This is a note to let you know that I've just added the patch titled KVM: x86: Fix potential preemption when get the current kvmclock timestamp to the 4.9-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: kvm-x86-fix-potential-preemption-when-get-the-current-kvmclock-timestamp.patch and it can be found in the queue-4.9 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From e2c2206a18993bc9f62393d49c7b2066c3845b25 Mon Sep 17 00:00:00 2001 From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx> Date: Thu, 11 May 2017 18:12:05 -0700 Subject: KVM: x86: Fix potential preemption when get the current kvmclock timestamp MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx> commit e2c2206a18993bc9f62393d49c7b2066c3845b25 upstream. BUG: using __this_cpu_read() in preemptible [00000000] code: qemu-system-x86/2809 caller is __this_cpu_preempt_check+0x13/0x20 CPU: 2 PID: 2809 Comm: qemu-system-x86 Not tainted 4.11.0+ #13 Call Trace: dump_stack+0x99/0xce check_preemption_disabled+0xf5/0x100 __this_cpu_preempt_check+0x13/0x20 get_kvmclock_ns+0x6f/0x110 [kvm] get_time_ref_counter+0x5d/0x80 [kvm] kvm_hv_process_stimers+0x2a1/0x8a0 [kvm] ? kvm_hv_process_stimers+0x2a1/0x8a0 [kvm] ? kvm_arch_vcpu_ioctl_run+0xac9/0x1ce0 [kvm] kvm_arch_vcpu_ioctl_run+0x5bf/0x1ce0 [kvm] kvm_vcpu_ioctl+0x384/0x7b0 [kvm] ? kvm_vcpu_ioctl+0x384/0x7b0 [kvm] ? __fget+0xf3/0x210 do_vfs_ioctl+0xa4/0x700 ? __fget+0x114/0x210 SyS_ioctl+0x79/0x90 entry_SYSCALL_64_fastpath+0x23/0xc2 RIP: 0033:0x7f9d164ed357 ? __this_cpu_preempt_check+0x13/0x20 This can be reproduced by run kvm-unit-tests/hyperv_stimer.flat w/ CONFIG_PREEMPT and CONFIG_DEBUG_PREEMPT enabled. Safe access to per-CPU data requires a couple of constraints, though: the thread working with the data cannot be preempted and it cannot be migrated while it manipulates per-CPU variables. If the thread is preempted, the thread that replaces it could try to work with the same variables; migration to another CPU could also cause confusion. However there is no preemption disable when reads host per-CPU tsc rate to calculate the current kvmclock timestamp. This patch fixes it by utilizing get_cpu/put_cpu pair to guarantee both __this_cpu_read() and rdtsc() are not preempted. Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> Cc: Radim Krčmář <rkrcmar@xxxxxxxxxx> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx> Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> Signed-off-by: Radim Krčmář <rkrcmar@xxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/x86/kvm/x86.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -1735,6 +1735,7 @@ static u64 __get_kvmclock_ns(struct kvm { struct kvm_arch *ka = &kvm->arch; struct pvclock_vcpu_time_info hv_clock; + u64 ret; spin_lock(&ka->pvclock_gtod_sync_lock); if (!ka->use_master_clock) { @@ -1746,10 +1747,17 @@ static u64 __get_kvmclock_ns(struct kvm hv_clock.system_time = ka->master_kernel_ns + ka->kvmclock_offset; spin_unlock(&ka->pvclock_gtod_sync_lock); + /* both __this_cpu_read() and rdtsc() should be on the same cpu */ + get_cpu(); + kvm_get_time_scale(NSEC_PER_SEC, __this_cpu_read(cpu_tsc_khz) * 1000LL, &hv_clock.tsc_shift, &hv_clock.tsc_to_system_mul); - return __pvclock_read_cycles(&hv_clock, rdtsc()); + ret = __pvclock_read_cycles(&hv_clock, rdtsc()); + + put_cpu(); + + return ret; } u64 get_kvmclock_ns(struct kvm *kvm) Patches currently in stable-queue which might be from wanpeng.li@xxxxxxxxxxx are queue-4.9/kvm-x86-fix-potential-preemption-when-get-the-current-kvmclock-timestamp.patch queue-4.9/kvm-x86-fix-load-damaged-ssex-mxcsr-register.patch queue-4.9/kvm-x86-fix-read-out-of-bounds-vulnerability-in-kvm-pio-emulation.patch