Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <59401856-0e45-0ee6-1e45-667c8e00cf21@redhat.com>
Date:   Fri, 22 Apr 2022 13:04:31 +0200
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.6.2
Subject: Re: [PATCH RFC 2/4] mm, personality: Implement
 memory-deny-write-execute as a personality flag
Content-Language: en-US
To:     Catalin Marinas <catalin.marinas@arm.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Christoph Hellwig <hch@infradead.org>,
        Lennart Poettering <lennart@poettering.net>,
        =?UTF-8?Q?Zbigniew_J=c4=99drzejewski-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>
References: <20220413134946.2732468-1-catalin.marinas@arm.com>
 <20220413134946.2732468-3-catalin.marinas@arm.com>
 <443d978a-7092-b5b1-22f3-ae3a997080ad@redhat.com> <YmKDaEaOpOaKl7m9@arm.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <YmKDaEaOpOaKl7m9@arm.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 22.04.22 12:28, Catalin Marinas wrote:
> On Thu, Apr 21, 2022 at 06:37:49PM +0100, David Hildenbrand wrote:
>> On 13.04.22 15:49, Catalin Marinas wrote:
>>> The aim of such policy is to prevent a user task from inadvertently
>>> creating an executable mapping that is or was writeable (and
>>> subsequently made read-only).
>>>
>>> An example of mmap() returning -EACCESS if the policy is enabled:
>>>
>>> 	mmap(0, size, PROT_READ | PROT_WRITE | PROT_EXEC, flags, 0, 0);
>>>
>>> Similarly, mprotect() would return -EACCESS below:
>>>
>>> 	addr = mmap(0, size, PROT_READ | PROT_EXEC, flags, 0, 0);
>>> 	mprotect(addr, size, PROT_READ | PROT_WRITE | PROT_EXEC);
>>>
>>> With the past vma writeable permission tracking, mprotect() below would
>>> also fail with -EACCESS:
>>>
>>> 	addr = mmap(0, size, PROT_READ | PROT_WRITE, flags, 0, 0);
>>> 	mprotect(addr, size, PROT_READ | PROT_EXEC);
>>>
>>> While the above could be achieved by checking PROT_WRITE & PROT_EXEC on
>>> mmap/mprotect and denying mprotect(PROT_EXEC) altogether (current
>>> systemd MDWE approach via SECCOMP BPF filters), we want the following
>>> scenario to succeed:
>>>
>>> 	addr = mmap(0, size, PROT_READ | PROT_EXEC, flags, 0, 0);
>>> 	mprotect(addr, size, PROT_READ | PROT_EXEC | PROT_BTI);
>>>
>>> where PROT_BTI enables branch tracking identification on arm64.
>>>
>>> The choice for a DENY_WRITE_EXEC personality flag, inherited on fork()
>>> and execve(), was made by analogy to READ_IMPLIES_EXEC.
>>>
>>> Note that it is sufficient to check for VM_WAS_WRITE in
>>> map_deny_write_exec() as this flag is always set on VM_WRITE mappings.
>>>
>>> Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
>>> Cc: Christoph Hellwig <hch@infradead.org>
>>> Cc: Andrew Morton <akpm@linux-foundation.org>
>>
>> How does this interact with get_user_pages(FOLL_WRITE|FOLL_FORCE) on a
>> VMA that is VM_MAYWRITE but not VM_WRITE? Is it handled accordingly?
> 
> For now, that's just about VM_WRITE. Most vmas are VM_MAYWRITE, so we
> can't really have MAYWRITE^EXEC. The basic feature aims to avoid user
> vulnerabilities where a buffer is mapped both writeable and executable.
> Of course, it can be expanded with additional prctl() flags to cover
> other cases.
> 
>> 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.

-- 
Thanks,

David / dhildenb