Re: [PATCH RFC 2/4] mm, personality: Implement memory-deny-write-execute as a personality flag

From: Catalin Marinas <catalin.marinas@arm.com>
To: David Hildenbrand <david@redhat.com>
Cc: "Andrew Morton" <akpm@linux-foundation.org>,
	"Christoph Hellwig" <hch@infradead.org>,
	"Lennart Poettering" <lennart@poettering.net>,
	"Zbigniew Jędrzejewski-Szmek" <zbyszek@in.waw.pl>,
	"Will Deacon" <will@kernel.org>,
	"Alexander Viro" <viro@zeniv.linux.org.uk>,
	"Eric Biederman" <ebiederm@xmission.com>,
	"Kees Cook" <keescook@chromium.org>,
	"Szabolcs Nagy" <Szabolcs.Nagy@arm.com>,
	"Mark Brown" <broonie@kernel.org>,
	"Jeremy Linton" <Jeremy.Linton@arm.com>,
	"Topi Miettinen" <toiwoton@gmail.com>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-abi-devel@lists.sourceforge.net"
	<linux-abi-devel@lists.sourceforge.net>
Subject: Re: [PATCH RFC 2/4] mm, personality: Implement memory-deny-write-execute as a personality flag
Date: Fri, 22 Apr 2022 14:12:21 +0100	[thread overview]
Message-ID: <YmKptQoIw2mkGdic@arm.com> (raw)
In-Reply-To: <59401856-0e45-0ee6-1e45-667c8e00cf21@redhat.com>

On Fri, Apr 22, 2022 at 01:04:31PM +0200, David Hildenbrand wrote:
> On 22.04.22 12:28, Catalin Marinas wrote:
> > On Thu, Apr 21, 2022 at 06:37:49PM +0100, David Hildenbrand wrote:
> >> Note that in the (FOLL_WRITE|FOLL_FORCE) we only require VM_MAYWRITE on
> >> the vma and trigger a write fault. As the VMA is not VM_WRITE, we won't
> >> actually map the PTE writable, but set it dirty. GUP will retry, find a
> >> R/O pte that is dirty and where it knows that it broke COW and will
> >> allow the read access, although the PTE is R/O.
> >>
> >> That mechanism is required to e.g., set breakpoints in R/O MAP_PRIVATE
> >> kernel sections, but it's used elsewhere for page pinning as well.
> >>
> >> My gut feeling is that GUP(FOLL_WRITE|FOLL_FORCE) could be used right
> >> now to bypass that mechanism, I might be wrong.
> > 
> > GUP can be used to bypass this. But if an attacker can trigger such GUP
> > paths via a syscall (e.g. ptrace(PTRACE_POKEDATA)), I think we need the
> > checks on those paths (and reject the syscall) rather than on
> > mmap/mprotect(). This would be covered by something like CAP_SYS_PTRACE.
> 
> I was told that RDMA uses FOLL_FORCE|FOLL_WRITE and is available to
> unprivileged users.

Ah, do they really need this? At a quick search, ib_umem_get() for
example:

	unsigned int gup_flags = FOLL_WRITE;
	...
	if (!umem->writable)
		gup_flags |= FOLL_FORCE;

I guess with a new MDWE flag we can make the GUP code ignore FOLL_FORCE
if VM_EXEC.

-- 
Catalin

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