linux-kernel - Re: [PATCH -next] riscv: lib: uaccess: fix CSR_STATUS SR

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Date:   Mon, 18 Jul 2022 16:10:22 +0100
From:   Ben Dooks <ben.dooks@...ethink.co.uk>
To:     Chen Lifu <chenlifu@...wei.com>, paul.walmsley@...ive.com,
        palmer@...belt.com, aou@...s.berkeley.edu,
        akira.tsukamoto@...il.com, jszhang@...nel.org,
        wangkefeng.wang@...wei.com, linux-riscv@...ts.infradead.org,
        linux-kernel@...r.kernel.org, alankao@...estech.com
Subject: Re: [PATCH -next] riscv: lib: uaccess: fix CSR_STATUS SR_SUM bit

On 15/06/2022 02:47, Chen Lifu wrote:
> Since commit 5d8544e2d007 ("RISC-V: Generic library routines and assembly")
> and commit ebcbd75e3962 ("riscv: Fix the bug in memory access fixup code"),
> if __clear_user and __copy_user return from an fixup branch,
> CSR_STATUS SR_SUM bit will be set, it is a vulnerability, so that
> S-mode memory accesses to pages that are accessible by U-mode will success.
> Disable S-mode access to U-mode memory should clear SR_SUM bit.
> 
> Fixes: 5d8544e2d007 ("RISC-V: Generic library routines and assembly")
> Fixes: ebcbd75e3962 ("riscv: Fix the bug in memory access fixup code")
> 
> Signed-off-by: Chen Lifu <chenlifu@...wei.com>

I've not run tested this, but it does look correct

Reviewed-by: Ben Dooks <ben.dooks@...ethink.co.uk>


> ---
>   arch/riscv/lib/uaccess.S | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/riscv/lib/uaccess.S b/arch/riscv/lib/uaccess.S
> index 8c475f4da308..ec486e5369d9 100644
> --- a/arch/riscv/lib/uaccess.S
> +++ b/arch/riscv/lib/uaccess.S
> @@ -173,11 +173,11 @@ ENTRY(__asm_copy_from_user)
>   	ret
>   
>   	/* Exception fixup code */
>   10:
>   	/* Disable access to user memory */
> -	csrs CSR_STATUS, t6
> +	csrc CSR_STATUS, t6
>   	mv a0, t5
>   	ret
>   ENDPROC(__asm_copy_to_user)
>   ENDPROC(__asm_copy_from_user)
>   EXPORT_SYMBOL(__asm_copy_to_user)
> @@ -225,10 +225,10 @@ ENTRY(__clear_user)
>   	j 3b
>   
>   	/* Exception fixup code */
>   11:
>   	/* Disable access to user memory */
> -	csrs CSR_STATUS, t6
> +	csrc CSR_STATUS, t6
>   	mv a0, a1
>   	ret
>   ENDPROC(__clear_user)
>   EXPORT_SYMBOL(__clear_user)


-- 
Ben Dooks				http://www.codethink.co.uk/
Senior Engineer				Codethink - Providing Genius

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