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Date: Mon, 19 Oct 2026 16:52:10 +0300
From: Mike Rapoport <rppt@kernel.org>
To: "David Hildenbrand (Arm)" <david@kernel.org>
Cc: "Ning, Hongyu" <hongyu.ning@linux.intel.com>, x86@kernel.org,
	shuah@kernel.org, rick.p.edgecombe@intel.com, tglx@kernel.org,
	mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
	hpa@zytor.com, ljs@kernel.org, akpm@linux-foundation.org,
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
	hongyu.ning@intel.com, jolsa@kernel.org, dwindsor@gmail.com
Subject: Re: [PATCH] selftests/x86: Fix userfaultfd test for shadow stack
 rejection
Message-ID: <atYginh9CIXA0baV@kernel.org>
References: <3b3e0a35-e664-40ab-84ad-afa3e31e3aea@linux.intel.com>
 <493158de-7b76-45ca-9d71-ad8981242c12@kernel.org>
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In-Reply-To: <493158de-7b76-45ca-9d71-ad8981242c12@kernel.org>

On Mon, Oct 19, 2026 at 03:03:43PM +0200, David Hildenbrand (Arm) wrote:
> On 10/19/26 03:03, Ning, Hongyu wrote:
> > Commit 3c58f641e813 ("userfaultfd: prevent registration of special
> > VMAs") prevents userfaultfd registration on shadow stack VMAs to avoid
> > bypassing shadow stack protections through UFFDIO_COPY.
> > 
> > The existing shadow stack selftest still expects UFFDIO_REGISTER to
> > succeed and uses UFFDIO_COPY to populate the shadow stack. This causes
> > the selftest to fail on Kernels containing this security fix.
> > 
> > Update the selftest to verify that UFFDIO_REGISTER rejects shadow stack
> > VMAs with EINVAL. Remove the obsolete userfaultfd fault handling thread
> > and UFFDIO_COPY test path.
> > 
> > Tested on: Intel shadow stack capable x86_64 platform
> > 
> > Before fix:
> > 	[FAIL]  Userfaultfd test with return code 1
> > 
> > After fix:
> > 	[OK]    Userfaultfd rejects shadow stack registration (EINVAL)
> 
> 
> Interesting, so we wouldn't be able to snapshot+restore an app that uses shadow
> stacks.
> 
> @Mike: I assume that's not a concern for CRIU etc, right?

CRIU can do snapshots without uffd and it can do shadow stacks in its own
inventive way :)
 
> > 
> > Fixes: 3c58f641e813 ("userfaultfd: prevent registration of special VMAs")
> > 
> > Signed-off-by: Hongyu Ning <hongyu.ning@linux.intel.com>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> 
> -- 
> Cheers,
> 
> David

-- 
Sincerely yours,
Mike.