3.16.78-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: Sean Christopherson sean.j.christopherson@intel.com
commit b63f20a778c88b6a04458ed6ffc69da953d3a109 upstream.
Use 'lea' instead of 'add' when adjusting %rsp in CALL_NOSPEC so as to avoid clobbering flags.
KVM's emulator makes indirect calls into a jump table of sorts, where the destination of the CALL_NOSPEC is a small blob of code that performs fast emulation by executing the target instruction with fixed operands.
adcb_al_dl: 0x000339f8 <+0>: adc %dl,%al 0x000339fa <+2>: ret
A major motiviation for doing fast emulation is to leverage the CPU to handle consumption and manipulation of arithmetic flags, i.e. RFLAGS is both an input and output to the target of CALL_NOSPEC. Clobbering flags results in all sorts of incorrect emulation, e.g. Jcc instructions often take the wrong path. Sans the nops...
asm("push %[flags]; popf; " CALL_NOSPEC " ; pushf; pop %[flags]\n" 0x0003595a <+58>: mov 0xc0(%ebx),%eax 0x00035960 <+64>: mov 0x60(%ebx),%edx 0x00035963 <+67>: mov 0x90(%ebx),%ecx 0x00035969 <+73>: push %edi 0x0003596a <+74>: popf 0x0003596b <+75>: call *%esi 0x000359a0 <+128>: pushf 0x000359a1 <+129>: pop %edi 0x000359a2 <+130>: mov %eax,0xc0(%ebx) 0x000359b1 <+145>: mov %edx,0x60(%ebx)
ctxt->eflags = (ctxt->eflags & ~EFLAGS_MASK) | (flags & EFLAGS_MASK); 0x000359a8 <+136>: mov -0x10(%ebp),%eax 0x000359ab <+139>: and $0x8d5,%edi 0x000359b4 <+148>: and $0xfffff72a,%eax 0x000359b9 <+153>: or %eax,%edi 0x000359bd <+157>: mov %edi,0x4(%ebx)
For the most part this has gone unnoticed as emulation of guest code that can trigger fast emulation is effectively limited to MMIO when running on modern hardware, and MMIO is rarely, if ever, accessed by instructions that affect or consume flags.
Breakage is almost instantaneous when running with unrestricted guest disabled, in which case KVM must emulate all instructions when the guest has invalid state, e.g. when the guest is in Big Real Mode during early BIOS.
Fixes: 776b043848fd2 ("x86/retpoline: Add initial retpoline support") Fixes: 1a29b5b7f347a ("KVM: x86: Make indirect calls in emulator speculation safe") Signed-off-by: Sean Christopherson sean.j.christopherson@intel.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Acked-by: Peter Zijlstra (Intel) peterz@infradead.org Link: https://lkml.kernel.org/r/20190822211122.27579-1-sean.j.christopherson@intel... Signed-off-by: Ben Hutchings ben@decadent.org.uk --- arch/x86/include/asm/nospec-branch.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -151,7 +151,7 @@ " lfence;\n" \ " jmp 902b;\n" \ " .align 16\n" \ - "903: addl $4, %%esp;\n" \ + "903: lea 4(%%esp), %%esp;\n" \ " pushl %[thunk_target];\n" \ " ret;\n" \ " .align 16\n" \