From fe83f5eae432ccc8e90082d6ed506d5233547473 Mon Sep 17 00:00:00 2001 From: Borislav Petkov Date: Wed, 16 Mar 2022 22:05:52 +0100 Subject: 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: ? 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 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 Signed-off-by: Borislav Petkov Acked-by: Peter Zijlstra (Intel) Tested-by: Jamie Heilman 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 --- arch/x86/kvm/emulate.c | 19 +++++++++++++++++-- 1 file 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 -- cgit