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

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Date:   Fri, 22 Apr 2022 14:12:21 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Lennart Poettering <lennart@...ttering.net>,
        Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>,
        Will Deacon <will@...nel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>,
        Mark Brown <broonie@...nel.org>,
        Jeremy Linton <Jeremy.Linton@....com>,
        Topi Miettinen <toiwoton@...il.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-abi-devel@...ts.sourceforge.net" 
        <linux-abi-devel@...ts.sourceforge.net>
Subject: Re: [PATCH RFC 2/4] mm, personality: Implement
 memory-deny-write-execute as a personality flag

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