Messages in this thread | | | Date | Fri, 3 Apr 2020 10:20:18 -0700 | From | Sean Christopherson <> | Subject | Re: [RFC PATCH] x86/split_lock: Disable SLD if an unaware (out-of-tree) module enables VMX |
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On Fri, Apr 03, 2020 at 06:42:44PM +0200, Peter Zijlstra wrote: > On Fri, Apr 03, 2020 at 09:30:07AM -0700, Sean Christopherson wrote: > > Hook into native CR4 writes to disable split-lock detection if CR4.VMXE > > is toggled on by an SDL-unaware entity, e.g. an out-of-tree hypervisor > > module. Most/all VMX-based hypervisors blindly reflect #AC exceptions > > into the guest, or don't intercept #AC in the first place. With SLD > > enabled, this results in unexpected #AC faults in the guest, leading to > > crashes in the guest and other undesirable behavior. > > > > Reported-by: "Kenneth R. Crudup" <kenny@panix.com> > > Cc: Peter Zijlstra <peterz@infradead.org> > > Cc: Thomas Gleixner <tglx@linutronix.de> > > Cc: Jessica Yu <jeyu@kernel.org> > > Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk> > > Cc: Kenneth R. Crudup <kenny@panix.com> > > Cc: Paolo Bonzini <pbonzini@redhat.com> > > Cc: Fenghua Yu <fenghua.yu@intel.com> > > Cc: Xiaoyao Li <xiaoyao.li@intel.com> > > Cc: Nadav Amit <namit@vmware.com> > > Cc: Thomas Hellstrom <thellstrom@vmware.com> > > Cc: Tony Luck <tony.luck@intel.com> > > Cc: Steven Rostedt <rostedt@goodmis.org> > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org> > > Cc: Jann Horn <jannh@google.com> > > Cc: Kees Cook <keescook@chromium.org> > > Cc: David Laight <David.Laight@ACULAB.COM> > > Cc: Doug Covelli <dcovelli@vmware.com> > > Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com> > > --- > > > > A bit ugly, but on the plus side the code is largely contained to intel.c. > > I think forgoing the on_all_cpus() remote kill is safe? > > How would it be safe? You can't control where the module text will be > ran, or how quickly.
Ugh, I forgot about the stupid core scope behavior.
CR4.VMXE needs to be set on every logical CPU before that CPU can do VMXON and enter a guest, so every CPU will come through this code and locally disable SLD.
But, a SMT sibling could race on the WRMSR and re-enable SLD on the CPU that just killed SLD. Waiting until other CPUs stop enabling SLD should work. Something like this? Disclaimer, memory ordering isn't my forte.
static atomic_t enabling_sld = ATOMIC_INIT(0);
static void sld_update_msr(bool on) { u64 test_ctrl_val = msr_test_ctrl_cache;
if (on && !sld_killed) test_ctrl_val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
if (test_ctrl_val & MSR_TEST_CTRL_SPLIT_LOCK_DETECT) atomic_inc(&enabling_sld);
wrmsrl(MSR_TEST_CTRL, test_ctrl_val);
if (test_ctrl_val & MSR_TEST_CTRL_SPLIT_LOCK_DETECT) atomic_dec(&enabling_sld); }
void split_lock_cr4_write(unsigned long val) { u64 ctrl;
/* * Out-of-tree hypervisors that aren't aware of split-lock will blindly * reflect split-lock #AC into their guests. Kill split-lock detection * if an unaware entity enables VMX. */ if (!static_cpu_has(X86_FEATURE_VMX) || !static_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT) || !(val & X86_CR4_VMXE) || atomic_read(&cr4_vmxe_split_lock_safe) || (native_read_cr4() & X86_CR4_VMXE)) return;
WARN_ON_ONCE(1);
/* * Set the global kill flag to prevent re-enabling SLD, e.g. via * switch_to_sld(). */ WRITE_ONCE(sld_killed, true);
/* * No need to forcefully disable SLD on other CPUs, they'll come here * if/when they set CR4.VMXE. But, wait until no other threads are * enabling SLD, i.e. have seen sld_killed, as the MSR may be shared * by SMT siblings. */ while (atomic_read(&enabling_sld)); sld_update_msr(false); }
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