[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <ZEAEDqVhS0qz6Wx1@casper.infradead.org>
Date: Wed, 19 Apr 2023 16:09:02 +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 Wed, Apr 19, 2023 at 11:07:04AM -0400, Waiman Long wrote:
> On 4/18/23 23:46, Matthew Wilcox wrote:
> > 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;
>
> I am not totally sure if the application changes the VMA to read-only first.
> Even if it does that, it highlights another possible issue when an anonymous
> VMA is merged with a stack VMA. Either the mprotect() to write-protect the
> VMA will fail or the application will segfault if it writes stuff to the
> stack. This particular issue is not related to SELinux. It provides another
> good idea why we should avoid merging stack VMA to anonymous VMA.
mprotect will split the VMA into two VMAs, one that is
PROT_READ|PROT_WRITE and one the is PROT_READ|PROT_EXEC.
Powered by blists - more mailing lists