[PATCH 5.18 145/231] x86/kvm: Fix SETcc emulation for return thunks

Greg Kroah-Hartman
2022-07-19T13:07:59Z

From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit af2e140f34208a5dfb6b7a8ad2d56bda88f0524d ]

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@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/kvm/emulate.c | 28 +++++++++++++++-------------
1 file changed, 15 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 89b11e7dca8a..b01437015f99 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -325,13 +325,15 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
#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")

@@ -435,16 +437,15 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
/*
* Depending on .config the SETcc functions look like:
*
- * ENDBR [4 bytes; CONFIG_X86_KERNEL_IBT]
- * SETcc %al [3 bytes]
- * RET [1 byte]
- * INT3 [1 byte; CONFIG_SLS]
- *
- * Which gives possible sizes 4, 5, 8 or 9. When rounded up to the
- * next power-of-two alignment they become 4, 8 or 16 resp.
+ * ENDBR [4 bytes; CONFIG_X86_KERNEL_IBT]
+ * SETcc %al [3 bytes]
+ * RET | JMP __x86_return_thunk [1,5 bytes; CONFIG_RETPOLINE]
+ * INT3 [1 byte; CONFIG_SLS]
*/
-#define SETCC_LENGTH (ENDBR_INSN_SIZE + 4 + IS_ENABLED(CONFIG_SLS))
-#define SETCC_ALIGN (4 << IS_ENABLED(CONFIG_SLS) << HAS_KERNEL_IBT)
+#define RET_LENGTH (1 + (4 * IS_ENABLED(CONFIG_RETPOLINE)) + \
+ IS_ENABLED(CONFIG_SLS))
+#define SETCC_LENGTH (ENDBR_INSN_SIZE + 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) \
@@ -453,9 +454,10 @@ static_assert(SETCC_LENGTH <= SETCC_ALIGN);
#op ": \n\t" \
ASM_ENDBR \
#op " %al \n\t" \
- __FOP_RET(#op)
+ __FOP_RET(#op) \
+ ".skip " __stringify(SETCC_ALIGN) " - (.-" #op "), 0xcc \n\t"

-FOP_START(setcc)
+__FOP_START(setcc, SETCC_ALIGN)
FOP_SETCC(seto)
FOP_SETCC(setno)
FOP_SETCC(setc)
--
2.35.1