A single fix for x86 which removes the RDPID usage from the paranoid entry
path and unconditionally uses LSL to retrieve the CPU number. RDPID depends
on MSR_TSX_AUX. 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. MSR_TSX_AUX is part of that lazy MSR set, so after writing the guest
value and before the lazy restore any exception using the paranoid entry
will read the guest value and use it as CPU number to retrieve the GSBASE
value for the current CPU when FSGSBASE is enabled. As RDPID is only used
in that particular entry path, there is no reason to burden VMENTER/EXIT
with two extra MSR writes. Remove the RDPID optimization, which is not even
backed by numbers from the paranoid entry path instead.
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: eaad981291ee3 ("x86/entry/64: Introduce the FIND_PERCPU_BASE macro")
Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
Debugged-by: Tom Lendacky <thomas.lendacky@amd.com>
Suggested-by: Andy Lutomirski <luto@kernel.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200821105229.18938-1-pbonzini@redhat.com
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