Re: [PATCH -next] riscv: lib: uaccess: fix CSR_STATUS SR_SUM bit

From: Palmer Dabbelt
Date: Wed Aug 10 2022 - 18:01:48 EST


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