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x86/entry/64: Do not use RDPID in paranoid entry to accomodate KVM
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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: eaad981 ("x86/entry/64: Introduce the FIND_PERCPU_BASE macro")
Reported-by: Tom Lendacky <[email protected]>
Debugged-by: Tom Lendacky <[email protected]>
Suggested-by: Andy Lutomirski <[email protected]>
Suggested-by: Peter Zijlstra <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
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Sean Christopherson authored and KAGA-KOKO committed Aug 21, 2020
1 parent 50f6c7d commit 6a3ea3e
Showing 1 changed file with 6 additions and 4 deletions.
10 changes: 6 additions & 4 deletions arch/x86/entry/calling.h
Original file line number Diff line number Diff line change
Expand Up @@ -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.
* 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.
*
* 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
ALTERNATIVE \
"LOAD_CPU_AND_NODE_SEG_LIMIT \reg", \
"RDPID \reg", \
X86_FEATURE_RDPID
LOAD_CPU_AND_NODE_SEG_LIMIT \reg
andq $VDSO_CPUNODE_MASK, \reg
movq __per_cpu_offset(, \reg, 8), \reg
.endm
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