[Xen-devel] [PATCH] x86: polish __{get,put}_user_{,no}check()

["Jan Beulich" <JBeulich@xxxxxxxx>]

The primary purpose is correcting a latent bug in __get_user_check()
(the macro has no active user at present): The access_ok() check should
be before the actual access, or else any PV guest could initiate MMIO
reads with side effects.

Clean up all four macros at once:
- all arguments evaluated exactly once
- build the "check" flavor using the "nocheck" ones, instead of open
  coding them
- "int" is wide enough for error codes
- name local variables without using underscores as prefixes
- avoid pointless parentheses
- add blanks after commas separating parameters or arguments
- consistently use tabs for indentation

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
This corrects the code which would have resulted in an XSA on Xen 4.2
and older, if those were still security supported. For that reason I at
least want to explore whether this is a change we want to take for 4.9.

--- a/xen/include/asm-x86/uaccess.h
+++ b/xen/include/asm-x86/uaccess.h
@@ -104,37 +104,35 @@ extern void __put_user_bad(void);
 #define __put_user(x,ptr) \
   __put_user_nocheck((__typeof__(*(ptr)))(x),(ptr),sizeof(*(ptr)))
 
-#define __put_user_nocheck(x,ptr,size)                         \
-({                                                             \
-       long __pu_err;                                          \
-       __put_user_size((x),(ptr),(size),__pu_err,-EFAULT);     \
-       __pu_err;                                               \
+#define __put_user_nocheck(x, ptr, size)                               \
+({                                                                     \
+       int err_;                                                       \
+       __put_user_size(x, ptr, size, err_, -EFAULT);                   \
+       err_;                                                           \
 })
 
-#define __put_user_check(x,ptr,size)                                   \
+#define __put_user_check(x, ptr, size)                                 \
 ({                                                                     \
-       long __pu_err = -EFAULT;                                        \
-       __typeof__(*(ptr)) __user *__pu_addr = (ptr);                   \
-       if (access_ok(__pu_addr,size))                                  \
-               __put_user_size((x),__pu_addr,(size),__pu_err,-EFAULT); \
-       __pu_err;                                                       \
+       __typeof__(*(ptr)) __user *ptr_ = (ptr);                        \
+       __typeof__(size) size_ = (size);                                \
+       access_ok(ptr_, size_) ? __put_user_nocheck(x, ptr_, size_)     \
+                              : -EFAULT;                               \
 })                                                     
 
-#define __get_user_nocheck(x,ptr,size)                          \
-({                                                              \
-       long __gu_err;                                          \
-       __get_user_size((x),(ptr),(size),__gu_err,-EFAULT);     \
-       __gu_err;                                               \
+#define __get_user_nocheck(x, ptr, size)                               \
+({                                                                     \
+       int err_;                                                       \
+       __get_user_size(x, ptr, size, err_, -EFAULT);                   \
+       err_;                                                           \
 })
 
-#define __get_user_check(x,ptr,size)                            \
-({                                                              \
-       long __gu_err;                                          \
-       __typeof__(*(ptr)) __user *__gu_addr = (ptr);           \
-       __get_user_size((x),__gu_addr,(size),__gu_err,-EFAULT); \
-       if (!access_ok(__gu_addr,size)) __gu_err = -EFAULT;     \
-       __gu_err;                                               \
-})                                                     
+#define __get_user_check(x, ptr, size)                                 \
+({                                                                     \
+       __typeof__(*(ptr)) __user *ptr_ = (ptr);                        \
+       __typeof__(size) size_ = (size);                                \
+       access_ok(ptr_, size_) ? __get_user_nocheck(x, ptr_, size_)     \
+                              : -EFAULT;                               \
+})
 
 struct __large_struct { unsigned long buf[100]; };
 #define __m(x) (*(const struct __large_struct *)(x))

Attachment: x86-get-put-user.patch
Description: Text document

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