kvm/emulate: Fix SETcc emulation function offsets with SLS

The commit in Fixes started adding INT3 after RETs as a mitigation
against straight-line speculation.

The fastop SETcc implementation in kvm's insn emulator uses macro magic
to generate all possible SETcc functions and to jump to them when
emulating the respective instruction.

However, it hardcodes the size and alignment of those functions to 4: a
three-byte SETcc insn and a single-byte RET. BUT, with SLS, there's an
INT3 that gets slapped after the RET, which brings the whole scheme out
of alignment:

  15:   0f 90 c0                seto   %al
  18:   c3                      ret
  19:   cc                      int3
  1a:   0f 1f 00                nopl   (%rax)
  1d:   0f 91 c0                setno  %al
  20:   c3                      ret
  21:   cc                      int3
  22:   0f 1f 00                nopl   (%rax)
  25:   0f 92 c0                setb   %al
  28:   c3                      ret
  29:   cc                      int3

and this explodes like this:

  int3: 0000 [#1] PREEMPT SMP PTI
  CPU: 0 PID: 2435 Comm: qemu-system-x86 Not tainted 5.17.0-rc8-sls #1
  Hardware name: Dell Inc. Precision WorkStation T3400  /0TP412, BIOS A14 04/30/2012
  RIP: 0010:setc+0x5/0x8 [kvm]
  Code: 00 00 0f 1f 00 0f b6 05 43 24 06 00 c3 cc 0f 1f 80 00 00 00 00 0f 90 c0 c3 cc 0f \
	  1f 00 0f 91 c0 c3 cc 0f 1f 00 0f 92 c0 c3 cc <0f> 1f 00 0f 93 c0 c3 cc 0f 1f 00 \
	  0f 94 c0 c3 cc 0f 1f 00 0f 95 c0
  Call Trace:
   <TASK>
   ? x86_emulate_insn [kvm]
   ? x86_emulate_instruction [kvm]
   ? vmx_handle_exit [kvm_intel]
   ? kvm_arch_vcpu_ioctl_run [kvm]
   ? kvm_vcpu_ioctl [kvm]
   ? __x64_sys_ioctl
   ? do_syscall_64
   ? entry_SYSCALL_64_after_hwframe
   </TASK>

Raise the alignment value when SLS is enabled and use a macro for that
instead of hard-coding naked numbers.

Fixes: e463a09af2f0 ("x86: Add straight-line-speculation mitigation")
Reported-by: Jamie Heilman <jamie@audible.transient.net>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Jamie Heilman <jamie@audible.transient.net>
Link: https://lore.kernel.org/r/YjGzJwjrvxg5YZ0Z@audible.transient.net
[Add a comment and a bit of safety checking, since this is going to be changed
 again for IBT support. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 17 insertions, 2 deletions

diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index 5719d8cfdbd9..e86d610dc6b7 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -429,8 +429,23 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
 	FOP_END
 
 /* Special case for SETcc - 1 instruction per cc */
+
+/*
+ * 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.
+ */
+#define SETCC_LENGTH	(4 + IS_ENABLED(CONFIG_SLS))
+#define SETCC_ALIGN	(4 << IS_ENABLED(CONFIG_SLS))
+static_assert(SETCC_LENGTH <= SETCC_ALIGN);
+
 #define FOP_SETCC(op) \
-	".align 4 \n\t" \
+	".align " __stringify(SETCC_ALIGN) " \n\t" \
 	".type " #op ", @function \n\t" \
 	#op ": \n\t" \
 	#op " %al \n\t" \
@@ -1047,7 +1062,7 @@ static int em_bsr_c(struct x86_emulate_ctxt *ctxt)
 static __always_inline u8 test_cc(unsigned int condition, unsigned long flags)
 {
 	u8 rc;
-	void (*fop)(void) = (void *)em_setcc + 4 * (condition & 0xf);
+	void (*fop)(void) = (void *)em_setcc + SETCC_ALIGN * (condition & 0xf);
 
 	flags = (flags & EFLAGS_MASK) | X86_EFLAGS_IF;
 	asm("push %[flags]; popf; " CALL_NOSPEC