Re: CVE-2022-49181: xen: fix is_xen_pmu()

Re: CVE-2022-49181: xen: fix is_xen_pmu()

[Juergen Gross]

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On 26.02.25 02:56, Greg Kroah-Hartman wrote:
> Description
> ===========
> 
> In the Linux kernel, the following vulnerability has been resolved:
> 
> xen: fix is_xen_pmu()
> 
> is_xen_pmu() is taking the cpu number as parameter, but it is not using
> it. Instead it just tests whether the Xen PMU initialization on the
> current cpu did succeed. As this test is done by checking a percpu
> pointer, preemption needs to be disabled in order to avoid switching
> the cpu while doing the test. While resuming from suspend() this seems
> not to be the case:
> 
> [   88.082751] ACPI: PM: Low-level resume complete
> [   88.087933] ACPI: EC: EC started
> [   88.091464] ACPI: PM: Restoring platform NVS memory
> [   88.097166] xen_acpi_processor: Uploading Xen processor PM info
> [   88.103850] Enabling non-boot CPUs ...
> [   88.108128] installing Xen timer for CPU 1
> [   88.112763] BUG: using smp_processor_id() in preemptible [00000000] code: systemd-sleep/7138
> [   88.122256] caller is is_xen_pmu+0x12/0x30
> [   88.126937] CPU: 0 PID: 7138 Comm: systemd-sleep Tainted: G        W         5.16.13-2.fc32.qubes.x86_64 #1
> [   88.137939] Hardware name: Star Labs StarBook/StarBook, BIOS 7.97 03/21/2022
> [   88.145930] Call Trace:
> [   88.148757]  <TASK>
> [   88.151193]  dump_stack_lvl+0x48/0x5e
> [   88.155381]  check_preemption_disabled+0xde/0xe0
> [   88.160641]  is_xen_pmu+0x12/0x30
> [   88.164441]  xen_smp_intr_init_pv+0x75/0x100
> 
> Fix that by replacing is_xen_pmu() by a simple boolean variable which
> reflects the Xen PMU initialization state on cpu 0.
> 
> Modify xen_pmu_init() to return early in case it is being called for a
> cpu other than cpu 0 and the boolean variable not being set.

Please revoke this CVE. The bug can happen only if running as a Xen PV
guest AND if the "vpmu" feature has been enabled in the Xen hypervisor.
Running with "vpmu" enabled is NOT security supported by Xen, so this
potential crash is by definition not a security issue.


Juergen

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