Commit bd89870c authored by Paul Mackerras's avatar Paul Mackerras Committed by Sasha Levin

KVM: PPC: Book3S HV: Save/restore TM state in H_CEDE

[ Upstream commit 93d17397 ]

It turns out that if the guest does a H_CEDE while the CPU is in
a transactional state, and the H_CEDE does a nap, and the nap
loses the architected state of the CPU (which is is allowed to do),
then we lose the checkpointed state of the virtual CPU.  In addition,
the transactional-memory state recorded in the MSR gets reset back
to non-transactional, and when we try to return to the guest, we take
a TM bad thing type of program interrupt because we are trying to
transition from non-transactional to transactional with a hrfid
instruction, which is not permitted.

The result of the program interrupt occurring at that point is that
the host CPU will hang in an infinite loop with interrupts disabled.
Thus this is a denial of service vulnerability in the host which can
be triggered by any guest (and depending on the guest kernel, it can
potentially triggered by unprivileged userspace in the guest).

This vulnerability has been assigned the ID CVE-2016-5412.

To fix this, we save the TM state before napping and restore it
on exit from the nap, when handling a H_CEDE in real mode.  The
case where H_CEDE exits to host virtual mode is already OK (as are
other hcalls which exit to host virtual mode) because the exit
path saves the TM state.

Cc: stable@vger.kernel.org # v3.15+
Signed-off-by: default avatarPaul Mackerras <paulus@ozlabs.org>
Signed-off-by: default avatarSasha Levin <alexander.levin@verizon.com>
parent ae40dadb
......@@ -1931,6 +1931,13 @@ _GLOBAL(kvmppc_h_cede) /* r3 = vcpu pointer, r11 = msr, r13 = paca */
/* save FP state */
bl kvmppc_save_fp
#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
BEGIN_FTR_SECTION
ld r9, HSTATE_KVM_VCPU(r13)
bl kvmppc_save_tm
END_FTR_SECTION_IFSET(CPU_FTR_TM)
#endif
/*
* Set DEC to the smaller of DEC and HDEC, so that we wake
* no later than the end of our timeslice (HDEC interrupts
......@@ -2007,6 +2014,12 @@ kvm_end_cede:
bl kvmhv_accumulate_time
#endif
#ifdef CONFIG_PPC_TRANSACTIONAL_MEM
BEGIN_FTR_SECTION
bl kvmppc_restore_tm
END_FTR_SECTION_IFSET(CPU_FTR_TM)
#endif
/* load up FP state */
bl kvmppc_load_fp
......
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