From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
commit af2e140f34208a5dfb6b7a8ad2d56bda88f0524d upstream.
Prepare the SETcc fastop stuff for when RET can be larger still.
The tricky bit here is that the expressions should not only be
constant C expressions, but also absolute GAS expressions. This means
no ?: and 'true' is ~0.
Also ensure em_setcc() has the same alignment as the actual FOP_SETCC()
ops, this ensures there cannot be an alignment hole between em_setcc()
and the first op.
Additionally, add a .skip directive to the FOP_SETCC() macro to fill
any remaining space with INT3 traps; however the primary purpose of
this directive is to generate AS warnings when the remaining space
goes negative. Which is a very good indication the alignment magic
went side-ways.
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
[cascardo: ignore ENDBR when computing SETCC_LENGTH]
[cascardo: conflict fixup]
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/kvm/emulate.c | 26 ++++++++++++++------------
1 file changed, 14 insertions(+), 12 deletions(-)
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -321,13 +321,15 @@ static int fastop(struct x86_emulate_ctx
#define FOP_RET(name) \
__FOP_RET(#name)
-#define FOP_START(op) \
+#define __FOP_START(op, align) \
extern void em_##op(struct fastop *fake); \
asm(".pushsection .text, \"ax\" \n\t" \
".global em_" #op " \n\t" \
- ".align " __stringify(FASTOP_SIZE) " \n\t" \
+ ".align " __stringify(align) " \n\t" \
"em_" #op ":\n\t"
+#define FOP_START(op) __FOP_START(op, FASTOP_SIZE)
+
#define FOP_END \
".popsection")
@@ -431,15 +433,14 @@ static int fastop(struct x86_emulate_ctx
/*
* Depending on .config the SETcc functions look like:
*
- * SETcc %al [3 bytes]
- * RET [1 byte]
- * INT3 [1 byte; CONFIG_SLS]
- *
- * Which gives possible sizes 4 or 5. When rounded up to the
- * next power-of-two alignment they become 4 or 8.
+ * SETcc %al [3 bytes]
+ * RET | JMP __x86_return_thunk [1,5 bytes; CONFIG_RETPOLINE]
+ * INT3 [1 byte; CONFIG_SLS]
*/
-#define SETCC_LENGTH (4 + IS_ENABLED(CONFIG_SLS))
-#define SETCC_ALIGN (4 << IS_ENABLED(CONFIG_SLS))
+#define RET_LENGTH (1 + (4 * IS_ENABLED(CONFIG_RETPOLINE)) + \
+ IS_ENABLED(CONFIG_SLS))
+#define SETCC_LENGTH (3 + RET_LENGTH)
+#define SETCC_ALIGN (4 << ((SETCC_LENGTH > 4) & 1) << ((SETCC_LENGTH > 8) & 1))
static_assert(SETCC_LENGTH <= SETCC_ALIGN);
#define FOP_SETCC(op) \
@@ -447,13 +448,14 @@ static_assert(SETCC_LENGTH <= SETCC_ALIG
".type " #op ", @function \n\t" \
#op ": \n\t" \
#op " %al \n\t" \
- __FOP_RET(#op)
+ __FOP_RET(#op) \
+ ".skip " __stringify(SETCC_ALIGN) " - (.-" #op "), 0xcc \n\t"
asm(".pushsection .fixup, \"ax\"\n"
"kvm_fastop_exception: xor %esi, %esi; " ASM_RET
".popsection");
-FOP_START(setcc)
+__FOP_START(setcc, SETCC_ALIGN)
FOP_SETCC(seto)
FOP_SETCC(setno)
FOP_SETCC(setc)
