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Subject[PATCH -v1.2] kvm/emulate: Fix SETcc emulation function offsets with SLS
On Thu, Mar 17, 2022 at 10:37:56AM +0100, Borislav Petkov wrote:
> Jamie, I'd appreciate testing this one too, pls, just in case.

Here's a version against -rc8 - the previous one was against tip/master
and had other contextual changes in it.

---
From: Borislav Petkov <bp@suse.de>

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>

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>
Link: https://lore.kernel.org/r/YjGzJwjrvxg5YZ0Z@audible.transient.net
---
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 5719d8cfdbd9..f321abb9a4a8 100644
--- a/arch/x86/kvm/emulate.c
+++ b/arch/x86/kvm/emulate.c
@@ -429,8 +429,11 @@ static int fastop(struct x86_emulate_ctxt *ctxt, fastop_t fop);
FOP_END

/* Special case for SETcc - 1 instruction per cc */
+
+#define SETCC_ALIGN (4 * (1 + IS_ENABLED(CONFIG_SLS)))
+
#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 +1050,7 @@ 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

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