CVE-2025-21839 — KVM: x86: Load DR6 with guest value only before entering .vcpu_run() loop

Affected: Linux

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

KVM: x86: Load DR6 with guest value only before entering .vcpu_run() loop

Move the conditional loading of hardware DR6 with the guest's DR6 value out of the core .vcpu_run() loop to fix a bug where KVM can load hardware with a stale vcpu->arch.dr6.

When the guest accesses a DR and host userspace isn't debugging the guest, KVM disables DR interception and loads the guest's values into hardware on VM-Enter and saves them on VM-Exit. This allows the guest to access DRs at will, e.g. so that a sequence of DR accesses to configure a breakpoint only generates one VM-Exit.

For DR0-DR3, the logic/behavior is identical between VMX and SVM, and also identical between KVM_DEBUGREG_BP_ENABLED (userspace debugging the guest) and KVM_DEBUGREG_WONT_EXIT (guest using DRs), and so KVM handles loading DR0-DR3 in common code, _outside_ of the core kvm_x86_ops.vcpu_run() loop.

But for DR6, the guest's value doesn't need to be loaded into hardware for KVM_DEBUGREG_BP_ENABLED, and SVM provides a dedicated VMCB field whereas VMX requires software to manually load the guest value, and so loading the guest's value into DR6 is handled by {svm,vmx}_vcpu_run(), i.e. is done _inside_ the core run loop.

Unfortunately, saving the guest values on VM-Exit is initiated by common x86, again outside of the core run loop. If the guest modifies DR6 (in hardware, when DR interception is disabled), and then the next VM-Exit is a fastpath V…

USN-7521-1 — linux, linux-azure, linux-azure-6.11, linux-gcp, linux-gcp-6.11, linux-hwe-6.11, linux-oem-6.11, linux-raspi, linux-rea…

Affected: Ubuntu:24.04:LTS: linux-azure-6.11, Ubuntu:24.04:LTS: linux-gcp-6.11, Ubuntu:24.04:LTS: linux-hwe-6.11, Ubuntu:24.04:LTS: linux-oem-6.11

Several security issues were discovered in the Linux kernel. An attacker could possibly use these to compromise the system. This update corrects flaws in the following subsystems: - ARM64 architecture; - PowerPC architecture; - x86 architecture; - Block layer subsystem; - Serial ATA and Parallel ATA drivers; - Drivers core; - Network block device driver; - Bluetooth drivers; - Character device driver; - TPM device driver; - Clock framework and drivers; - Hardware crypto device drivers; - FireWire subsystem; - EFI core; - Qualcomm firmware drivers; - GPIO subsystem; - GPU drivers; - HID subsystem; - I2C subsystem; - I3C subsystem; - InfiniBand drivers; - Input Device (Mouse) drivers; - IOMMU subsystem; - Multiple devices driver; - Media drivers; - NVIDIA Tegra memory controller driver; - Fastrpc Driver; - Network drivers; - Operating Performance Points (OPP) driver; - PCI subsystem; - x86 platform drivers; - PPS (Pulse Per Second) driver; - PTP clock framework; - Remote Processor subsystem; - Real Time Clock drivers; - S/390 drivers; - SCSI subsystem; - QCOM SoC drivers; - SPI subsystem; - Media staging drivers; - Trusted Execution Environment drivers; - TTY drivers; - UFS subsystem; - USB Device Class drivers; - USB core drivers; - USB Gadget drivers; - USB Host Controller drivers; - Framebuffer layer; - File systems infrastructure; - BTRFS file system; - Ceph distributed fi…

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CVE-2025-21839 — KVM: x86: Load DR6 with guest value only before entering .vcpu_run() loop

USN-7521-1 — linux, linux-azure, linux-azure-6.11, linux-gcp, linux-gcp-6.11, linux-hwe-6.11, linux-oem-6.11, linux-raspi, linux-rea…