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