diff mbox series

[v2,1/2] x86/uaccess: rework user access speculative harden guards

Message ID 20241126093508.6966-2-roger.pau@citrix.com (mailing list archive)
State New
Headers show
Series x86/misra: fix remaining violation of rule 20.7 | expand

Commit Message

Roger Pau Monne Nov. 26, 2024, 9:35 a.m. UTC 
The current guards to select whether user accesses should be speculative
hardened violate Misra rule 20.7, as the UA_KEEP() macro doesn't (and can't)
parenthesize the 'args' argument.

Change the logic so the guard is implemented inside the assembly block using
the .if assembly directive.  This results in some register spilling if the
guest_access_mask_ptr assembly macro is removed from the final assembly.

No functional change intended.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Changes xince v1:
 - Drop unneeded __maybe_unused.
 - Add comment in GUARD definitions.
 - Mention register spilling in the comment message.
---
 xen/arch/x86/include/asm/uaccess.h | 45 +++++++++++++++---------------
 xen/arch/x86/usercopy.c            | 26 ++++++++---------
 2 files changed, 35 insertions(+), 36 deletions(-)

Comments

Jan Beulich Nov. 26, 2024, 9:58 a.m. UTC | #1 
On 26.11.2024 10:35, Roger Pau Monne wrote:
> The current guards to select whether user accesses should be speculative
> hardened violate Misra rule 20.7, as the UA_KEEP() macro doesn't (and can't)
> parenthesize the 'args' argument.

For my own education: This definitely isn't the only place where we use a
macro with variable arguments, and where the use of the respective macro
parameter can't be parenthesized. Given patch 2, why is e.g.

#define emulate_fpu_insn_stub(bytes...)                                 \
do {                                                                    \
    unsigned int nr_ = sizeof((uint8_t[]){ bytes });                    \
    memcpy(get_stub(stub), ((uint8_t[]){ bytes, 0xc3 }), nr_ + 1);      \
    invoke_stub("", "", "=m" (dummy) : "i" (0));                        \
    put_stub(stub);                                                     \
} while (0)

not an issue? The first use of "bytes" is in figure braces, so probably
fine. Yet the second use is followed by a comma, so unlikely to be okay.

Somewhat similarly for

#define AMD_OSVW_ERRATUM(osvw_id, ...)  osvw_id, __VA_ARGS__, 0

where we're using the C99 form rather than the GNU extension, and where
hence __VA_ARGS__ would - by extrapolation of the Misra rule - need
parenthesizing, when it isn't and can't be.

Isn't it rather the case that variable argument macros need a more general
deviation, if not an adjustment to the Misra rule? Extending the Cc list
some ...

Jan
diff mbox series

Patch

diff --git a/xen/arch/x86/include/asm/uaccess.h b/xen/arch/x86/include/asm/uaccess.h
index 2d01669b9610..6b8150ac221a 100644
--- a/xen/arch/x86/include/asm/uaccess.h
+++ b/xen/arch/x86/include/asm/uaccess.h
@@ -24,9 +24,6 @@  unsigned int copy_from_unsafe_ll(void *to, const void *from, unsigned int n);
 void noreturn __get_user_bad(void);
 void noreturn __put_user_bad(void);
 
-#define UA_KEEP(args...) args
-#define UA_DROP(args...)
-
 /**
  * get_guest: - Get a simple variable from guest space.
  * @x:   Variable to store result.
@@ -104,7 +101,7 @@  void noreturn __put_user_bad(void);
 #define put_unsafe(x, ptr)						\
 ({									\
 	int err_; 							\
-	put_unsafe_size(x, ptr, sizeof(*(ptr)), UA_DROP, err_, -EFAULT);\
+	put_unsafe_size(x, ptr, sizeof(*(ptr)), 0, err_, -EFAULT);      \
 	err_;								\
 })
 
@@ -126,7 +123,7 @@  void noreturn __put_user_bad(void);
 #define get_unsafe(x, ptr)						\
 ({									\
 	int err_; 							\
-	get_unsafe_size(x, ptr, sizeof(*(ptr)), UA_DROP, err_, -EFAULT);\
+	get_unsafe_size(x, ptr, sizeof(*(ptr)), 0, err_, -EFAULT);	\
 	err_;								\
 })
 
@@ -155,9 +152,9 @@  struct __large_struct { unsigned long buf[100]; };
  */
 #define put_unsafe_asm(x, addr, GUARD, err, itype, rtype, ltype, errret) \
 	__asm__ __volatile__(						\
-		GUARD(							\
+		".if " STR(GUARD) "\n"					\
 		"	guest_access_mask_ptr %[ptr], %[scr1], %[scr2]\n" \
-		)							\
+		".endif\n"						\
 		"1:	mov"itype" %"rtype"[val], (%[ptr])\n"		\
 		"2:\n"							\
 		".section .fixup,\"ax\"\n"				\
@@ -165,16 +162,16 @@  struct __large_struct { unsigned long buf[100]; };
 		"	jmp 2b\n"					\
 		".previous\n"						\
 		_ASM_EXTABLE(1b, 3b)					\
-		: [ret] "+r" (err), [ptr] "=&r" (dummy_)		\
-		  GUARD(, [scr1] "=&r" (dummy_), [scr2] "=&r" (dummy_))	\
+		: [ret] "+r" (err), [ptr] "=&r" (dummy_),		\
+		  [scr1] "=&r" (dummy_), [scr2] "=&r" (dummy_)		\
 		: [val] ltype (x), "m" (__m(addr)),			\
 		  "[ptr]" (addr), [errno] "i" (errret))
 
 #define get_unsafe_asm(x, addr, GUARD, err, rtype, ltype, errret)	\
 	__asm__ __volatile__(						\
-		GUARD(							\
+		".if " STR(GUARD) "\n"					\
 		"	guest_access_mask_ptr %[ptr], %[scr1], %[scr2]\n" \
-		)							\
+		".endif\n"						\
 		"1:	mov (%[ptr]), %"rtype"[val]\n"			\
 		"2:\n"							\
 		".section .fixup,\"ax\"\n"				\
@@ -184,14 +181,15 @@  struct __large_struct { unsigned long buf[100]; };
 		".previous\n"						\
 		_ASM_EXTABLE(1b, 3b)					\
 		: [ret] "+r" (err), [val] ltype (x),			\
-		  [ptr] "=&r" (dummy_)					\
-		  GUARD(, [scr1] "=&r" (dummy_), [scr2] "=&r" (dummy_))	\
+		  [ptr] "=&r" (dummy_),					\
+		  [scr1] "=&r" (dummy_), [scr2] "=&r" (dummy_)		\
 		: "m" (__m(addr)), "[ptr]" (addr),			\
 		  [errno] "i" (errret))
 
 #define put_unsafe_size(x, ptr, size, grd, retval, errret)                 \
 do {                                                                       \
     retval = 0;                                                            \
+    BUILD_BUG_ON((grd) != 0 && (grd) != 1);                                \
     stac();                                                                \
     switch ( size )                                                        \
     {                                                                      \
@@ -214,11 +212,12 @@  do {                                                                       \
 } while ( false )
 
 #define put_guest_size(x, ptr, size, retval, errret) \
-    put_unsafe_size(x, ptr, size, UA_KEEP, retval, errret)
+    put_unsafe_size(x, ptr, size, 1, retval, errret)
 
 #define get_unsafe_size(x, ptr, size, grd, retval, errret)                 \
 do {                                                                       \
     retval = 0;                                                            \
+    BUILD_BUG_ON((grd) != 0 && (grd) != 1);                                \
     stac();                                                                \
     switch ( size )                                                        \
     {                                                                      \
@@ -233,7 +232,7 @@  do {                                                                       \
 } while ( false )
 
 #define get_guest_size(x, ptr, size, retval, errret)                       \
-    get_unsafe_size(x, ptr, size, UA_KEEP, retval, errret)
+    get_unsafe_size(x, ptr, size, 1, retval, errret)
 
 /**
  * __copy_to_guest_pv: - Copy a block of data into guest space, with less
@@ -333,16 +332,16 @@  copy_to_unsafe(void __user *to, const void *from, unsigned int n)
 
         switch (n) {
         case 1:
-            put_unsafe_size(*(const uint8_t *)from, to, 1, UA_DROP, ret, 1);
+            put_unsafe_size(*(const uint8_t *)from, to, 1, 0, ret, 1);
             return ret;
         case 2:
-            put_unsafe_size(*(const uint16_t *)from, to, 2, UA_DROP, ret, 2);
+            put_unsafe_size(*(const uint16_t *)from, to, 2, 0, ret, 2);
             return ret;
         case 4:
-            put_unsafe_size(*(const uint32_t *)from, to, 4, UA_DROP, ret, 4);
+            put_unsafe_size(*(const uint32_t *)from, to, 4, 0, ret, 4);
             return ret;
         case 8:
-            put_unsafe_size(*(const uint64_t *)from, to, 8, UA_DROP, ret, 8);
+            put_unsafe_size(*(const uint64_t *)from, to, 8, 0, ret, 8);
             return ret;
         }
     }
@@ -374,16 +373,16 @@  copy_from_unsafe(void *to, const void __user *from, unsigned int n)
         switch ( n )
         {
         case 1:
-            get_unsafe_size(*(uint8_t *)to, from, 1, UA_DROP, ret, 1);
+            get_unsafe_size(*(uint8_t *)to, from, 1, 0, ret, 1);
             return ret;
         case 2:
-            get_unsafe_size(*(uint16_t *)to, from, 2, UA_DROP, ret, 2);
+            get_unsafe_size(*(uint16_t *)to, from, 2, 0, ret, 2);
             return ret;
         case 4:
-            get_unsafe_size(*(uint32_t *)to, from, 4, UA_DROP, ret, 4);
+            get_unsafe_size(*(uint32_t *)to, from, 4, 0, ret, 4);
             return ret;
         case 8:
-            get_unsafe_size(*(uint64_t *)to, from, 8, UA_DROP, ret, 8);
+            get_unsafe_size(*(uint64_t *)to, from, 8, 0, ret, 8);
             return ret;
         }
     }
diff --git a/xen/arch/x86/usercopy.c b/xen/arch/x86/usercopy.c
index 7ab2009efe4c..286080f68f7e 100644
--- a/xen/arch/x86/usercopy.c
+++ b/xen/arch/x86/usercopy.c
@@ -11,23 +11,23 @@ 
 #include <asm/uaccess.h>
 
 #ifndef GUARD
-# define GUARD UA_KEEP
+# define GUARD 1 /* Keep speculative harden guards. */
 #endif
 
 unsigned int copy_to_guest_ll(void __user *to, const void *from, unsigned int n)
 {
-    GUARD(unsigned dummy);
+    unsigned dummy;
 
     stac();
     asm volatile (
-        GUARD(
+        ".if " STR(GUARD) "\n"
         "    guest_access_mask_ptr %[to], %q[scratch1], %q[scratch2]\n"
-        )
+        ".endif\n"
         "1:  rep movsb\n"
         "2:\n"
         _ASM_EXTABLE(1b, 2b)
-        : [cnt] "+c" (n), [to] "+D" (to), [from] "+S" (from)
-          GUARD(, [scratch1] "=&r" (dummy), [scratch2] "=&r" (dummy))
+        : [cnt] "+c" (n), [to] "+D" (to), [from] "+S" (from),
+          [scratch1] "=&r" (dummy), [scratch2] "=&r" (dummy)
         :: "memory" );
     clac();
 
@@ -40,9 +40,9 @@  unsigned int copy_from_guest_ll(void *to, const void __user *from, unsigned int
 
     stac();
     asm volatile (
-        GUARD(
+        ".if " STR(GUARD) "\n"
         "    guest_access_mask_ptr %[from], %q[scratch1], %q[scratch2]\n"
-        )
+        ".endif\n"
         "1:  rep movsb\n"
         "2:\n"
         ".section .fixup,\"ax\"\n"
@@ -56,15 +56,15 @@  unsigned int copy_from_guest_ll(void *to, const void __user *from, unsigned int
         ".previous\n"
         _ASM_EXTABLE(1b, 6b)
         : [cnt] "+c" (n), [to] "+D" (to), [from] "+S" (from),
-          [aux] "=&r" (dummy)
-          GUARD(, [scratch1] "=&r" (dummy), [scratch2] "=&r" (dummy))
+          [aux] "=&r" (dummy),
+          [scratch1] "=&r" (dummy), [scratch2] "=&r" (dummy)
         :: "memory" );
     clac();
 
     return n;
 }
 
-#if GUARD(1) + 0
+#if GUARD
 
 /**
  * copy_to_guest_pv: - Copy a block of data into PV guest space.
@@ -140,14 +140,14 @@  unsigned int copy_from_guest_pv(void *to, const void __user *from,
 }
 
 # undef GUARD
-# define GUARD UA_DROP
+# define GUARD 0 /* Drop speculative harden guards. */
 # define copy_to_guest_ll copy_to_unsafe_ll
 # define copy_from_guest_ll copy_from_unsafe_ll
 # undef __user
 # define __user
 # include __FILE__
 
-#endif /* GUARD(1) */
+#endif /* GUARD */
 
 /*
  * Local variables: