linux-kernel - Re: [PATCH] mm/mmap: Map MAP_STACK to VM

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Date:   Wed, 19 Apr 2023 04:46:49 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Joe Mario <jmario@...hat.com>,
        Barry Marson <bmarson@...hat.com>,
        Rafael Aquini <aquini@...hat.com>
Subject: Re: [PATCH] mm/mmap: Map MAP_STACK to VM_STACK

On Tue, Apr 18, 2023 at 09:16:37PM -0400, Waiman Long wrote:
>  1) App runs creating lots of threads.
>  2) It mmap's 256K pages of anonymous memory.
>  3) It writes executable code to that memory.
>  4) It calls mprotect() with PROT_EXEC on that memory so
>     it can subsequently execute the code.
> 
> The above mprotect() will fail if the mmap'd region's VMA gets merged with
> the VMA for one of the thread stacks.  That's because the default RHEL
> SELinux policy is to not allow executable stacks.

By the way, this is a daft policy.  The policy you really want is
EXEC|WRITE is not allowed.  A non-writable stack is useless, so it's
actually a superset of your current policy.  Forbidding _simultaneous_
write and executable is just good programming.  This way, you don't need
to care about the underlying VMA's current permissions, you just need
to do:

	if ((prot & (PROT_EXEC|PROT_WRITE)) == (PROT_EXEC|PROT_WRITE))
		return -EACCESS;