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

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Message-ID: <443d978a-7092-b5b1-22f3-ae3a997080ad@redhat.com>
Date:   Thu, 21 Apr 2022 19:37:49 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Lennart Poettering <lennart@...ttering.net>,
        Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>
Cc:     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-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-abi-devel@...ts.sourceforge.net
Subject: Re: [PATCH RFC 2/4] mm, personality: Implement
 memory-deny-write-execute as a personality flag

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@....com>
> Cc: Christoph Hellwig <hch@...radead.org>
> Cc: Andrew Morton <akpm@...ux-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?

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.

-- 
Thanks,

David / dhildenb