[PATCH v2] riscv: Limit uaccess speculation using guard page

[Lukas Gerlach <lukas.gerlach@cispa.de>]

User pointers passed to uaccess routines can be speculatively used
before access_ok() validates them, potentially leaking kernel memory.

Clamp any address >= TASK_SIZE to the guard page at TASK_SIZE-1, which
will always fault. The clamp is branchless to prevent speculative bypass.

Unlike the v1 approach of clearing the sign bit, this works with all
paging modes (Sv39/Sv48/Sv57) and does not interfere with the pointer
masking extension (Smnpm).

Similar to commit 4d8efc2d5ee4 ("arm64: Use pointer masking to limit
uaccess speculation").

Signed-off-by: Lukas Gerlach <lukas.gerlach@cispa.de>
---
 arch/riscv/include/asm/uaccess.h | 38 +++++++++++++++++++++++++++++---------
 1 file changed, 29 insertions(+), 9 deletions(-)

diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h
index 11c9886c3b70..df31df3bd55c 100644
--- a/arch/riscv/include/asm/uaccess.h
+++ b/arch/riscv/include/asm/uaccess.h
@@ -74,6 +74,20 @@ static inline unsigned long __untagged_addr_remote(struct mm_struct *mm, unsigne
 #define __typefits(x, type, not) \
 	__builtin_choose_expr(sizeof(x) <= sizeof(type), (unsigned type)0, not)
 
+/*
+ * Sanitize a uaccess pointer such that it cannot reach any kernel address.
+ * Branchlessly clamp any address >= TASK_SIZE to the unmapped guard page
+ * at TASK_SIZE-1, which will always fault on access.
+ */
+#define uaccess_mask_ptr(ptr) ((__typeof__(ptr))__uaccess_mask_ptr(ptr))
+static inline void __user *__uaccess_mask_ptr(const void __user *ptr)
+{
+	unsigned long p = (unsigned long)ptr;
+	unsigned long mask = (unsigned long)((long)(TASK_SIZE - 1 - p) >> 63);
+
+	return (void __user *)((p & ~mask) | ((TASK_SIZE - 1) & mask));
+}
+
 /*
  * The exception table consists of pairs of addresses: the first is the
  * address of an instruction that is allowed to fault, and the second is
@@ -245,7 +259,8 @@ __gu_failed:								\
  */
 #define __get_user(x, ptr)					\
 ({								\
-	const __typeof__(*(ptr)) __user *__gu_ptr = untagged_addr(ptr); \
+	const __typeof__(*(ptr)) __user *__gu_ptr =		\
+		uaccess_mask_ptr(untagged_addr(ptr));		\
 	long __gu_err = 0;					\
 	__typeof__(x) __gu_val;					\
 								\
@@ -376,7 +391,8 @@ err_label:							\
  */
 #define __put_user(x, ptr)					\
 ({								\
-	__typeof__(*(ptr)) __user *__gu_ptr = untagged_addr(ptr); \
+	__typeof__(*(ptr)) __user *__gu_ptr =			\
+		uaccess_mask_ptr(untagged_addr(ptr));		\
 	__typeof__(*__gu_ptr) __val = (x);			\
 	long __pu_err = 0;					\
 								\
@@ -423,13 +439,15 @@ unsigned long __must_check __asm_copy_from_user(void *to,
 static inline unsigned long
 raw_copy_from_user(void *to, const void __user *from, unsigned long n)
 {
-	return __asm_copy_from_user(to, untagged_addr(from), n);
+	return __asm_copy_from_user(to,
+		uaccess_mask_ptr(untagged_addr(from)), n);
 }
 
 static inline unsigned long
 raw_copy_to_user(void __user *to, const void *from, unsigned long n)
 {
-	return __asm_copy_to_user(untagged_addr(to), from, n);
+	return __asm_copy_to_user(
+		uaccess_mask_ptr(untagged_addr(to)), from, n);
 }
 
 extern long strncpy_from_user(char *dest, const char __user *src, long count);
@@ -444,7 +462,7 @@ unsigned long __must_check clear_user(void __user *to, unsigned long n)
 {
 	might_fault();
 	return access_ok(to, n) ?
-		__clear_user(untagged_addr(to), n) : n;
+		__clear_user(uaccess_mask_ptr(untagged_addr(to)), n) : n;
 }
 
 #define arch_get_kernel_nofault(dst, src, type, err_label)			\
@@ -471,20 +489,22 @@ static inline void user_access_restore(unsigned long enabled) { }
  * the error labels - thus the macro games.
  */
 #define arch_unsafe_put_user(x, ptr, label)				\
-	__put_user_nocheck(x, (ptr), label)
+	__put_user_nocheck(x, uaccess_mask_ptr(ptr), label)
 
 #define arch_unsafe_get_user(x, ptr, label)	do {			\
 	__inttype(*(ptr)) __gu_val;					\
-	__get_user_nocheck(__gu_val, (ptr), label);			\
+	__get_user_nocheck(__gu_val, uaccess_mask_ptr(ptr), label);	\
 	(x) = (__force __typeof__(*(ptr)))__gu_val;			\
 } while (0)
 
 #define unsafe_copy_to_user(_dst, _src, _len, label)			\
-	if (__asm_copy_to_user_sum_enabled(_dst, _src, _len))		\
+	if (__asm_copy_to_user_sum_enabled(				\
+		uaccess_mask_ptr(_dst), _src, _len))			\
 		goto label;
 
 #define unsafe_copy_from_user(_dst, _src, _len, label)			\
-	if (__asm_copy_from_user_sum_enabled(_dst, _src, _len))		\
+	if (__asm_copy_from_user_sum_enabled(				\
+		_dst, uaccess_mask_ptr(_src), _len))			\
 		goto label;
 
 #else /* CONFIG_MMU */

---
base-commit: f4d0ec0aa20d49f09dc01d82894ce80d72de0560
change-id: 20260226-uaccess-guard-v2-7a3358bee742

Best regards,
-- 
Lukas Gerlach <lukas.gerlach@cispa.de>