Re: [PATCH 5/5] fix kvm's use of __pa() on percpu areas

["H. Peter Anvin" <hpa@zytor.com>]

Final question: are any of these done in frequent paths?  (I believe no, but...)

Dave Hansen <dave@linux.vnet.ibm.com> wrote:

>
>In short, it is illegal to call __pa() on an address holding
>a percpu variable.  The times when this actually matters are
>pretty obscure (certain 32-bit NUMA systems), but it _does_
>happen.  It is important to keep KVM guests working on these
>systems because the real hardware is getting harder and
>harder to find.
>
>This bug manifested first by me seeing a plain hang at boot
>after this message:
>
>	CPU 0 irqstacks, hard=f3018000 soft=f301a000
>
>or, sometimes, it would actually make it out to the console:
>
>[    0.000000] BUG: unable to handle kernel paging request at ffffffff
>
>I eventually traced it down to the KVM async pagefault code.
>This can be worked around by disabling that code either at
>compile-time, or on the kernel command-line.
>
>The kvm async pagefault code was injecting page faults in
>to the guest which the guest misinterpreted because its
>"reason" was not being properly sent from the host.
>
>The guest passes a physical address of an per-cpu async page
>fault structure via an MSR to the host.  Since __pa() is
>broken on percpu data, the physical address it sent was
>bascially bogus and the host went scribbling on random data.
>The guest never saw the real reason for the page fault (it
>was injected by the host), assumed that the kernel had taken
>a _real_ page fault, and panic()'d.  The behavior varied,
>though, depending on what got corrupted by the bad write.
>
>Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
>Acked-by: Rik van Riel <riel@redhat.com>
>---
>
> linux-2.6.git-dave/arch/x86/kernel/kvm.c      |    9 +++++----
> linux-2.6.git-dave/arch/x86/kernel/kvmclock.c |    4 ++--
> 2 files changed, 7 insertions(+), 6 deletions(-)
>
>diff -puN arch/x86/kernel/kvm.c~fix-kvm-__pa-use-on-percpu-areas
>arch/x86/kernel/kvm.c
>---
>linux-2.6.git/arch/x86/kernel/kvm.c~fix-kvm-__pa-use-on-percpu-areas	2013-01-17
>10:22:26.914436992 -0800
>+++ linux-2.6.git-dave/arch/x86/kernel/kvm.c	2013-01-17
>10:22:26.922437062 -0800
>@@ -289,9 +289,9 @@ static void kvm_register_steal_time(void
> 
> 	memset(st, 0, sizeof(*st));
> 
>-	wrmsrl(MSR_KVM_STEAL_TIME, (__pa(st) | KVM_MSR_ENABLED));
>+	wrmsrl(MSR_KVM_STEAL_TIME, (slow_virt_to_phys(st) |
>KVM_MSR_ENABLED));
> 	printk(KERN_INFO "kvm-stealtime: cpu %d, msr %lx\n",
>-		cpu, __pa(st));
>+		cpu, slow_virt_to_phys(st));
> }
> 
>static DEFINE_PER_CPU(unsigned long, kvm_apic_eoi) =
>KVM_PV_EOI_DISABLED;
>@@ -316,7 +316,7 @@ void __cpuinit kvm_guest_cpu_init(void)
> 		return;
> 
> 	if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF) && kvmapf) {
>-		u64 pa = __pa(&__get_cpu_var(apf_reason));
>+		u64 pa = slow_virt_to_phys(&__get_cpu_var(apf_reason));
> 
> #ifdef CONFIG_PREEMPT
> 		pa |= KVM_ASYNC_PF_SEND_ALWAYS;
>@@ -332,7 +332,8 @@ void __cpuinit kvm_guest_cpu_init(void)
> 		/* Size alignment is implied but just to make it explicit. */
> 		BUILD_BUG_ON(__alignof__(kvm_apic_eoi) < 4);
> 		__get_cpu_var(kvm_apic_eoi) = 0;
>-		pa = __pa(&__get_cpu_var(kvm_apic_eoi)) | KVM_MSR_ENABLED;
>+		pa = slow_virt_to_phys(&__get_cpu_var(kvm_apic_eoi))
>+			| KVM_MSR_ENABLED;
> 		wrmsrl(MSR_KVM_PV_EOI_EN, pa);
> 	}
> 
>diff -puN arch/x86/kernel/kvmclock.c~fix-kvm-__pa-use-on-percpu-areas
>arch/x86/kernel/kvmclock.c
>---
>linux-2.6.git/arch/x86/kernel/kvmclock.c~fix-kvm-__pa-use-on-percpu-areas	2013-01-17
>10:22:26.918437028 -0800
>+++ linux-2.6.git-dave/arch/x86/kernel/kvmclock.c	2013-01-17
>10:22:26.922437062 -0800
>@@ -162,8 +162,8 @@ int kvm_register_clock(char *txt)
> 	int low, high, ret;
> 	struct pvclock_vcpu_time_info *src = &hv_clock[cpu].pvti;
> 
>-	low = (int)__pa(src) | 1;
>-	high = ((u64)__pa(src) >> 32);
>+	low = (int)slow_virt_to_phys(src) | 1;
>+	high = ((u64)slow_virt_to_phys(src) >> 32);
> 	ret = native_write_msr_safe(msr_kvm_system_time, low, high);
> 	printk(KERN_INFO "kvm-clock: cpu %d, msr %x:%x, %s\n",
> 	       cpu, high, low, txt);
>_

-- 
Sent from my mobile phone. Please excuse brevity and lack of formatting.

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