Commit 6a3ea3e6 authored by Sean Christopherson's avatar Sean Christopherson Committed by Thomas Gleixner

x86/entry/64: Do not use RDPID in paranoid entry to accomodate KVM

KVM has an optmization to avoid expensive MRS read/writes on
VMENTER/EXIT. It caches the MSR values and restores them either when
leaving the run loop, on preemption or when going out to user space.

The affected MSRs are not required for kernel context operations. This
changed with the recently introduced mechanism to handle FSGSBASE in the
paranoid entry code which has to retrieve the kernel GSBASE value by
accessing per CPU memory. The mechanism needs to retrieve the CPU number
and uses either LSL or RDPID if the processor supports it.

Unfortunately RDPID uses MSR_TSC_AUX which is in the list of cached and
lazily restored MSRs, which means between the point where the guest value
is written and the point of restore, MSR_TSC_AUX contains a random number.

If an NMI or any other exception which uses the paranoid entry path happens
in such a context, then RDPID returns the random guest MSR_TSC_AUX value.

As a consequence this reads from the wrong memory location to retrieve the
kernel GSBASE value. Kernel GS is used to for all regular this_cpu_*()
operations. If the GSBASE in the exception handler points to the per CPU
memory of a different CPU then this has the obvious consequences of data
corruption and crashes.

As the paranoid entry path is the only place which accesses MSR_TSX_AUX
(via RDPID) and the fallback via LSL is not significantly slower, remove
the RDPID alternative from the entry path and always use LSL.

The alternative would be to write MSR_TSC_AUX on every VMENTER and VMEXIT
which would be inflicting massive overhead on that code path.

[ tglx: Rewrote changelog ]

Fixes: eaad9812 ("x86/entry/64: Introduce the FIND_PERCPU_BASE macro")
Reported-by: default avatarTom Lendacky <thomas.lendacky@amd.com>
Debugged-by: default avatarTom Lendacky <thomas.lendacky@amd.com>
Suggested-by: default avatarAndy Lutomirski <luto@kernel.org>
Suggested-by: default avatarPeter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarSean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200821105229.18938-1-pbonzini@redhat.com
parent 50f6c7db
...@@ -374,12 +374,14 @@ For 32-bit we have the following conventions - kernel is built with ...@@ -374,12 +374,14 @@ For 32-bit we have the following conventions - kernel is built with
* Fetch the per-CPU GSBASE value for this processor and put it in @reg. * Fetch the per-CPU GSBASE value for this processor and put it in @reg.
* We normally use %gs for accessing per-CPU data, but we are setting up * We normally use %gs for accessing per-CPU data, but we are setting up
* %gs here and obviously can not use %gs itself to access per-CPU data. * %gs here and obviously can not use %gs itself to access per-CPU data.
*
* Do not use RDPID, because KVM loads guest's TSC_AUX on vm-entry and
* may not restore the host's value until the CPU returns to userspace.
* Thus the kernel would consume a guest's TSC_AUX if an NMI arrives
* while running KVM's run loop.
*/ */
.macro GET_PERCPU_BASE reg:req .macro GET_PERCPU_BASE reg:req
ALTERNATIVE \ LOAD_CPU_AND_NODE_SEG_LIMIT \reg
"LOAD_CPU_AND_NODE_SEG_LIMIT \reg", \
"RDPID \reg", \
X86_FEATURE_RDPID
andq $VDSO_CPUNODE_MASK, \reg andq $VDSO_CPUNODE_MASK, \reg
movq __per_cpu_offset(, \reg, 8), \reg movq __per_cpu_offset(, \reg, 8), \reg
.endm .endm
......
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