linux-kernel - Re: [PATCH] MIPS: Prevent READ_IMPLIES

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Date:   Wed, 8 Jul 2020 15:51:24 +0800
From:   Tiezhu Yang <yangtiezhu@...ngson.cn>
To:     "Maciej W. Rozycki" <macro@....com>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Juxin Gao <gaojuxin@...ngson.cn>,
        "Maciej W. Rozycki" <macro@...ux-mips.org>
Subject: Re: [PATCH] MIPS: Prevent READ_IMPLIES_EXEC propagation

On 07/08/2020 03:45 AM, Maciej W. Rozycki wrote:
> On Tue, 7 Jul 2020, Tiezhu Yang wrote:
>
>> In the MIPS architecture, we should clear the security-relevant
>> flag READ_IMPLIES_EXEC in the function SET_PERSONALITY2() of the
>> file arch/mips/include/asm/elf.h.
>>
>> Otherwise, with this flag set, PROT_READ implies PROT_EXEC for
>> mmap to make memory executable that is not safe, because this
>> condition allows an attacker to simply jump to and execute bytes
>> that are considered to be just data [1].
>   Why isn't the arrangement made with `mips_elf_read_implies_exec'
> sufficient?

We inherit the READ_IMPLIES_EXEC personality flag across fork().
If we do not explicitly clear this flag in SET_PERSONALITY2(),
PROT_READ implies PROT_EXEC for mmap to make memory executable
even if used with the GCC option "-z noexecstack" when compile.

By the way, we can see some other reasons in the following commit:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=48f99c8ec0b2

>
>    Maciej