[PATCH v2] KVM: arm64: Make vcpu flag updates non-preemptible

[PATCH v2] KVM: arm64: Make vcpu flag updates non-preemptible

[Marc Zyngier]

Per-vcpu flags are updated using a non-atomic RMW operation.
Which means it is possible to get preempted between the read and
write operations.

Another interesting thing to note is that preemption also updates
flags, as we have some flag manipulation in both the load and put
operations.

It is thus possible to lose information communicated by either
load or put, as the preempted flag update will overwrite the flags
when the thread is resumed. This is specially critical if either
load or put has stored information which depends on the physical
CPU the vcpu runs on.

This results in really elusive bugs, and kudos must be given to
Mostafa for the long hours of debugging, and finally spotting
the problem.

Fix it by disabling preemption during the RMW operation, which
ensures that the state stays consistent. Also upgrade vcpu_get_flag
path to use READ_ONCE() to make sure the field is always atomically
accessed.

Fixes: e87abb73e594 ("KVM: arm64: Add helpers to manipulate vcpu flags among a set")
Reported-by: Mostafa Saleh <smostafa@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
---

Notes:
    v2: add READ_ONCE() on the read path, expand commit message

 arch/arm64/include/asm/kvm_host.h | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index bcd774d74f34..3dd691c85ca0 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -576,9 +576,22 @@ struct kvm_vcpu_arch {
 	({							\
 		__build_check_flag(v, flagset, f, m);		\
 								\
-		v->arch.flagset & (m);				\
+		READ_ONCE(v->arch.flagset) & (m);		\
 	})
 
+/*
+ * Note that the set/clear accessors must be preempt-safe in order to
+ * avoid nesting them with load/put which also manipulate flags...
+ */
+#ifdef __KVM_NVHE_HYPERVISOR__
+/* the nVHE hypervisor is always non-preemptible */
+#define __vcpu_flags_preempt_disable()
+#define __vcpu_flags_preempt_enable()
+#else
+#define __vcpu_flags_preempt_disable()	preempt_disable()
+#define __vcpu_flags_preempt_enable()	preempt_enable()
+#endif
+
 #define __vcpu_set_flag(v, flagset, f, m)			\
 	do {							\
 		typeof(v->arch.flagset) *fset;			\
@@ -586,9 +599,11 @@ struct kvm_vcpu_arch {
 		__build_check_flag(v, flagset, f, m);		\
 								\
 		fset = &v->arch.flagset;			\
+		__vcpu_flags_preempt_disable();			\
 		if (HWEIGHT(m) > 1)				\
 			*fset &= ~(m);				\
 		*fset |= (f);					\
+		__vcpu_flags_preempt_enable();			\
 	} while (0)
 
 #define __vcpu_clear_flag(v, flagset, f, m)			\
@@ -598,7 +613,9 @@ struct kvm_vcpu_arch {
 		__build_check_flag(v, flagset, f, m);		\
 								\
 		fset = &v->arch.flagset;			\
+		__vcpu_flags_preempt_disable();			\
 		*fset &= ~(m);					\
+		__vcpu_flags_preempt_enable();			\
 	} while (0)
 
 #define vcpu_get_flag(v, ...)	__vcpu_get_flag((v), __VA_ARGS__)
-- 
2.34.1


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Re: [PATCH v2] KVM: arm64: Make vcpu flag updates non-preemptible

[Oliver Upton]

On Tue, 18 Apr 2023 13:57:37 +0100, Marc Zyngier wrote:
> Per-vcpu flags are updated using a non-atomic RMW operation.
> Which means it is possible to get preempted between the read and
> write operations.
> 
> Another interesting thing to note is that preemption also updates
> flags, as we have some flag manipulation in both the load and put
> operations.
> 
> [...]

Applied to kvmarm/fixes, thanks!

[1/1] KVM: arm64: Make vcpu flag updates non-preemptible
      https://git.kernel.org/kvmarm/kvmarm/c/35dcb3ac663a

--
Best,
Oliver

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Re: [PATCH v2] KVM: arm64: Make vcpu flag updates non-preemptible

[Will Deacon]

On Tue, Apr 18, 2023 at 01:57:37PM +0100, Marc Zyngier wrote:
> Per-vcpu flags are updated using a non-atomic RMW operation.
> Which means it is possible to get preempted between the read and
> write operations.
> 
> Another interesting thing to note is that preemption also updates
> flags, as we have some flag manipulation in both the load and put
> operations.
> 
> It is thus possible to lose information communicated by either
> load or put, as the preempted flag update will overwrite the flags
> when the thread is resumed. This is specially critical if either
> load or put has stored information which depends on the physical
> CPU the vcpu runs on.
> 
> This results in really elusive bugs, and kudos must be given to
> Mostafa for the long hours of debugging, and finally spotting
> the problem.
> 
> Fix it by disabling preemption during the RMW operation, which
> ensures that the state stays consistent. Also upgrade vcpu_get_flag
> path to use READ_ONCE() to make sure the field is always atomically
> accessed.
> 
> Fixes: e87abb73e594 ("KVM: arm64: Add helpers to manipulate vcpu flags among a set")
> Reported-by: Mostafa Saleh <smostafa@google.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> Cc: stable@vger.kernel.org
> ---
> 
> Notes:
>     v2: add READ_ONCE() on the read path, expand commit message

Acked-by: Will Deacon <will@kernel.org>

Will

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