[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH 3/3] x86/spec-ctrl: Fix NMI race condition with VT-x MSR_SPEC_CTRL handling
On 13/01/2022 16:38, Andrew Cooper wrote: > The logic was based on a mistaken understanding of how NMI blocking on vmexit > works. NMIs are only blocked for EXIT_REASON_NMI, and not for general exits. > Therefore, an NMI can in general hit early in the vmx_asm_vmexit_handler path, > and the guest's value will be clobbered before it is saved. > > Switch to using MSR load/save lists. This causes the guest value to be saved > atomically with respect to NMIs/MCEs/etc. > > First, update vmx_cpuid_policy_changed() to configure the load/save lists at > the same time as configuring the intercepts. This function is always used in > remote context, so extend the vmx_vmcs_{enter,exit}() block to cover the whole > function, rather than having multiple remote acquisitions of the same VMCS. > > vmx_add_guest_msr() can fail, but only in ways which are entirely fatal to the > guest, so handle failures using domain_crash(). vmx_del_msr() can fail with > -ESRCH but we don't matter, and this path will be taken during domain create > on a system lacking IBRSB. > > Second, update vmx_msr_{read,write}_intercept() to use the load/save lists > rather than vcpu_msrs, and update the comment to describe the new state > location. > > Finally, adjust the entry/exit asm. Drop DO_SPEC_CTRL_ENTRY_FROM_HVM > entirely, and use a shorter code sequence to simply reload Xen's setting from > the top-of-stack block. > > Because the guest values are loaded/saved atomically, we do not need to use > the shadowing logic to cope with late NMIs/etc, so we can omit > DO_SPEC_CTRL_EXIT_TO_GUEST entirely and VMRESUME/VMLAUNCH with Xen's value in > context. Furthermore, we can drop the SPEC_CTRL_ENTRY_FROM_PV too, as there's > no need to switch back to Xen's context on an early failure. > > Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> > --- > CC: Jan Beulich <JBeulich@xxxxxxxx> > CC: Roger Pau Monné <roger.pau@xxxxxxxxxx> > CC: Wei Liu <wl@xxxxxxx> > CC: Jun Nakajima <jun.nakajima@xxxxxxxxx> > CC: Kevin Tian <kevin.tian@xxxxxxxxx> > > Needs backporting as far as people can tolerate. > > If the entry/exit logic were in C, I'd ASSERT() that shadow tracking is off, > but this is awkard to arrange in asm. Actually, it's just occurred to me that an ASSERT is actually quite easy here. I'm proposing this additional delta (totally untested). diff --git a/xen/arch/x86/hvm/vmx/entry.S b/xen/arch/x86/hvm/vmx/entry.S index 297ed8685126..f569c3259b32 100644 --- a/xen/arch/x86/hvm/vmx/entry.S +++ b/xen/arch/x86/hvm/vmx/entry.S @@ -41,6 +41,13 @@ ENTRY(vmx_asm_vmexit_handler) movzbl CPUINFO_xen_spec_ctrl(%rsp), %eax xor %edx, %edx wrmsr + +#ifdef CONFIG_DEBUG + testb $SCF_use_shadow, CPUINFO_spec_ctrl_flags(%rsp) + jz 1f + ASSERT_FAILED("MSR_SPEC_CTRL shadowing active") +1: +#endif .endm ALTERNATIVE "", restore_spec_ctrl, X86_FEATURE_SC_MSR_HVM /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */ ~Andrew
|
Lists.xenproject.org is hosted with RackSpace, monitoring our |