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