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

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Date:   Wed, 10 Aug 2022 15:01:38 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     chenlifu@...wei.com
CC:     Paul Walmsley <paul.walmsley@...ive.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,
        chenlifu@...wei.com
Subject:     Re: [PATCH -next] riscv: lib: uaccess: fix CSR_STATUS SR_SUM bit

On Tue, 14 Jun 2022 18:47:14 PDT (-0700), chenlifu@...wei.com 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>
> ---
>  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)

Thanks, this is on for-next (still for 5.20).