[PATCH 1/2] KVM: SEV: Nullify "have run CPUs" mask pointer when freeing it

[Sean Christopherson <seanjc@google.com>]

Nullify have_run_cpus when freeing the mask, particularly in the error path
of __sev_guest_init(), so that KVM doesn't have to subtly use sev->active
to track whether or not the mask has been freed.  As pointed out by Sashiko,
blindly freeing the mask in sev_vm_destroy() results in a double-free if
the mask is freed if __sev_guest_init() fails.

Throw the logic in a helper as nullifying the pointer is frustratingly
difficult and weird due to have_run_cpus being a single-entry array when
CPUMASK_OFFSTACK=n.  Deliberately don't use CPUMASK_VAR_NULL, as it's not
directly assignable when the cpumask is on-stack, e.g. requires using a
local variable and a memcpy(), which is beyond ridiculous.  Furthermore,
while clearing the on-stack bitmask is an unnecessary and arguably unwanted
side effect, KVM absolutely relies on '0' being the "null" value given that
the struct is zero-allocated.

Opportunistically add an alloc() helper to pair with free(); there are just
enough call sites to make doing so worthwhile.

Fixes: 12c1f6e03f94 ("KVM: SEV: Free have_run_cpus during VM destruction even if VM is no longer SEV")
Cc: stable@vger.kernel.org
Reported-by: Sashiko Bot <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260923165349.CAAF01F000FF@smtp.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/svm/sev.c | 33 ++++++++++++++++++++++-----------
 1 file changed, 22 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 3448d56520c6..d3a2e6a51efc 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -488,6 +488,20 @@ static void snp_guest_req_cleanup(struct kvm *kvm)
 	sev->guest_resp_buf = NULL;
 }
 
+static int sev_alloc_have_run_cpus(struct kvm_sev_info *sev)
+{
+	if (!zalloc_cpumask_var(&sev->have_run_cpus, GFP_KERNEL_ACCOUNT))
+		return -ENOMEM;
+
+	return 0;
+}
+
+static void sev_free_have_run_cpus(struct kvm_sev_info *sev)
+{
+	free_cpumask_var(sev->have_run_cpus);
+	memset(&sev->have_run_cpus, 0, sizeof(sev->have_run_cpus));
+}
+
 static int __sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp,
 			    struct kvm_sev_init *data,
 			    unsigned long vm_type)
@@ -545,10 +559,9 @@ static int __sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp,
 	if (ret)
 		goto e_free_asid;
 
-	if (!zalloc_cpumask_var(&sev->have_run_cpus, GFP_KERNEL_ACCOUNT)) {
-		ret = -ENOMEM;
+	ret = sev_alloc_have_run_cpus(sev);
+	if (ret)
 		goto e_free_asid;
-	}
 
 	/* This needs to happen after SEV/SNP firmware initialization. */
 	if (snp_active) {
@@ -566,7 +579,7 @@ static int __sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp,
 	return 0;
 
 e_free:
-	free_cpumask_var(sev->have_run_cpus);
+	sev_free_have_run_cpus(sev);
 e_free_asid:
 	argp->error = init_args.error;
 	sev_asid_free(sev);
@@ -2191,10 +2204,9 @@ int sev_vm_move_enc_context_from(struct kvm *kvm, unsigned int source_fd)
 	 * does not, i.e. KVM could skip flushes if memory is reclaimed from
 	 * the old VM but not the new VM.
 	 */
-	if (!zalloc_cpumask_var(&dst_sev->have_run_cpus, GFP_KERNEL_ACCOUNT)) {
-		ret = -ENOMEM;
+	ret = sev_alloc_have_run_cpus(dst_sev);
+	if (ret)
 		goto out_source_vcpu;
-	}
 
 	sev_migrate_from(kvm, source_kvm);
 	kvm_vm_dead(source_kvm);
@@ -2888,10 +2900,9 @@ int sev_vm_copy_enc_context_from(struct kvm *kvm, unsigned int source_fd)
 	}
 
 	mirror_sev = to_kvm_sev_info(kvm);
-	if (!zalloc_cpumask_var(&mirror_sev->have_run_cpus, GFP_KERNEL_ACCOUNT)) {
-		ret = -ENOMEM;
+	ret = sev_alloc_have_run_cpus(mirror_sev);
+	if (ret)
 		goto e_unlock;
-	}
 
 	/*
 	 * The mirror kvm holds an enc_context_owner ref so its asid can't
@@ -2984,7 +2995,7 @@ void sev_vm_destroy(struct kvm *kvm)
 	 * Free the mask even if the VM is not *currently* an SEV VM, as it may
 	 * have been an SEV VM prior to intra-host migration.
 	 */
-	free_cpumask_var(sev->have_run_cpus);
+	sev_free_have_run_cpus(sev);
 
 	if (!sev_guest(kvm))
 		return;
-- 
2.56.0.rc1.315.gc6ed9934b7-goog