[PATCH v2 1/6] x86/uaccess: Avoid barrier_nospec() in copy_from_user()

[Josh Poimboeuf]

For x86-64, the barrier_nospec() in copy_from_user() is overkill and
painfully slow.  Instead, use pointer masking to force the user pointer
to a non-kernel value in speculative paths.

To avoid regressing powerpc, move the barrier_nospec() to the powerpc
raw_copy_from_user() implementation so there's no functional change.

Signed-off-by: Josh Poimboeuf <jpoimboe at kernel.org>
---
 arch/powerpc/include/asm/uaccess.h | 2 ++
 arch/x86/include/asm/uaccess_64.h  | 7 ++++---
 arch/x86/lib/getuser.S             | 2 +-
 arch/x86/lib/putuser.S             | 2 +-
 include/linux/uaccess.h            | 6 ------
 5 files changed, 8 insertions(+), 11 deletions(-)

diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
index 4f5a46a77fa2..12abb8bf5eda 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -7,6 +7,7 @@
 #include <asm/extable.h>
 #include <asm/kup.h>
 #include <asm/asm-compat.h>
+#include <asm/barrier.h>
 
 #ifdef __powerpc64__
 /* We use TASK_SIZE_USER64 as TASK_SIZE is not constant */
@@ -341,6 +342,7 @@ static inline unsigned long raw_copy_from_user(void *to,
 {
 	unsigned long ret;
 
+	barrier_nospec();
 	allow_read_from_user(from, n);
 	ret = __copy_tofrom_user((__force void __user *)to, from, n);
 	prevent_read_from_user(from, n);
diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
index afce8ee5d7b7..61693028ea2b 100644
--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -54,11 +54,11 @@ static inline unsigned long __untagged_addr_remote(struct mm_struct *mm,
 #define valid_user_address(x) ((__force long)(x) >= 0)
 
 /*
- * Masking the user address is an alternative to a conditional
- * user_access_begin that can avoid the fencing. This only works
- * for dense accesses starting at the address.
+ * If it's a kernel address, force it to all 1's.  This prevents a mispredicted
+ * access_ok() from speculatively accessing kernel space.
  */
 #define mask_user_address(x) ((typeof(x))((long)(x)|((long)(x)>>63)))
+
 #define masked_user_access_begin(x) ({				\
 	__auto_type __masked_ptr = (x);				\
 	__masked_ptr = mask_user_address(__masked_ptr);		\
@@ -133,6 +133,7 @@ copy_user_generic(void *to, const void *from, unsigned long len)
 static __always_inline __must_check unsigned long
 raw_copy_from_user(void *dst, const void __user *src, unsigned long size)
 {
+	src = mask_user_address(src);
 	return copy_user_generic(dst, (__force void *)src, size);
 }
 
diff --git a/arch/x86/lib/getuser.S b/arch/x86/lib/getuser.S
index d066aecf8aeb..094224ec9dca 100644
--- a/arch/x86/lib/getuser.S
+++ b/arch/x86/lib/getuser.S
@@ -39,7 +39,7 @@
 
 .macro check_range size:req
 .if IS_ENABLED(CONFIG_X86_64)
-	mov %rax, %rdx
+	mov %rax, %rdx		/* mask_user_address() */
 	sar $63, %rdx
 	or %rdx, %rax
 .else
diff --git a/arch/x86/lib/putuser.S b/arch/x86/lib/putuser.S
index 975c9c18263d..09b7e37934ab 100644
--- a/arch/x86/lib/putuser.S
+++ b/arch/x86/lib/putuser.S
@@ -34,7 +34,7 @@
 
 .macro check_range size:req
 .if IS_ENABLED(CONFIG_X86_64)
-	mov %rcx, %rbx
+	mov %rcx, %rbx		/* mask_user_address() */
 	sar $63, %rbx
 	or %rbx, %rcx
 .else
diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
index 39c7cf82b0c2..dda9725a9559 100644
--- a/include/linux/uaccess.h
+++ b/include/linux/uaccess.h
@@ -160,12 +160,6 @@ _inline_copy_from_user(void *to, const void __user *from, unsigned long n)
 	unsigned long res = n;
 	might_fault();
 	if (!should_fail_usercopy() && likely(access_ok(from, n))) {
-		/*
-		 * Ensure that bad access_ok() speculation will not
-		 * lead to nasty side effects *after* the copy is
-		 * finished:
-		 */
-		barrier_nospec();
 		instrument_copy_from_user_before(to, from, n);
 		res = raw_copy_from_user(to, from, n);
 		instrument_copy_from_user_after(to, from, n, res);
-- 
2.47.0