linux-kernel - Re: [PATCH v2 1/2] mm: Implement memory-deny-write-execute as a prctl

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Date:   Mon, 23 Jan 2023 12:45:50 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Joey Gouly <joey.gouly@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Lennart Poettering <lennart@...ttering.net>,
        Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        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, nd@....com, shuah@...nel.org
Subject: Re: [PATCH v2 1/2] mm: Implement memory-deny-write-execute as a prctl

On 19.01.23 17:03, Joey Gouly wrote:
> The aim of such policy is to prevent a user task from creating an
> executable mapping that is also writeable.
> 
> 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);
> 
> The BPF filter that systemd MDWE uses is stateless, and disallows
> mprotect() with PROT_EXEC completely. This new prctl allows PROT_EXEC to
> be enabled if it was already PROT_EXEC, which allows the following case:
> 
> 	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.
> 
> Signed-off-by: Joey Gouly <joey.gouly@....com>
> Co-developed-by: Catalin Marinas <catalin.marinas@....com>
> Signed-off-by: Catalin Marinas <catalin.marinas@....com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> ---
>   include/linux/mman.h           | 34 ++++++++++++++++++++++++++++++++++
>   include/linux/sched/coredump.h |  6 +++++-
>   include/uapi/linux/prctl.h     |  6 ++++++
>   kernel/sys.c                   | 33 +++++++++++++++++++++++++++++++++
>   mm/mmap.c                      | 10 ++++++++++
>   mm/mprotect.c                  |  5 +++++
>   6 files changed, 93 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/mman.h b/include/linux/mman.h
> index 58b3abd457a3..cee1e4b566d8 100644
> --- a/include/linux/mman.h
> +++ b/include/linux/mman.h
> @@ -156,4 +156,38 @@ calc_vm_flag_bits(unsigned long flags)
>   }
>   
>   unsigned long vm_commit_limit(void);
> +
> +/*
> + * Denies creating a writable executable mapping or gaining executable permissions.
> + *
> + * This denies the following:
> + *
> + * 	a)	mmap(PROT_WRITE | PROT_EXEC)
> + *
> + *	b)	mmap(PROT_WRITE)
> + *		mprotect(PROT_EXEC)
> + *
> + *	c)	mmap(PROT_WRITE)
> + *		mprotect(PROT_READ)
> + *		mprotect(PROT_EXEC)
> + *
> + * But allows the following:
> + *
> + *	d)	mmap(PROT_READ | PROT_EXEC)
> + *		mmap(PROT_READ | PROT_EXEC | PROT_BTI)
> + */

Shouldn't we clear VM_MAYEXEC at mmap() time such that we cannot set 
VM_EXEC anymore? In an ideal world, there would be no further mprotect 
changes required.

-- 
Thanks,

David / dhildenb