linux-cve-announce - CVE-2025-21839: KVM: x86: Load DR6 with guest value only before entering .vcpu

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Message-ID: <2025030707-CVE-2025-21839-dfd9@gregkh>
Date: Fri,  7 Mar 2025 10:10:09 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: CVE-2025-21839: KVM: x86: Load DR6 with guest value only before entering .vcpu_run() loop

Description
===========

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 VM-Exit, KVM will reload hardware DR6 with vcpu->arch.dr6 and
clobber the guest's actual value.

The bug shows up primarily with nested VMX because KVM handles the VMX
preemption timer in the fastpath, and the window between hardware DR6
being modified (in guest context) and DR6 being read by guest software is
orders of magnitude larger in a nested setup.  E.g. in non-nested, the
VMX preemption timer would need to fire precisely between #DB injection
and the #DB handler's read of DR6, whereas with a KVM-on-KVM setup, the
window where hardware DR6 is "dirty" extends all the way from L1 writing
DR6 to VMRESUME (in L1).

    L1's view:
    ==========
    <L1 disables DR interception>
           CPU 0/KVM-7289    [023] d....  2925.640961: kvm_entry: vcpu 0
 A:  L1 Writes DR6
           CPU 0/KVM-7289    [023] d....  2925.640963: <hack>: Set DRs, DR6 = 0xffff0ff1

 B:        CPU 0/KVM-7289    [023] d....  2925.640967: kvm_exit: vcpu 0 reason EXTERNAL_INTERRUPT intr_info 0x800000ec

 D: L1 reads DR6, arch.dr6 = 0
           CPU 0/KVM-7289    [023] d....  2925.640969: <hack>: Sync DRs, DR6 = 0xffff0ff0

           CPU 0/KVM-7289    [023] d....  2925.640976: kvm_entry: vcpu 0
    L2 reads DR6, L1 disables DR interception
           CPU 0/KVM-7289    [023] d....  2925.640980: kvm_exit: vcpu 0 reason DR_ACCESS info1 0x0000000000000216
           CPU 0/KVM-7289    [023] d....  2925.640983: kvm_entry: vcpu 0

           CPU 0/KVM-7289    [023] d....  2925.640983: <hack>: Set DRs, DR6 = 0xffff0ff0

    L2 detects failure
           CPU 0/KVM-7289    [023] d....  2925.640987: kvm_exit: vcpu 0 reason HLT
    L1 reads DR6 (confirms failure)
           CPU 0/KVM-7289    [023] d....  2925.640990: <hack>: Sync DRs, DR6 = 0xffff0ff0

    L0's view:
    ==========
    L2 reads DR6, arch.dr6 = 0
          CPU 23/KVM-5046    [001] d....  3410.005610: kvm_exit: vcpu 23 reason DR_ACCESS info1 0x0000000000000216
          CPU 23/KVM-5046    [001] .....  3410.005610: kvm_nested_vmexit: vcpu 23 reason DR_ACCESS info1 0x0000000000000216

    L2 => L1 nested VM-Exit
          CPU 23/KVM-5046    [001] .....  3410.005610: kvm_nested_vmexit_inject: reason: DR_ACCESS ext_inf1: 0x0000000000000216

          CPU 23/KVM-5046    [001] d....  3410.005610: kvm_entry: vcpu 23
          CPU 23/KVM-5046    [001] d....  3410.005611: kvm_exit: vcpu 23 reason VMREAD
          CPU 23/KVM-5046    [001] d....  3410.005611: kvm_entry: vcpu 23
          CPU 23/KVM-5046    [001] d....  3410.005612: kvm_exit: vcpu 23 reason VMREAD
          CPU 23/KVM-5046    [001] d....  3410.005612: kvm_entry: vcpu 23

    L1 writes DR7, L0 disables DR interception
          CPU 23/KVM-5046    [001] d....  3410.005612: kvm_exit: vcpu 23 reason DR_ACCESS info1 0x0000000000000007
          CPU 23/KVM-5046    [001] d....  3410.005613: kvm_entry: vcpu 23

    L0 writes DR6 = 0 (arch.dr6)
          CPU 23/KVM-5046    [001] d....  3410.005613: <hack>: Set DRs, DR6 = 0xffff0ff0

 A: <L1 writes DR6 = 1, no interception, arch.dr6 is still '0'>

 B:       CPU 23/KVM-5046    [001] d....  3410.005614: kvm_exit: vcpu 23 reason PREEMPTION_TIMER
          CPU 23/KVM-5046    [001] d....  3410.005614: kvm_entry: vcpu 23

 C: L0 writes DR6 = 0 (arch.dr6)
          CPU 23/KVM-5046    [001] d....  3410.005614: <hack>: Set DRs, DR6 = 0xffff0ff0

    L1 => L2 nested VM-Enter
          CPU 23/KVM-5046    [001] d....  3410.005616: kvm_exit: vcpu 23 reason VMRESUME

    L0 reads DR6, arch.dr6 = 0

The Linux kernel CVE team has assigned CVE-2025-21839 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.7 with commit d67668e9dd76d98136048935723947156737932b and fixed in 6.12.16 with commit 4eb063de686bfcdfd03a8c801d1bbe87d2d5eb55
	Issue introduced in 5.7 with commit d67668e9dd76d98136048935723947156737932b and fixed in 6.13.4 with commit d456de38d9eb753a4e9fde053c18d4ef8e485339
	Issue introduced in 5.7 with commit d67668e9dd76d98136048935723947156737932b and fixed in 6.14-rc3 with commit c2fee09fc167c74a64adb08656cb993ea475197e

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2025-21839
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	arch/x86/include/asm/kvm-x86-ops.h
	arch/x86/include/asm/kvm_host.h
	arch/x86/kvm/svm/svm.c
	arch/x86/kvm/vmx/main.c
	arch/x86/kvm/vmx/vmx.c
	arch/x86/kvm/vmx/x86_ops.h
	arch/x86/kvm/x86.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/4eb063de686bfcdfd03a8c801d1bbe87d2d5eb55
	https://git.kernel.org/stable/c/d456de38d9eb753a4e9fde053c18d4ef8e485339
	https://git.kernel.org/stable/c/c2fee09fc167c74a64adb08656cb993ea475197e