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

[Sean Christopherson <seanjc@google.com>]

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
-- 


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