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KVM: x86: fix escape of guest dr6 to the host
commit efdab99281
upstream.
syzkaller reported:
WARNING: CPU: 0 PID: 12927 at arch/x86/kernel/traps.c:780 do_debug+0x222/0x250
CPU: 0 PID: 12927 Comm: syz-executor Tainted: G OE 4.15.0-rc2+ #16
RIP: 0010:do_debug+0x222/0x250
Call Trace:
<#DB>
debug+0x3e/0x70
RIP: 0010:copy_user_enhanced_fast_string+0x10/0x20
</#DB>
_copy_from_user+0x5b/0x90
SyS_timer_create+0x33/0x80
entry_SYSCALL_64_fastpath+0x23/0x9a
The testcase sets a watchpoint (with perf_event_open) on a buffer that is
passed to timer_create() as the struct sigevent argument. In timer_create(),
copy_from_user()'s rep movsb triggers the BP. The testcase also sets
the debug registers for the guest.
However, KVM only restores host debug registers when the host has active
watchpoints, which triggers a race condition when running the testcase with
multiple threads. The guest's DR6.BS bit can escape to the host before
another thread invokes timer_create(), and do_debug() complains.
The fix is to respect do_debug()'s dr6 invariant when leaving KVM.
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
parent
2cc50a1946
commit
7b5ec55059
1 changed files with 6 additions and 0 deletions
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@ -2926,6 +2926,12 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
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kvm_x86_ops->vcpu_put(vcpu);
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kvm_put_guest_fpu(vcpu);
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vcpu->arch.last_host_tsc = rdtsc();
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/*
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* If userspace has set any breakpoints or watchpoints, dr6 is restored
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* on every vmexit, but if not, we might have a stale dr6 from the
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* guest. do_debug expects dr6 to be cleared after it runs, do the same.
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*/
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set_debugreg(0, 6);
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}
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static int kvm_vcpu_ioctl_get_lapic(struct kvm_vcpu *vcpu,
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