From 67d0662ce9116eb6a5aa62ffd3cf61fce452d4be Mon Sep 17 00:00:00 2001
From: Tiezhu Yang <yangtiezhu@loongson.cn>
Date: Tue, 7 Jul 2020 17:39:01 +0800
Subject: MIPS: Prevent READ_IMPLIES_EXEC propagation

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>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
---
 arch/mips/include/asm/elf.h | 1 +
 1 file changed, 1 insertion(+)

(limited to 'arch/mips/include/asm/elf.h')

diff --git a/arch/mips/include/asm/elf.h b/arch/mips/include/asm/elf.h
index 5aa29ced6970..71c7622025d1 100644
--- a/arch/mips/include/asm/elf.h
+++ b/arch/mips/include/asm/elf.h
@@ -410,6 +410,7 @@ do {									\
 	clear_thread_flag(TIF_32BIT_FPREGS);				\
 	clear_thread_flag(TIF_HYBRID_FPREGS);				\
 	clear_thread_flag(TIF_32BIT_ADDR);				\
+	current->personality &= ~READ_IMPLIES_EXEC;			\
 									\
 	if ((ex).e_ident[EI_CLASS] == ELFCLASS32)			\
 		__SET_PERSONALITY32(ex, state);				\
-- 
cgit v1.2.3