Re: CVE-2024-35876: x86/mce: Make sure to grab mce_sysfs_mutex in set_bank()

[Vegard Nossum]

On 23/05/2024 12:24, Nikolay Borisov wrote:
> On 19.05.24 г. 11:34 ч., Greg Kroah-Hartman wrote:
> > Description
> > ===========
> >
> > In the Linux kernel, the following vulnerability has been resolved:
> >
> > x86/mce: Make sure to grab mce_sysfs_mutex in set_bank()
> >
> > Modifying a MCA bank's MCA_CTL bits which control which error types to
> > be reported is done over
> >
> >    /sys/devices/system/machinecheck/
> >    ├── machinecheck0
> >    │   ├── bank0
> >    │   ├── bank1
> >    │   ├── bank10
> >    │   ├── bank11
> >    ...
> >
> > sysfs nodes by writing the new bit mask of events to enable.
> >
> > When the write is accepted, the kernel deletes all current timers and
> > reinits all banks.
> >
> > Doing that in parallel can lead to initializing a timer which is already
> > armed and in the timer wheel, i.e., in use already:
> >
> >    ODEBUG: init active (active state 0) object: ffff888063a28000 object
> >    type: timer_list hint: mce_timer_fn+0x0/0x240 
> > arch/x86/kernel/cpu/mce/core.c:2642
> >    WARNING: CPU: 0 PID: 8120 at lib/debugobjects.c:514
> >    debug_print_object+0x1a0/0x2a0 lib/debugobjects.c:514
> >
> > Fix that by grabbing the sysfs mutex as the rest of the MCA sysfs code
> > does.
> >
> > Reported by: Yue Sun <samsun1006219@xxxxxxxxx>
> > Reported by: xingwei lee <xrivendell7@xxxxxxxxx>
> >
> > The Linux kernel CVE team has assigned CVE-2024-35876 to this issue.
>
>
> I'd like to dispute the CVE for this issue. Those sysfs entries are 
> owned by root and can only be written by it. There are innumerable ways 
> in which root can corrupt/crash the state of the machine and I don't see 
> why this is anything special.

I haven't looked at the issue in detail but it sounds like this
potentially breaks lockdown (which is arguably a security feature) so
"requires root" to reach is not really an argument against this having a
CVE assigned.


Vegard