CVE-2026-31592 in Linux
要約
〜によって VulDB • 2026年05月24日
Based on the provided register dump and syzlang reproducer, here is the analysis and the corresponding C code fix using `guard()` to simplify error handling.
### Analysis
1. **Context**: The crash occurs in the KVM subsystem, specifically involving `KVM_SET_GUEST_DEBUG` and `KVM_RUN` on a VCPU (`r5`) created from VM `r4`. 2. **Reproducer Clues**: * `r5 = ioctl$KVM_CREATE_VCPU(r4, 0xae41, 0x2)`: Creates VCPU 2. * `ioctl$KVM_SET_GUEST_DEBUG(r5, ...)`: Sets up guest debug. * `ioctl$KVM_RUN(r5, ...)`: Runs the VCPU. * The crash likely happens inside `kvm_arch_vcpu_ioctl_set_guest_debug` or during the `KVM_RUN` when the debug state is processed. 3. **Register Dump**: * `RBP: 00007f34ea000d69`: Stack pointer. * `R14: 00007f34ea1a6080`: Likely a pointer to a structure (e.g., `struct kvm_vcpu` or `struct kvm_run`). * The crash is likely a NULL pointer dereference or invalid memory access within the KVM debug handling path.
### Fix Strategy
The prompt asks to "opportunistically use `guard()` to avoid having to define a new error label and goto usage." This implies refactoring a function that currently uses `goto err;` patterns to use C11 `_Generic` or a custom `guard` macro (common in Linux kernel development, e.g., `guard(mutex)`, `guard(rcu_read_lock)`, or custom cleanup labels).
However, since the crash is in `kvm_arch_vcpu_ioctl_set_guest_debug` or related, and the reproducer shows a specific sequence, the bug is likely in how the guest debug state is validated or applied. A common issue in KVM guest debug is improper handling of breakpoints when the VCPU is in a specific state or when memory regions are not properly set up.
Looking at the syzlang: `ioctl$KVM_SET_GUEST_DEBUG(r5, 0x4048ae9b, &(0x7f0000000300)={0x4376ea830d46549b, 0x0, [0x46, 0x0, 0x0, 0x0, 0x0, 0x1000]})`
The structure passed is `struct kvm_guest_debug`. The fields are: * `control`: `0x4376ea830d46549b` * `nr_bp`: `0x0` * `reserved`: `[0x46, 0x0, ...]`
The `control` field has `KVM_GUESTDBG_ENABLE` (bit 0) and `KVM_GUESTDBG_USE_HW_BP` (bit 1) set? Let's decode `0x4376ea830d46549b`. Actually, the exact bit values don't matter as much as the fact that the code path is taken.
The most likely fix involves ensuring that when `KVM_SET_GUEST_DEBUG` is called, the kernel properly validates the input or handles the case where hardware breakpoints are requested but not supported or improperly initialized.
However, the prompt specifically asks to use `guard()` to simplify error handling. This suggests the bug might be in a function that has complex error paths, and the fix involves using `guard()` to automatically clean up resources (like locks or allocated memory) on error, preventing leaks or inconsistent states that lead to the crash.
Given the lack of specific source code, I will provide a generic example of how `guard()` is used in the Linux kernel to replace `goto err` patterns, applied to a hypothetical function in the KVM code that handles guest debug.
### Code Fix Example
Assume the buggy code looks like this (simplified):
```c static int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu, struct kvm_guest_debug *dbg) {
int r; struct kvm_x86_ops *ops = vcpu->kvm->arch.x86_ops;
if (!ops->set_guest_debug) return -EOPNOTSUPP;
r = mutex_lock_interruptible(&vcpu->mutex); if (r) goto out;
// ... complex logic ...
r = ops->set_guest_debug(vcpu, dbg); if (r) goto out_unlock;
// ... more logic ...
mutex_unlock(&vcpu->mutex); return 0;
out_unlock: mutex_unlock(&vcpu->mutex); out: return r; }
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