From: David Hildenbrand <david@redhat.com>
To: "Catalin Marinas" <catalin.marinas@arm.com>,
"Andrew Morton" <akpm@linux-foundation.org>,
"Christoph Hellwig" <hch@infradead.org>,
"Lennart Poettering" <lennart@poettering.net>,
"Zbigniew Jędrzejewski-Szmek" <zbyszek@in.waw.pl>
Cc: 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-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org,
linux-abi-devel@lists.sourceforge.net
Subject: Re: [PATCH RFC 2/4] mm, personality: Implement memory-deny-write-execute as a personality flag
Date: Thu, 21 Apr 2022 19:37:49 +0200 [thread overview]
Message-ID: <443d978a-7092-b5b1-22f3-ae3a997080ad@redhat.com> (raw)
In-Reply-To: <20220413134946.2732468-3-catalin.marinas@arm.com>
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?
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
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next prev parent reply other threads:[~2022-04-21 17:39 UTC|newest]
Thread overview: 25+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-04-13 13:49 [PATCH RFC 0/4] mm, arm64: In-kernel support for memory-deny-write-execute (MDWE) Catalin Marinas
2022-04-13 13:49 ` [PATCH RFC 1/4] mm: Track previously writeable vma permission Catalin Marinas
2022-04-13 13:49 ` [PATCH RFC 2/4] mm, personality: Implement memory-deny-write-execute as a personality flag Catalin Marinas
2022-04-21 17:37 ` David Hildenbrand [this message]
2022-04-22 10:28 ` Catalin Marinas
2022-04-22 11:04 ` David Hildenbrand
2022-04-22 13:12 ` Catalin Marinas
2022-04-22 17:41 ` David Hildenbrand
2022-04-13 13:49 ` [PATCH RFC 3/4] fs/binfmt_elf: Tell user-space about the DENY_WRITE_EXEC " Catalin Marinas
2022-04-13 13:49 ` [PATCH RFC 4/4] arm64: Select ARCH_ENABLE_DENY_WRITE_EXEC Catalin Marinas
2022-04-13 18:39 ` [PATCH RFC 0/4] mm, arm64: In-kernel support for memory-deny-write-execute (MDWE) Topi Miettinen
2022-04-14 13:49 ` Catalin Marinas
2022-04-14 18:52 ` Kees Cook
2022-04-15 20:01 ` Topi Miettinen
2022-04-20 13:01 ` Catalin Marinas
2022-04-20 17:44 ` Kees Cook
2022-04-20 19:34 ` Topi Miettinen
2022-04-20 23:21 ` Kees Cook
2022-04-21 15:35 ` Catalin Marinas
2022-04-21 16:42 ` Kees Cook
2022-04-21 17:24 ` Catalin Marinas
2022-04-21 17:41 ` Kees Cook
2022-04-21 18:33 ` Catalin Marinas
2022-04-21 16:48 ` Topi Miettinen
2022-04-21 17:28 ` Catalin Marinas
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