CVE-2024-58083

In the Linux kernel, the following vulnerability has been resolved:

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 shouldnt 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 its fully online will allow reverting commit afb2acb2e3a3, without re-introducing the vcpu_array[0] UAF race.

Weakness

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

Potential Mitigations

References

• https://git.kernel.org/stable/c/09d50ccf0b2d739db4a485b08afe7520a4402a63
• https://git.kernel.org/stable/c/125da53b3c0c9d7f58353aea0076e9efd6498ba7
• https://git.kernel.org/stable/c/1e7381f3617d14b3c11da80ff5f8a93ab14cfc46
• https://git.kernel.org/stable/c/5cce2ed69b00e022b5cdf0c49c82986abd2941a8
• https://git.kernel.org/stable/c/7c4899239d0f70f88ac42665b3da51678d122480
• https://git.kernel.org/stable/c/ca8da90ed1432ff3d000de4f1e2275d4e7d21b96
• https://git.kernel.org/stable/c/d817e510662fd1c9797952408d94806f97a5fffd
• https://git.kernel.org/stable/c/f2f805ada63b536bc192458a7098388286568ad4