Re: CVE-2022-49101: xen: delay xen_hvm_init_time_ops() if kdump is boot on vcpu>=32

[Juergen Gross <jgross@suse.com>]

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On 26.02.25 02:55, Greg Kroah-Hartman wrote:
> Description
> ===========
> 
> In the Linux kernel, the following vulnerability has been resolved:
> 
> xen: delay xen_hvm_init_time_ops() if kdump is boot on vcpu>=32
> 
> The sched_clock() can be used very early since commit 857baa87b642
> ("sched/clock: Enable sched clock early"). In addition, with commit
> 38669ba205d1 ("x86/xen/time: Output xen sched_clock time from 0"), kdump
> kernel in Xen HVM guest may panic at very early stage when accessing
> &__this_cpu_read(xen_vcpu)->time as in below:
> 
> setup_arch()
>   -> init_hypervisor_platform()
>       -> x86_init.hyper.init_platform = xen_hvm_guest_init()
>           -> xen_hvm_init_time_ops()
>               -> xen_clocksource_read()
>                   -> src = &__this_cpu_read(xen_vcpu)->time;
> 
> This is because Xen HVM supports at most MAX_VIRT_CPUS=32 'vcpu_info'
> embedded inside 'shared_info' during early stage until xen_vcpu_setup() is
> used to allocate/relocate 'vcpu_info' for boot cpu at arbitrary address.
> 
> However, when Xen HVM guest panic on vcpu >= 32, since
> xen_vcpu_info_reset(0) would set per_cpu(xen_vcpu, cpu) = NULL when
> vcpu >= 32, xen_clocksource_read() on vcpu >= 32 would panic.
> 
> This patch calls xen_hvm_init_time_ops() again later in
> xen_hvm_smp_prepare_boot_cpu() after the 'vcpu_info' for boot vcpu is
> registered when the boot vcpu is >= 32.
> 
> This issue can be reproduced on purpose via below command at the guest
> side when kdump/kexec is enabled:
> 
> "taskset -c 33 echo c > /proc/sysrq-trigger"

Please revoke this CVE. The bug is happening only AFTER the guest has
crashed already.


Juergen

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