linux-kernel - [PATCH] powerpc/uaccess: Perform barrier

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Message-Id: <c72f014730823b413528e90ab6c4d3bcb79f8497.1612692067.git.christophe.leroy@csgroup.eu>
Date:   Sun,  7 Feb 2021 10:08:11 +0000 (UTC)
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, cmr@...efail.de
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH] powerpc/uaccess: Perform barrier_nospec() in KUAP allowance
 helpers

barrier_nospec() in uaccess helpers is there to protect against
speculative accesses around access_ok().

When using user_access_begin() sequences together with
unsafe_get_user() like macros, barrier_nospec() is called for
every single read although we know the access_ok() is done
onece.

Since all user accesses must be granted by a call to either
allow_read_from_user() or allow_read_write_user() which will
always happen after the access_ok() check, move the barrier_nospec()
there.

Reported-by: Christopher M. Riedl <cmr@...efail.de>
Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 arch/powerpc/include/asm/kup.h     |  2 ++
 arch/powerpc/include/asm/uaccess.h | 12 +-----------
 2 files changed, 3 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/include/asm/kup.h b/arch/powerpc/include/asm/kup.h
index bf221a2a523e..7ec21af49a45 100644
--- a/arch/powerpc/include/asm/kup.h
+++ b/arch/powerpc/include/asm/kup.h
@@ -91,6 +91,7 @@ static __always_inline void setup_kup(void)
 
 static inline void allow_read_from_user(const void __user *from, unsigned long size)
 {
+	barrier_nospec();
 	allow_user_access(NULL, from, size, KUAP_READ);
 }
 
@@ -102,6 +103,7 @@ static inline void allow_write_to_user(void __user *to, unsigned long size)
 static inline void allow_read_write_user(void __user *to, const void __user *from,
 					 unsigned long size)
 {
+	barrier_nospec();
 	allow_user_access(to, from, size, KUAP_READ_WRITE);
 }
 
diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
index 501c9a79038c..46123ae6a4c9 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -315,7 +315,6 @@ do {								\
 	__chk_user_ptr(__gu_addr);				\
 	if (!is_kernel_addr((unsigned long)__gu_addr))		\
 		might_fault();					\
-	barrier_nospec();					\
 	if (do_allow)								\
 		__get_user_size(__gu_val, __gu_addr, __gu_size, __gu_err);	\
 	else									\
@@ -333,10 +332,8 @@ do {								\
 	__typeof__(size) __gu_size = (size);				\
 									\
 	might_fault();							\
-	if (access_ok(__gu_addr, __gu_size)) {				\
-		barrier_nospec();					\
+	if (access_ok(__gu_addr, __gu_size))				\
 		__get_user_size(__gu_val, __gu_addr, __gu_size, __gu_err); \
-	}								\
 	(x) = (__force __typeof__(*(ptr)))__gu_val;				\
 									\
 	__gu_err;							\
@@ -350,7 +347,6 @@ do {								\
 	__typeof__(size) __gu_size = (size);			\
 								\
 	__chk_user_ptr(__gu_addr);				\
-	barrier_nospec();					\
 	__get_user_size(__gu_val, __gu_addr, __gu_size, __gu_err); \
 	(x) = (__force __typeof__(*(ptr)))__gu_val;			\
 								\
@@ -395,7 +391,6 @@ raw_copy_in_user(void __user *to, const void __user *from, unsigned long n)
 {
 	unsigned long ret;
 
-	barrier_nospec();
 	allow_read_write_user(to, from, n);
 	ret = __copy_tofrom_user(to, from, n);
 	prevent_read_write_user(to, from, n);
@@ -412,19 +407,15 @@ static inline unsigned long raw_copy_from_user(void *to,
 
 		switch (n) {
 		case 1:
-			barrier_nospec();
 			__get_user_size(*(u8 *)to, from, 1, ret);
 			break;
 		case 2:
-			barrier_nospec();
 			__get_user_size(*(u16 *)to, from, 2, ret);
 			break;
 		case 4:
-			barrier_nospec();
 			__get_user_size(*(u32 *)to, from, 4, ret);
 			break;
 		case 8:
-			barrier_nospec();
 			__get_user_size(*(u64 *)to, from, 8, ret);
 			break;
 		}
@@ -432,7 +423,6 @@ static inline unsigned long raw_copy_from_user(void *to,
 			return 0;
 	}
 
-	barrier_nospec();
 	allow_read_from_user(from, n);
 	ret = __copy_tofrom_user((__force void __user *)to, from, n);
 	prevent_read_from_user(from, n);
-- 
2.25.0