diff mbox series

[v2,3/8] x86/PV: harden guest memory accesses against speculative abuse

Message ID ad99a113-ecc5-2548-c91e-81fa4fe6154a@suse.com (mailing list archive)
State New
Headers show
Series x86/PV: avoid speculation abuse through guest accessors | expand

Commit Message

Jan Beulich Feb. 17, 2021, 8:20 a.m. UTC
Inspired by
https://lore.kernel.org/lkml/f12e7d3cecf41b2c29734ea45a393be21d4a8058.1597848273.git.jpoimboe@redhat.com/
and prior work in that area of x86 Linux, suppress speculation with
guest specified pointer values by suitably masking the addresses to
non-canonical space in case they fall into Xen's virtual address range.

Introduce a new Kconfig control.

Note that it is necessary in such code to avoid using "m" kind operands:
If we didn't, there would be no guarantee that the register passed to
guest_access_mask_ptr is also the (base) one used for the memory access.

As a minor unrelated change in get_unsafe_asm() the unnecessary "itype"
parameter gets dropped and the XOR on the fixup path gets changed to be
a 32-bit one in all cases: This way we avoid pointless REX.W or operand
size overrides, or writes to partial registers.

Requested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
---
v2: Add comment to assembler macro.
---
The insn sequence chosen is certainly up for discussion; I've picked
this one despite the RCR because alternatives I could come up with,
like

	mov	$(HYPERVISOR_VIRT_END), %rax
	mov	$~0, %rdx
	mov	$0x7fffffffffffffff, %rcx
	cmp	%rax, %rdi
	cmovb	%rcx, %rdx
	and	%rdx, %rdi

weren't necessarily better: Either, as above, they are longer and
require a 3rd scratch register, or they also utilize the carry flag in
some similar way.
---
Judging from the comment ahead of put_unsafe_asm() we might as well not
tell gcc at all anymore about the memory access there, now that there's
no use of the operand anymore in the assembly code.
diff mbox series

Patch

--- a/xen/arch/x86/usercopy.c
+++ b/xen/arch/x86/usercopy.c
@@ -10,12 +10,19 @@ 
 #include <xen/sched.h>
 #include <asm/uaccess.h>
 
-unsigned __copy_to_user_ll(void __user *to, const void *from, unsigned n)
+#ifndef GUARD
+# define GUARD UA_KEEP
+#endif
+
+unsigned int copy_to_guest_ll(void __user *to, const void *from, unsigned int n)
 {
     unsigned dummy;
 
     stac();
     asm volatile (
+        GUARD(
+        "    guest_access_mask_ptr %[to], %q[scratch1], %q[scratch2]\n"
+        )
         "    cmp  $"STR(2*BYTES_PER_LONG-1)", %[cnt]\n"
         "    jbe  1f\n"
         "    mov  %k[to], %[cnt]\n"
@@ -42,6 +49,7 @@  unsigned __copy_to_user_ll(void __user *
         _ASM_EXTABLE(1b, 2b)
         : [cnt] "+c" (n), [to] "+D" (to), [from] "+S" (from),
           [aux] "=&r" (dummy)
+          GUARD(, [scratch1] "=&r" (dummy), [scratch2] "=&r" (dummy))
         : "[aux]" (n)
         : "memory" );
     clac();
@@ -49,12 +57,15 @@  unsigned __copy_to_user_ll(void __user *
     return n;
 }
 
-unsigned __copy_from_user_ll(void *to, const void __user *from, unsigned n)
+unsigned int copy_from_guest_ll(void *to, const void __user *from, unsigned int n)
 {
     unsigned dummy;
 
     stac();
     asm volatile (
+        GUARD(
+        "    guest_access_mask_ptr %[from], %q[scratch1], %q[scratch2]\n"
+        )
         "    cmp  $"STR(2*BYTES_PER_LONG-1)", %[cnt]\n"
         "    jbe  1f\n"
         "    mov  %k[to], %[cnt]\n"
@@ -87,6 +98,7 @@  unsigned __copy_from_user_ll(void *to, c
         _ASM_EXTABLE(1b, 6b)
         : [cnt] "+c" (n), [to] "+D" (to), [from] "+S" (from),
           [aux] "=&r" (dummy)
+          GUARD(, [scratch1] "=&r" (dummy), [scratch2] "=&r" (dummy))
         : "[aux]" (n)
         : "memory" );
     clac();
@@ -94,6 +106,8 @@  unsigned __copy_from_user_ll(void *to, c
     return n;
 }
 
+#if GUARD(1) + 0
+
 /**
  * copy_to_user: - Copy a block of data into user space.
  * @to:   Destination address, in user space.
@@ -128,8 +142,11 @@  unsigned clear_user(void __user *to, uns
 {
     if ( access_ok(to, n) )
     {
+        long dummy;
+
         stac();
         asm volatile (
+            "    guest_access_mask_ptr %[to], %[scratch1], %[scratch2]\n"
             "0:  rep stos"__OS"\n"
             "    mov  %[bytes], %[cnt]\n"
             "1:  rep stosb\n"
@@ -140,7 +157,8 @@  unsigned clear_user(void __user *to, uns
             ".previous\n"
             _ASM_EXTABLE(0b,3b)
             _ASM_EXTABLE(1b,2b)
-            : [cnt] "=&c" (n), [to] "+D" (to)
+            : [cnt] "=&c" (n), [to] "+D" (to), [scratch1] "=&r" (dummy),
+              [scratch2] "=&r" (dummy)
             : [bytes] "r" (n & (BYTES_PER_LONG - 1)),
               [longs] "0" (n / BYTES_PER_LONG), "a" (0) );
         clac();
@@ -174,6 +192,16 @@  unsigned copy_from_user(void *to, const
     return n;
 }
 
+# undef GUARD
+# define GUARD UA_DROP
+# 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) */
+
 /*
  * Local variables:
  * mode: C
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -458,6 +458,8 @@  UNLIKELY_START(g, create_bounce_frame_ba
         jmp   asm_domain_crash_synchronous  /* Does not return */
 __UNLIKELY_END(create_bounce_frame_bad_sp)
 
+        guest_access_mask_ptr %rsi, %rax, %rcx
+
 #define STORE_GUEST_STACK(reg, n) \
 0:      movq  %reg,(n)*8(%rsi); \
         _ASM_EXTABLE(0b, domain_crash_page_fault_ ## n ## x8)
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -111,6 +111,24 @@  config SPECULATIVE_HARDEN_BRANCH
 
 	  If unsure, say Y.
 
+config SPECULATIVE_HARDEN_GUEST_ACCESS
+	bool "Speculative PV Guest Memory Access Hardening"
+	default y
+	depends on PV
+	help
+	  Contemporary processors may use speculative execution as a
+	  performance optimisation, but this can potentially be abused by an
+	  attacker to leak data via speculative sidechannels.
+
+	  One source of data leakage is via speculative accesses to hypervisor
+	  memory through guest controlled values used to access guest memory.
+
+	  When enabled, code paths accessing PV guest memory will have guest
+	  controlled addresses massaged such that memory accesses through them
+	  won't touch hypervisor address space.
+
+	  If unsure, say Y.
+
 endmenu
 
 config HYPFS
--- a/xen/include/asm-x86/asm-defns.h
+++ b/xen/include/asm-x86/asm-defns.h
@@ -56,3 +56,23 @@ 
 .macro INDIRECT_JMP arg:req
     INDIRECT_BRANCH jmp \arg
 .endm
+
+.macro guest_access_mask_ptr ptr:req, scratch1:req, scratch2:req
+#if defined(CONFIG_SPECULATIVE_HARDEN_GUEST_ACCESS)
+    /*
+     * Here we want
+     *
+     * ptr &= ~0ull >> (ptr < HYPERVISOR_VIRT_END);
+     *
+     * but guaranteed without any conditional branches (hence in assembly).
+     */
+    mov $(HYPERVISOR_VIRT_END - 1), \scratch1
+    mov $~0, \scratch2
+    cmp \ptr, \scratch1
+    rcr $1, \scratch2
+    and \scratch2, \ptr
+#elif defined(CONFIG_DEBUG) && defined(CONFIG_PV)
+    xor $~\@, \scratch1
+    xor $~\@, \scratch2
+#endif
+.endm
--- a/xen/include/asm-x86/uaccess.h
+++ b/xen/include/asm-x86/uaccess.h
@@ -12,13 +12,19 @@ 
 unsigned copy_to_user(void *to, const void *from, unsigned len);
 unsigned clear_user(void *to, unsigned len);
 unsigned copy_from_user(void *to, const void *from, unsigned len);
+
 /* Handles exceptions in both to and from, but doesn't do access_ok */
-unsigned __copy_to_user_ll(void __user*to, const void *from, unsigned n);
-unsigned __copy_from_user_ll(void *to, const void __user *from, unsigned n);
+unsigned int copy_to_guest_ll(void __user*to, const void *from, unsigned int n);
+unsigned int copy_from_guest_ll(void *to, const void __user *from, unsigned int n);
+unsigned int copy_to_unsafe_ll(void *to, const void *from, unsigned int n);
+unsigned int copy_from_unsafe_ll(void *to, const void *from, unsigned int n);
 
 extern long __get_user_bad(void);
 extern void __put_user_bad(void);
 
+#define UA_KEEP(args...) args
+#define UA_DROP(args...)
+
 /**
  * get_user: - Get a simple variable from user space.
  * @x:   Variable to store result.
@@ -77,7 +83,6 @@  extern void __put_user_bad(void);
  * On error, the variable @x is set to zero.
  */
 #define __get_guest(x, ptr) get_guest_nocheck(x, ptr, sizeof(*(ptr)))
-#define get_unsafe __get_guest
 
 /**
  * __put_guest: - Write a simple value into guest space, with less checking.
@@ -98,7 +103,13 @@  extern void __put_user_bad(void);
  */
 #define __put_guest(x, ptr) \
     put_guest_nocheck((__typeof__(*(ptr)))(x), ptr, sizeof(*(ptr)))
-#define put_unsafe __put_guest
+
+#define put_unsafe(x, ptr)						\
+({									\
+	int err_; 							\
+	put_unsafe_size(x, ptr, sizeof(*(ptr)), UA_DROP, err_, -EFAULT);\
+	err_;								\
+})
 
 #define put_guest_nocheck(x, ptr, size)					\
 ({									\
@@ -115,6 +126,13 @@  extern void __put_user_bad(void);
 			       : -EFAULT;				\
 })
 
+#define get_unsafe(x, ptr)						\
+({									\
+	int err_; 							\
+	get_unsafe_size(x, ptr, sizeof(*(ptr)), UA_DROP, err_, -EFAULT);\
+	err_;								\
+})
+
 #define get_guest_nocheck(x, ptr, size)					\
 ({									\
 	int err_; 							\
@@ -138,62 +156,87 @@  struct __large_struct { unsigned long bu
  * we do not write to any memory gcc knows about, so there are no
  * aliasing issues.
  */
-#define put_unsafe_asm(x, addr, err, itype, rtype, ltype, errret)	\
+#define put_unsafe_asm(x, addr, GUARD, err, itype, rtype, ltype, errret) \
 	stac();								\
 	__asm__ __volatile__(						\
-		"1:	mov"itype" %"rtype"1,%2\n"			\
+		GUARD(							\
+		"	guest_access_mask_ptr %[ptr], %[scr1], %[scr2]\n" \
+		)							\
+		"1:	mov"itype" %"rtype"[val], (%[ptr])\n"		\
 		"2:\n"							\
 		".section .fixup,\"ax\"\n"				\
-		"3:	mov %3,%0\n"					\
+		"3:	mov %[errno], %[ret]\n"				\
 		"	jmp 2b\n"					\
 		".previous\n"						\
 		_ASM_EXTABLE(1b, 3b)					\
-		: "=r"(err)						\
-		: ltype (x), "m"(__m(addr)), "i"(errret), "0"(err));	\
+		: [ret] "+r" (err), [ptr] "=&r" (dummy_)		\
+		  GUARD(, [scr1] "=&r" (dummy_), [scr2] "=&r" (dummy_))	\
+		: [val] ltype (x), "m" (__m(addr)),			\
+		  "[ptr]" (addr), [errno] "i" (errret));		\
 	clac()
 
-#define get_unsafe_asm(x, addr, err, itype, rtype, ltype, errret)	\
+#define get_unsafe_asm(x, addr, GUARD, err, rtype, ltype, errret)	\
 	stac();								\
 	__asm__ __volatile__(						\
-		"1:	mov"itype" %2,%"rtype"1\n"			\
+		GUARD(							\
+		"	guest_access_mask_ptr %[ptr], %[scr1], %[scr2]\n" \
+		)							\
+		"1:	mov (%[ptr]), %"rtype"[val]\n"			\
 		"2:\n"							\
 		".section .fixup,\"ax\"\n"				\
-		"3:	mov %3,%0\n"					\
-		"	xor"itype" %"rtype"1,%"rtype"1\n"		\
+		"3:	mov %[errno], %[ret]\n"				\
+		"	xor %k[val], %k[val]\n"				\
 		"	jmp 2b\n"					\
 		".previous\n"						\
 		_ASM_EXTABLE(1b, 3b)					\
-		: "=r"(err), ltype (x)					\
-		: "m"(__m(addr)), "i"(errret), "0"(err));		\
+		: [ret] "+r" (err), [val] ltype (x),			\
+		  [ptr] "=&r" (dummy_)					\
+		  GUARD(, [scr1] "=&r" (dummy_), [scr2] "=&r" (dummy_))	\
+		: "m" (__m(addr)), "[ptr]" (addr),			\
+		  [errno] "i" (errret));				\
 	clac()
 
-#define put_unsafe_size(x, ptr, size, retval, errret)                      \
+#define put_unsafe_size(x, ptr, size, grd, retval, errret)                 \
 do {                                                                       \
     retval = 0;                                                            \
     switch ( size )                                                        \
     {                                                                      \
-    case 1: put_unsafe_asm(x, ptr, retval, "b", "b", "iq", errret); break; \
-    case 2: put_unsafe_asm(x, ptr, retval, "w", "w", "ir", errret); break; \
-    case 4: put_unsafe_asm(x, ptr, retval, "l", "k", "ir", errret); break; \
-    case 8: put_unsafe_asm(x, ptr, retval, "q",  "", "ir", errret); break; \
+    long dummy_;                                                           \
+    case 1:                                                                \
+        put_unsafe_asm(x, ptr, grd, retval, "b", "b", "iq", errret);       \
+        break;                                                             \
+    case 2:                                                                \
+        put_unsafe_asm(x, ptr, grd, retval, "w", "w", "ir", errret);       \
+        break;                                                             \
+    case 4:                                                                \
+        put_unsafe_asm(x, ptr, grd, retval, "l", "k", "ir", errret);       \
+        break;                                                             \
+    case 8:                                                                \
+        put_unsafe_asm(x, ptr, grd, retval, "q",  "", "ir", errret);       \
+        break;                                                             \
     default: __put_user_bad();                                             \
     }                                                                      \
 } while ( false )
-#define put_guest_size put_unsafe_size
 
-#define get_unsafe_size(x, ptr, size, retval, errret)                      \
+#define put_guest_size(x, ptr, size, retval, errret) \
+    put_unsafe_size(x, ptr, size, UA_KEEP, retval, errret)
+
+#define get_unsafe_size(x, ptr, size, grd, retval, errret)                 \
 do {                                                                       \
     retval = 0;                                                            \
     switch ( size )                                                        \
     {                                                                      \
-    case 1: get_unsafe_asm(x, ptr, retval, "b", "b", "=q", errret); break; \
-    case 2: get_unsafe_asm(x, ptr, retval, "w", "w", "=r", errret); break; \
-    case 4: get_unsafe_asm(x, ptr, retval, "l", "k", "=r", errret); break; \
-    case 8: get_unsafe_asm(x, ptr, retval, "q",  "", "=r", errret); break; \
+    long dummy_;                                                           \
+    case 1: get_unsafe_asm(x, ptr, grd, retval, "b", "=q", errret); break; \
+    case 2: get_unsafe_asm(x, ptr, grd, retval, "w", "=r", errret); break; \
+    case 4: get_unsafe_asm(x, ptr, grd, retval, "k", "=r", errret); break; \
+    case 8: get_unsafe_asm(x, ptr, grd, retval,  "", "=r", errret); break; \
     default: __get_user_bad();                                             \
     }                                                                      \
 } while ( false )
-#define get_guest_size get_unsafe_size
+
+#define get_guest_size(x, ptr, size, retval, errret)                       \
+    get_unsafe_size(x, ptr, size, UA_KEEP, retval, errret)
 
 /**
  * __copy_to_guest_pv: - Copy a block of data into guest space, with less
@@ -229,9 +272,8 @@  __copy_to_guest_pv(void __user *to, cons
             return ret;
         }
     }
-    return __copy_to_user_ll(to, from, n);
+    return copy_to_guest_ll(to, from, n);
 }
-#define copy_to_unsafe __copy_to_guest_pv
 
 /**
  * __copy_from_guest_pv: - Copy a block of data from guest space, with less
@@ -270,9 +312,87 @@  __copy_from_guest_pv(void *to, const voi
             return ret;
         }
     }
-    return __copy_from_user_ll(to, from, n);
+    return copy_from_guest_ll(to, from, n);
+}
+
+/**
+ * copy_to_unsafe: - Copy a block of data to unsafe space, with exception
+ *                   checking
+ * @to:   Unsafe destination address.
+ * @from: Safe source address, in hypervisor space.
+ * @n:    Number of bytes to copy.
+ *
+ * Copy data from hypervisor space to a potentially unmapped area.
+ *
+ * Returns number of bytes that could not be copied.
+ * On success, this will be zero.
+ */
+static always_inline unsigned int
+copy_to_unsafe(void __user *to, const void *from, unsigned int n)
+{
+    if (__builtin_constant_p(n)) {
+        unsigned long ret;
+
+        switch (n) {
+        case 1:
+            put_unsafe_size(*(const uint8_t *)from, to, 1, UA_DROP, ret, 1);
+            return ret;
+        case 2:
+            put_unsafe_size(*(const uint16_t *)from, to, 2, UA_DROP, ret, 2);
+            return ret;
+        case 4:
+            put_unsafe_size(*(const uint32_t *)from, to, 4, UA_DROP, ret, 4);
+            return ret;
+        case 8:
+            put_unsafe_size(*(const uint64_t *)from, to, 8, UA_DROP, ret, 8);
+            return ret;
+        }
+    }
+
+    return copy_to_unsafe_ll(to, from, n);
+}
+
+/**
+ * copy_from_unsafe: - Copy a block of data from unsafe space, with exception
+ *                     checking
+ * @to:   Safe destination address, in hypervisor space.
+ * @from: Unsafe source address.
+ * @n:    Number of bytes to copy.
+ *
+ * Copy data from a potentially unmapped area space to hypervisor space.
+ *
+ * Returns number of bytes that could not be copied.
+ * On success, this will be zero.
+ *
+ * If some data could not be copied, this function will pad the copied
+ * data to the requested size using zero bytes.
+ */
+static always_inline unsigned int
+copy_from_unsafe(void *to, const void __user *from, unsigned int n)
+{
+    if ( __builtin_constant_p(n) )
+    {
+        unsigned long ret;
+
+        switch ( n )
+        {
+        case 1:
+            get_unsafe_size(*(uint8_t *)to, from, 1, UA_DROP, ret, 1);
+            return ret;
+        case 2:
+            get_unsafe_size(*(uint16_t *)to, from, 2, UA_DROP, ret, 2);
+            return ret;
+        case 4:
+            get_unsafe_size(*(uint32_t *)to, from, 4, UA_DROP, ret, 4);
+            return ret;
+        case 8:
+            get_unsafe_size(*(uint64_t *)to, from, 8, UA_DROP, ret, 8);
+            return ret;
+        }
+    }
+
+    return copy_from_unsafe_ll(to, from, n);
 }
-#define copy_from_unsafe __copy_from_guest_pv
 
 /*
  * The exception table consists of pairs of addresses: the first is the