.gov>,
Andrew Morton <akpm@...ux-foundation.org>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] init: Fix false positives in W+X checking
* Jeffrey Hugo <jhugo@...eaurora.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@...eaurora.org>
> Reported-by: Jan Glauber <jan.glauber@...iumnetworks.com>
> Fixes: e1a58320a38d ("x86/mm: Warn on W^X mappings")
> Signed-off-by: Jeffrey Hugo <jhugo@...eaurora.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@...nel.org>
Thanks,
Ingo
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