Re: [PATCH] x86/SMP: guard socket_cpumask[] access in cpu_smpboot_free()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Jun 30, 2025 at 01:42:31PM +0200, Jan Beulich wrote:
> Before CPU identification has run (and it may not have run at all e.g.
> when AP bringup failed altogether), cpu_data[].phys_proc_id (which is
> what cpu_to_socket() resolves to) can't really be used. The use of
> cpu_to_socket()'s result as an array index cpu_smpboot_free() therefore
> needs guarding, as the function will also be invoked upon AP bringup
> failure, in which case CPU identification may not have run.
> 
> Without "x86/CPU: re-work populating of cpu_data[]" [1] the issue is
> less pronounced: The field starts out as zero, then has the BSP value
> (likely again zero) copied into it, and it is properly invalidated only
> in cpu_smpboot_free(). Still it is clearly wrong to use the BSP's socket
> number here.
> 
> Making the guard work with and without the above patch applied turns out
> interesting: Prior to that patch, the sole invalidation done is that in
> cpu_smpboot_free(). Upon a later bringup attempt, the fields invalidated
> are overwritten by the BSP values again, though. Hence compare APIC IDs,
> as they cannot validly be the same once CPU identification has run.
> 
> [1] https://lists.xen.org/archives/html/xen-devel/2024-02/msg00727.html
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks, Roger.