From 1e7381f3617d14b3c11da80ff5f8a93ab14cfc46 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Wed, 9 Oct 2024 08:04:50 -0700 Subject: KVM: Explicitly verify target vCPU is online in kvm_get_vcpu() Explicitly verify the target vCPU is fully online _prior_ to clamping the index in kvm_get_vcpu(). If the index is "bad", the nospec clamping will generate '0', i.e. KVM will return vCPU0 instead of NULL. In practice, the bug is unlikely to cause problems, as it will only come into play if userspace or the guest is buggy or misbehaving, e.g. KVM may send interrupts to vCPU0 instead of dropping them on the floor. However, returning vCPU0 when it shouldn't exist per online_vcpus is problematic now that KVM uses an xarray for the vCPUs array, as KVM needs to insert into the xarray before publishing the vCPU to userspace (see commit c5b077549136 ("KVM: Convert the kvm->vcpus array to a xarray")), i.e. before vCPU creation is guaranteed to succeed. As a result, incorrectly providing access to vCPU0 will trigger a use-after-free if vCPU0 is dereferenced and kvm_vm_ioctl_create_vcpu() bails out of vCPU creation due to an error and frees vCPU0. Commit afb2acb2e3a3 ("KVM: Fix vcpu_array[0] races") papered over that issue, but in doing so introduced an unsolvable teardown conundrum. Preventing accesses to vCPU0 before it's fully online will allow reverting commit afb2acb2e3a3, without re-introducing the vcpu_array[0] UAF race. Fixes: 1d487e9bf8ba ("KVM: fix spectrev1 gadgets") Cc: stable@vger.kernel.org Cc: Will Deacon Cc: Michal Luczaj Reviewed-by: Pankaj Gupta Acked-by: Will Deacon Link: https://lore.kernel.org/r/20241009150455.1057573-2-seanjc@google.com Signed-off-by: Sean Christopherson --- include/linux/kvm_host.h | 9 +++++++++ 1 file changed, 9 insertions(+) (limited to 'include') diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index 401439bb21e3..b0b38744c4b0 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -963,6 +963,15 @@ static inline struct kvm_io_bus *kvm_get_bus(struct kvm *kvm, enum kvm_bus idx) static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i) { int num_vcpus = atomic_read(&kvm->online_vcpus); + + /* + * Explicitly verify the target vCPU is online, as the anti-speculation + * logic only limits the CPU's ability to speculate, e.g. given a "bad" + * index, clamping the index to 0 would return vCPU0, not NULL. + */ + if (i >= num_vcpus) + return NULL; + i = array_index_nospec(i, num_vcpus); /* Pairs with smp_wmb() in kvm_vm_ioctl_create_vcpu. */ -- cgit v1.2.3 From 0664dc74e9d004c36b4400081811df795169809a Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Wed, 9 Oct 2024 08:04:51 -0700 Subject: KVM: Verify there's at least one online vCPU when iterating over all vCPUs Explicitly check that there is at least online vCPU before iterating over all vCPUs. Because the max index is an unsigned long, passing "0 - 1" in the online_vcpus==0 case results in xa_for_each_range() using an unlimited max, i.e. allows it to access vCPU0 when it shouldn't. This will allow KVM to safely _erase_ from vcpu_array if the last stages of vCPU creation fail, i.e. without generating a use-after-free if a different task happens to be concurrently iterating over all vCPUs. Note, because xa_for_each_range() is a macro, kvm_for_each_vcpu() subtly reloads online_vcpus after each iteration, i.e. adding an extra load doesn't meaningfully impact the total cost of iterating over all vCPUs. And because online_vcpus is never decremented, there is no risk of a reload triggering a walk of the entire xarray. Cc: Will Deacon Cc: Michal Luczaj Acked-by: Will Deacon Link: https://lore.kernel.org/r/20241009150455.1057573-3-seanjc@google.com Signed-off-by: Sean Christopherson --- include/linux/kvm_host.h | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) (limited to 'include') diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h index b0b38744c4b0..92f192bec07b 100644 --- a/include/linux/kvm_host.h +++ b/include/linux/kvm_host.h @@ -979,9 +979,10 @@ static inline struct kvm_vcpu *kvm_get_vcpu(struct kvm *kvm, int i) return xa_load(&kvm->vcpu_array, i); } -#define kvm_for_each_vcpu(idx, vcpup, kvm) \ - xa_for_each_range(&kvm->vcpu_array, idx, vcpup, 0, \ - (atomic_read(&kvm->online_vcpus) - 1)) +#define kvm_for_each_vcpu(idx, vcpup, kvm) \ + if (atomic_read(&kvm->online_vcpus)) \ + xa_for_each_range(&kvm->vcpu_array, idx, vcpup, 0, \ + (atomic_read(&kvm->online_vcpus) - 1)) static inline struct kvm_vcpu *kvm_get_vcpu_by_id(struct kvm *kvm, int id) { -- cgit v1.2.3