+ kvm folks.
On Wed, Mar 16, 2022 at 08:15:43PM +0000, Jamie Heilman wrote:
> It does indeed!
Thanks, here's a proper patch. I've added your Reported-by and Tested-by
tags - I hope that's ok.
---
From: Borislav Petkov <bp@xxxxxxx>
Date: Wed, 16 Mar 2022 22:05:52 +0100
Subject: [PATCH] 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+0x40/0xa0
? entry_SYSCALL_64_after_hwframe+0x44/0xae
</TASK>
Align everything to 8 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@xxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Tested-by: Jamie Heilman <jamie@xxxxxxxxxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/YjGzJwjrvxg5YZ0Z@xxxxxxxxxxxxxxxxxxxxx
---
arch/x86/kvm/emulate.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
index f667bd8df533..e88ce4171c4a 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -430,8 +430,11 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
FOP_END
/* Special case for SETcc - 1 instruction per cc */
+
+#define SETCC_ALIGN 8
+
#define FOP_SETCC(op) \
- ".align 4 \n\t" \
+ ".align " __stringify(SETCC_ALIGN) " \n\t" \
".type " #op ", @function \n\t" \
#op ": \n\t" \
ASM_ENDBR \
@@ -1049,7 +1052,7 @@ 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
--
2.29.2
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
