diff mbox series

[v2,7/7] KVM: selftests: Avoid infinite loop if ucall_alloc() fails

Message ID 20221209015307.1781352-8-oliver.upton@linux.dev
State New
Headers show
Series [v2,1/7] KVM: selftests: Fix build due to ucall_uninit() removal | expand

Commit Message

Oliver Upton Dec. 9, 2022, 1:53 a.m. UTC 
Guest assertions depend on successfully allocating a ucall structure. As
such, the use of guest assertions when ucall_alloc() fails simply leads
to an infinite loop in guest code.

Use GUEST_UCALL_NONE() to indicate failure instead. Though not
technically necessary, use a goto to have a single callsite and an
associated comment about why assertions don't work here. It isn't
perfect, at least the poor developer gets some signal out of the
guest...

Fixes: 426729b2cf2e ("KVM: selftests: Add ucall pool based implementation")
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
 tools/testing/selftests/kvm/lib/ucall_common.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

Comments

Sean Christopherson Dec. 9, 2022, 9:03 p.m. UTC | #1 
On Fri, Dec 09, 2022, Oliver Upton wrote:
> Guest assertions depend on successfully allocating a ucall structure. As
> such, the use of guest assertions when ucall_alloc() fails simply leads
> to an infinite loop in guest code.
> 
> Use GUEST_UCALL_NONE() to indicate failure instead. Though not
> technically necessary, use a goto to have a single callsite and an
> associated comment about why assertions don't work here. It isn't
> perfect, at least the poor developer gets some signal out of the
> guest...
> 
> Fixes: 426729b2cf2e ("KVM: selftests: Add ucall pool based implementation")
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---
>  tools/testing/selftests/kvm/lib/ucall_common.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/lib/ucall_common.c b/tools/testing/selftests/kvm/lib/ucall_common.c
> index 0cc0971ce60e..e8370da3de24 100644
> --- a/tools/testing/selftests/kvm/lib/ucall_common.c
> +++ b/tools/testing/selftests/kvm/lib/ucall_common.c
> @@ -41,7 +41,8 @@ static struct ucall *ucall_alloc(void)
>  	struct ucall *uc;
>  	int i;
>  
> -	GUEST_ASSERT(ucall_pool);
> +	if (!ucall_pool)
> +		goto out;
>  
>  	for (i = 0; i < KVM_MAX_VCPUS; ++i) {
>  		if (!test_and_set_bit(i, ucall_pool->in_use)) {
> @@ -51,7 +52,14 @@ static struct ucall *ucall_alloc(void)
>  		}
>  	}
>  
> -	GUEST_ASSERT(0);
> +out:
> +	/*
> +	 * If the guest cannot grab a ucall structure from the pool then the
> +	 * only option to get out to userspace is a bare ucall. This is probably
> +	 * a good time to mention that guest assertions depend on ucalls with
> +	 * arguments too.
> +	 */
> +	GUEST_UCALL_NONE();

UCALL_NONE isn't much better than infinite stack recursion, e.g. a test might end
up passing by dumb luck, or go in the wrong direction because it sometimes handles
UCALL_NONE.

How about this?

From: Sean Christopherson <seanjc@google.com>
Date: Fri, 9 Dec 2022 12:55:44 -0800
Subject: [PATCH] KVM: selftests: Use magic value to signal ucall_alloc()
 failure

Use a magic value to signal a ucall_alloc() failure instead of simply
doing GUEST_ASSERT().  GUEST_ASSERT() relies on ucall_alloc() and so a
failure puts the guest into an infinite loop.

Use -1 as the magic value, as a real ucall struct should never wrap.

Reported-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 tools/testing/selftests/kvm/lib/ucall_common.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/kvm/lib/ucall_common.c b/tools/testing/selftests/kvm/lib/ucall_common.c
index 0cc0971ce60e..2f0e2ea941cc 100644
--- a/tools/testing/selftests/kvm/lib/ucall_common.c
+++ b/tools/testing/selftests/kvm/lib/ucall_common.c
@@ -4,6 +4,8 @@
 #include "linux/bitmap.h"
 #include "linux/atomic.h"
 
+#define GUEST_UCALL_FAILED -1
+
 struct ucall_header {
 	DECLARE_BITMAP(in_use, KVM_MAX_VCPUS);
 	struct ucall ucalls[KVM_MAX_VCPUS];
@@ -41,7 +43,8 @@ static struct ucall *ucall_alloc(void)
 	struct ucall *uc;
 	int i;
 
-	GUEST_ASSERT(ucall_pool);
+	if (!ucall_pool)
+		goto ucall_failed;
 
 	for (i = 0; i < KVM_MAX_VCPUS; ++i) {
 		if (!test_and_set_bit(i, ucall_pool->in_use)) {
@@ -51,7 +54,13 @@ static struct ucall *ucall_alloc(void)
 		}
 	}
 
-	GUEST_ASSERT(0);
+ucall_failed:
+	/*
+	 * If the vCPU cannot grab a ucall structure, make a bare ucall with a
+	 * magic value to signal to get_ucall() that things went sideways.
+	 * GUEST_ASSERT() depends on ucall_alloc() and so cannot be used here.
+	 */
+	ucall_arch_do_ucall(GUEST_UCALL_FAILED);
 	return NULL;
 }
 
@@ -93,6 +102,9 @@ uint64_t get_ucall(struct kvm_vcpu *vcpu, struct ucall *uc)
 
 	addr = ucall_arch_get_ucall(vcpu);
 	if (addr) {
+		TEST_ASSERT(addr != (void *)GUEST_UCALL_FAILED,
+			    "Guest failed to allocate ucall struct");
+
 		memcpy(uc, addr, sizeof(*uc));
 		vcpu_run_complete_io(vcpu);
 	} else {

base-commit: dc2efbe4813e0dc4368779bc36c5f0e636cb8eb2
--
Oliver Upton Dec. 9, 2022, 9:35 p.m. UTC | #2 
On Fri, Dec 09, 2022 at 09:03:45PM +0000, Sean Christopherson wrote:

[...]

> > -	GUEST_ASSERT(0);
> > +out:
> > +	/*
> > +	 * If the guest cannot grab a ucall structure from the pool then the
> > +	 * only option to get out to userspace is a bare ucall. This is probably
> > +	 * a good time to mention that guest assertions depend on ucalls with
> > +	 * arguments too.
> > +	 */
> > +	GUEST_UCALL_NONE();
> 
> UCALL_NONE isn't much better than infinite stack recursion, e.g. a test might end
> up passing by dumb luck, or go in the wrong direction because it sometimes handles
> UCALL_NONE.

Oh, I was just seeking an end to my misery. Yeah, we can use a magic
value to signal this instead.

> How about this?

LGTM.

--
Thanks,
Oliver

> From: Sean Christopherson <seanjc@google.com>
> Date: Fri, 9 Dec 2022 12:55:44 -0800
> Subject: [PATCH] KVM: selftests: Use magic value to signal ucall_alloc()
>  failure
> 
> Use a magic value to signal a ucall_alloc() failure instead of simply
> doing GUEST_ASSERT().  GUEST_ASSERT() relies on ucall_alloc() and so a
> failure puts the guest into an infinite loop.
> 
> Use -1 as the magic value, as a real ucall struct should never wrap.
> 
> Reported-by: Oliver Upton <oliver.upton@linux.dev>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  tools/testing/selftests/kvm/lib/ucall_common.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/lib/ucall_common.c b/tools/testing/selftests/kvm/lib/ucall_common.c
> index 0cc0971ce60e..2f0e2ea941cc 100644
> --- a/tools/testing/selftests/kvm/lib/ucall_common.c
> +++ b/tools/testing/selftests/kvm/lib/ucall_common.c
> @@ -4,6 +4,8 @@
>  #include "linux/bitmap.h"
>  #include "linux/atomic.h"
>  
> +#define GUEST_UCALL_FAILED -1
> +
>  struct ucall_header {
>  	DECLARE_BITMAP(in_use, KVM_MAX_VCPUS);
>  	struct ucall ucalls[KVM_MAX_VCPUS];
> @@ -41,7 +43,8 @@ static struct ucall *ucall_alloc(void)
>  	struct ucall *uc;
>  	int i;
>  
> -	GUEST_ASSERT(ucall_pool);
> +	if (!ucall_pool)
> +		goto ucall_failed;
>  
>  	for (i = 0; i < KVM_MAX_VCPUS; ++i) {
>  		if (!test_and_set_bit(i, ucall_pool->in_use)) {
> @@ -51,7 +54,13 @@ static struct ucall *ucall_alloc(void)
>  		}
>  	}
>  
> -	GUEST_ASSERT(0);
> +ucall_failed:
> +	/*
> +	 * If the vCPU cannot grab a ucall structure, make a bare ucall with a
> +	 * magic value to signal to get_ucall() that things went sideways.
> +	 * GUEST_ASSERT() depends on ucall_alloc() and so cannot be used here.
> +	 */
> +	ucall_arch_do_ucall(GUEST_UCALL_FAILED);
>  	return NULL;
>  }
>  
> @@ -93,6 +102,9 @@ uint64_t get_ucall(struct kvm_vcpu *vcpu, struct ucall *uc)
>  
>  	addr = ucall_arch_get_ucall(vcpu);
>  	if (addr) {
> +		TEST_ASSERT(addr != (void *)GUEST_UCALL_FAILED,
> +			    "Guest failed to allocate ucall struct");
> +
>  		memcpy(uc, addr, sizeof(*uc));
>  		vcpu_run_complete_io(vcpu);
>  	} else {
> 
> base-commit: dc2efbe4813e0dc4368779bc36c5f0e636cb8eb2
> -- 
>
Paolo Bonzini Dec. 12, 2022, 10:38 a.m. UTC | #3 
On 12/9/22 22:03, Sean Christopherson wrote:
> From: Sean Christopherson<seanjc@google.com>
> Date: Fri, 9 Dec 2022 12:55:44 -0800
> Subject: [PATCH] KVM: selftests: Use magic value to signal ucall_alloc()
>   failure
> 
> Use a magic value to signal a ucall_alloc() failure instead of simply
> doing GUEST_ASSERT().  GUEST_ASSERT() relies on ucall_alloc() and so a
> failure puts the guest into an infinite loop.
> 
> Use -1 as the magic value, as a real ucall struct should never wrap.
> 
> Reported-by: Oliver Upton<oliver.upton@linux.dev>
> Signed-off-by: Sean Christopherson<seanjc@google.com>
> ---
>   tools/testing/selftests/kvm/lib/ucall_common.c | 16 ++++++++++++++--
>   1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/lib/ucall_common.c b/tools/testing/selftests/kvm/lib/ucall_common.c
> index 0cc0971ce60e..2f0e2ea941cc 100644
> --- a/tools/testing/selftests/kvm/lib/ucall_common.c
> +++ b/tools/testing/selftests/kvm/lib/ucall_common.c
> @@ -4,6 +4,8 @@
>   #include "linux/bitmap.h"
>   #include "linux/atomic.h"
>   
> +#define GUEST_UCALL_FAILED -1
> +
>   struct ucall_header {
>   	DECLARE_BITMAP(in_use, KVM_MAX_VCPUS);
>   	struct ucall ucalls[KVM_MAX_VCPUS];
> @@ -41,7 +43,8 @@ static struct ucall *ucall_alloc(void)
>   	struct ucall *uc;
>   	int i;
>   
> -	GUEST_ASSERT(ucall_pool);
> +	if (!ucall_pool)
> +		goto ucall_failed;
>   
>   	for (i = 0; i < KVM_MAX_VCPUS; ++i) {
>   		if (!test_and_set_bit(i, ucall_pool->in_use)) {
> @@ -51,7 +54,13 @@ static struct ucall *ucall_alloc(void)
>   		}
>   	}
>   
> -	GUEST_ASSERT(0);
> +ucall_failed:
> +	/*
> +	 * If the vCPU cannot grab a ucall structure, make a bare ucall with a
> +	 * magic value to signal to get_ucall() that things went sideways.
> +	 * GUEST_ASSERT() depends on ucall_alloc() and so cannot be used here.
> +	 */
> +	ucall_arch_do_ucall(GUEST_UCALL_FAILED);
>   	return NULL;
>   }
>   
> @@ -93,6 +102,9 @@ uint64_t get_ucall(struct kvm_vcpu *vcpu, struct ucall *uc)
>   
>   	addr = ucall_arch_get_ucall(vcpu);
>   	if (addr) {
> +		TEST_ASSERT(addr != (void *)GUEST_UCALL_FAILED,
> +			    "Guest failed to allocate ucall struct");
> +
>   		memcpy(uc, addr, sizeof(*uc));
>   		vcpu_run_complete_io(vcpu);
>   	} else {
> 
> base-commit: dc2efbe4813e0dc4368779bc36c5f0e636cb8eb2
> -- 

Queued, thanks.

Paolo
diff mbox series

Patch

diff --git a/tools/testing/selftests/kvm/lib/ucall_common.c b/tools/testing/selftests/kvm/lib/ucall_common.c
index 0cc0971ce60e..e8370da3de24 100644
--- a/tools/testing/selftests/kvm/lib/ucall_common.c
+++ b/tools/testing/selftests/kvm/lib/ucall_common.c
@@ -41,7 +41,8 @@  static struct ucall *ucall_alloc(void)
 	struct ucall *uc;
 	int i;
 
-	GUEST_ASSERT(ucall_pool);
+	if (!ucall_pool)
+		goto out;
 
 	for (i = 0; i < KVM_MAX_VCPUS; ++i) {
 		if (!test_and_set_bit(i, ucall_pool->in_use)) {
@@ -51,7 +52,14 @@  static struct ucall *ucall_alloc(void)
 		}
 	}
 
-	GUEST_ASSERT(0);
+out:
+	/*
+	 * If the guest cannot grab a ucall structure from the pool then the
+	 * only option to get out to userspace is a bare ucall. This is probably
+	 * a good time to mention that guest assertions depend on ucalls with
+	 * arguments too.
+	 */
+	GUEST_UCALL_NONE();
 	return NULL;
 }