linux-kernel - [PATCH 2/2] riscv: uaccess: do not do misaligned accesses in get/put

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Message-ID: <20250530205658.624195-3-cleger@rivosinc.com>
Date: Fri, 30 May 2025 22:56:58 +0200
From: Clément Léger <cleger@...osinc.com>
To: linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Cc: Clément Léger <cleger@...osinc.com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Alexandre Ghiti <alex@...ti.fr>
Subject: [PATCH 2/2] riscv: uaccess: do not do misaligned accesses in get/put_user()

Doing misaligned access to userspace memory would make a trap on
platform where it is emulated. Latest fixes removed the kernel
capability to do unaligned accesses to userspace memory safely since
interrupts are kept disabled at all time during that. Thus doing so
would crash the kernel.

Such behavior was detected with GET_UNALIGN_CTL() that was doing
a put_user() with an unsigned long* address that should have been an
unsigned int*. Reenabling kernel misaligned access emulation is a bit
risky and it would also degrade performances. Rather than doing that,
we will try to avoid any misaligned accessed by using copy_from/to_user()
which does not do any misaligned accesses. This can be done only for
!CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS and thus allows to only generate
a bit more code for this config.

Signed-off-by: Clément Léger <cleger@...osinc.com>
---
 arch/riscv/include/asm/uaccess.h | 28 ++++++++++++++++++++++------
 1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
index 046de7ced09c..b542c05f394f 100644
--- a/arch/riscv/include/asm/uaccess.h
+++ b/arch/riscv/include/asm/uaccess.h
@@ -169,8 +169,21 @@ do {								\
 
 #endif /* CONFIG_64BIT */
 
+unsigned long __must_check __asm_copy_to_user(void __user *to,
+	const void *from, unsigned long n);
+unsigned long __must_check __asm_copy_from_user(void *to,
+	const void __user *from, unsigned long n);
+
 #define __get_user_nocheck(x, __gu_ptr, label)			\
 do {								\
+	if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) {			\
+		if (!IS_ALIGNED((uintptr_t)__gu_ptr, sizeof(*__gu_ptr))) {		\
+			if (__asm_copy_from_user(&(x), __gu_ptr, sizeof(*__gu_ptr)))	\
+				goto label;			\
+			else					\
+				break;				\
+		}						\
+	}							\
 	switch (sizeof(*__gu_ptr)) {				\
 	case 1:							\
 		__get_user_asm("lb", (x), __gu_ptr, label);	\
@@ -297,6 +310,15 @@ do {								\
 
 #define __put_user_nocheck(x, __gu_ptr, label)			\
 do {								\
+	if (!IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS)) {			\
+		if (!IS_ALIGNED((uintptr_t)__gu_ptr, sizeof(*__gu_ptr))) {		\
+			unsigned long val = (unsigned long)(x);				\
+			if (__asm_copy_to_user(__gu_ptr, &(val), sizeof(*__gu_ptr)))	\
+				goto label;			\
+			else					\
+				break;				\
+		}						\
+	}							\
 	switch (sizeof(*__gu_ptr)) {				\
 	case 1:							\
 		__put_user_asm("sb", (x), __gu_ptr, label);	\
@@ -385,12 +407,6 @@ err_label:							\
 		-EFAULT;					\
 })
 
-
-unsigned long __must_check __asm_copy_to_user(void __user *to,
-	const void *from, unsigned long n);
-unsigned long __must_check __asm_copy_from_user(void *to,
-	const void __user *from, unsigned long n);
-
 static inline unsigned long
 raw_copy_from_user(void *to, const void __user *from, unsigned long n)
 {
-- 
2.49.0