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Date:   Wed, 8 Jul 2020 16:26:09 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Tiezhu Yang <yangtiezhu@loongson.cn>
Cc:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        linux-mips@vger.kernel.org, linux-kernel@vger.kernel.org,
        Xuefeng Li <lixuefeng@loongson.cn>,
        Juxin Gao <gaojuxin@loongson.cn>
Subject: Re: [PATCH] MIPS: Prevent READ_IMPLIES_EXEC propagation
Message-ID: <202007081624.82FA0CC1EA@keescook>
References: <1594114741-26852-1-git-send-email-yangtiezhu@loongson.cn>
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On Tue, Jul 07, 2020 at 05:39:01PM +0800, 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].
> 
> In mm/mmap.c:
> unsigned long do_mmap(struct file *file, unsigned long addr,
> 			unsigned long len, unsigned long prot,
> 			unsigned long flags, vm_flags_t vm_flags,
> 			unsigned long pgoff, unsigned long *populate,
> 			struct list_head *uf)
> {
> 	[...]
> 	if ((prot & PROT_READ) && (current->personality & READ_IMPLIES_EXEC))
> 		if (!(file && path_noexec(&file->f_path)))
> 			prot |= PROT_EXEC;
> 	[...]
> }
> 
> By the way, x86 and ARM64 have done the similar thing.
> 
> After commit 250c22777fe1 ("x86_64: move kernel"), in the file
> arch/x86/kernel/process_64.c:
> void set_personality_64bit(void)
> {
> 	[...]
> 	current->personality &= ~READ_IMPLIES_EXEC;
> }
> 
> After commit 48f99c8ec0b2 ("arm64: Preventing READ_IMPLIES_EXEC
> propagation"), in the file arch/arm64/include/asm/elf.h:
> #define SET_PERSONALITY(ex)						\
> ({									\
> 	clear_thread_flag(TIF_32BIT);					\
> 	current->personality &= ~READ_IMPLIES_EXEC;			\
> })
> 
> [1] https://insights.sei.cmu.edu/cert/2014/02/feeling-insecure-blame-your-parent.html
> 
> Reported-by: Juxin Gao <gaojuxin@loongson.cn>
> Co-developed-by: Juxin Gao <gaojuxin@loongson.cn>
> Signed-off-by: Juxin Gao <gaojuxin@loongson.cn>
> Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>

This seems correct to me.

Reviewed-by: Kees Cook <keescook@chromium.org>

BTW, does MIPS also need similar changes to this series:
https://lore.kernel.org/lkml/20200327064820.12602-1-keescook@chromium.org/

Quoting from there "MIPS may need adjusting but the history of CPU
features and toolchain behavior is very unclear to me."

-- 
Kees Cook