[Xen-devel] [PATCH v11 6/11] x86/entry: Organise the clobbering of the RSB/RAS on entry to Xen

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

ret instructions are speculated directly to values recorded in the Return
Stack Buffer/Return Address Stack, as there is no uncertainty in well-formed
code.  Guests can take advantage of this in two ways:

  1) If they can find a path in Xen which executes more ret instructions than
     call instructions.  (At least one in the waitqueue infrastructure,
     probably others.)

  2) Use the fact that the RSB/RAS in hardware is actually a circular stack
     without a concept of empty.  (When it logically empties, stale values
     will start being used.)

To mitigate, overwrite the RSB on entry to Xen with gadgets which will capture
and contain rogue speculation.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>

v10:
 * Spelling/comment improvements.
 * Split to fit around IST safety.  Other half of the patch moved into
   "x86/boot: Calculate the most appropriate BTI mitigation to use"
 * Avoid using numeric labels in DO_OVERWRITE_RSB
v11:
 * Use pause;lfence;jmp to capture speculation, for AMD processors.
---
 xen/include/asm-x86/cpufeatures.h   |  2 ++
 xen/include/asm-x86/nops.h          |  1 +
 xen/include/asm-x86/spec_ctrl_asm.h | 44 +++++++++++++++++++++++++++++++++++++
 3 files changed, 47 insertions(+)

diff --git a/xen/include/asm-x86/cpufeatures.h 
b/xen/include/asm-x86/cpufeatures.h
index dd2388f..0ee4a1f 100644
--- a/xen/include/asm-x86/cpufeatures.h
+++ b/xen/include/asm-x86/cpufeatures.h
@@ -28,3 +28,5 @@ XEN_CPUFEATURE(IND_THUNK_JMP,   (FSCAPINTS+0)*32+14) /* Use 
IND_THUNK_JMP */
 XEN_CPUFEATURE(XEN_IBPB,        (FSCAPINTS+0)*32+15) /* IBRSB || IBPB */
 XEN_CPUFEATURE(XEN_IBRS_SET,    (FSCAPINTS+0)*32+16) /* IBRSB && IRBS set in 
Xen */
 XEN_CPUFEATURE(XEN_IBRS_CLEAR,  (FSCAPINTS+0)*32+17) /* IBRSB && IBRS clear in 
Xen */
+XEN_CPUFEATURE(RSB_NATIVE,      (FSCAPINTS+0)*32+18) /* RSB overwrite needed 
for native */
+XEN_CPUFEATURE(RSB_VMEXIT,      (FSCAPINTS+0)*32+20) /* RSB overwrite needed 
for vmexit */
diff --git a/xen/include/asm-x86/nops.h b/xen/include/asm-x86/nops.h
index 80126c1..61319cc 100644
--- a/xen/include/asm-x86/nops.h
+++ b/xen/include/asm-x86/nops.h
@@ -70,6 +70,7 @@
 #define ASM_NOP24 ASM_NOP8; ASM_NOP8; ASM_NOP8
 #define ASM_NOP29 ASM_NOP8; ASM_NOP8; ASM_NOP8; ASM_NOP5
 #define ASM_NOP32 ASM_NOP8; ASM_NOP8; ASM_NOP8; ASM_NOP8
+#define ASM_NOP40 ASM_NOP8; ASM_NOP8; ASM_NOP8; ASM_NOP8; ASM_NOP8
 
 #define ASM_NOP_MAX 9
 
diff --git a/xen/include/asm-x86/spec_ctrl_asm.h 
b/xen/include/asm-x86/spec_ctrl_asm.h
index ba55574..e27ea2b 100644
--- a/xen/include/asm-x86/spec_ctrl_asm.h
+++ b/xen/include/asm-x86/spec_ctrl_asm.h
@@ -74,6 +74,44 @@
  *  - SPEC_CTRL_EXIT_TO_GUEST
  */
 
+.macro DO_OVERWRITE_RSB
+/*
+ * Requires nothing
+ * Clobbers %rax, %rcx
+ *
+ * Requires 256 bytes of stack space, but %rsp has no net change. Based on
+ * Google's performance numbers, the loop is unrolled to 16 iterations and two
+ * calls per iteration.
+ *
+ * The call filling the RSB needs a nonzero displacement.  A nop would do, but
+ * we use "1: pause; lfence; jmp 1b" to safely contains any ret-based
+ * speculation, even if the loop is speculatively executed prematurely.
+ *
+ * %rsp is preserved by using an extra GPR because a) we've got plenty spare,
+ * b) the two movs are shorter to encode than `add $32*8, %rsp`, and c) can be
+ * optimised with mov-elimination in modern cores.
+ */
+    mov $16, %ecx                   /* 16 iterations, two calls per loop */
+    mov %rsp, %rax                  /* Store the current %rsp */
+
+.L\@_fill_rsb_loop:
+
+    .irp n, 1, 2                    /* Unrolled twice. */
+    call .L\@_insert_rsb_entry_\n   /* Create an RSB entry. */
+
+.L\@_capture_speculation_\n:
+    pause
+    lfence
+    jmp .L\@_capture_speculation_\n /* Capture rogue speculation. */
+
+.L\@_insert_rsb_entry_\n:
+    .endr
+
+    sub $1, %ecx
+    jnz .L\@_fill_rsb_loop
+    mov %rax, %rsp                  /* Restore old %rsp */
+.endm
+
 .macro DO_SPEC_CTRL_ENTRY_FROM_VMEXIT ibrs_val:req
 /*
  * Requires %rbx=current, %rsp=regs/cpuinfo
@@ -173,6 +211,8 @@
 
 /* Use after a VMEXIT from an HVM guest. */
 #define SPEC_CTRL_ENTRY_FROM_VMEXIT                                     \
+    ALTERNATIVE __stringify(ASM_NOP40),                                 \
+        DO_OVERWRITE_RSB, X86_FEATURE_RSB_VMEXIT;                       \
     ALTERNATIVE_2 __stringify(ASM_NOP32),                               \
         __stringify(DO_SPEC_CTRL_ENTRY_FROM_VMEXIT                      \
                     ibrs_val=SPEC_CTRL_IBRS),                           \
@@ -183,6 +223,8 @@
 
 /* Use after an entry from PV context (syscall/sysenter/int80/int82/etc). */
 #define SPEC_CTRL_ENTRY_FROM_PV                                         \
+    ALTERNATIVE __stringify(ASM_NOP40),                                 \
+        DO_OVERWRITE_RSB, X86_FEATURE_RSB_NATIVE;                       \
     ALTERNATIVE_2 __stringify(ASM_NOP21),                               \
         __stringify(DO_SPEC_CTRL_ENTRY maybexen=0                       \
                     ibrs_val=SPEC_CTRL_IBRS),                           \
@@ -192,6 +234,8 @@
 
 /* Use in interrupt/exception context.  May interrupt Xen or PV context. */
 #define SPEC_CTRL_ENTRY_FROM_INTR                                       \
+    ALTERNATIVE __stringify(ASM_NOP40),                                 \
+        DO_OVERWRITE_RSB, X86_FEATURE_RSB_NATIVE;                       \
     ALTERNATIVE_2 __stringify(ASM_NOP29),                               \
         __stringify(DO_SPEC_CTRL_ENTRY maybexen=1                       \
                     ibrs_val=SPEC_CTRL_IBRS),                           \
-- 
2.1.4


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