Re: [PATCH v2] x86/boot: Disable interrupts when establishing SSP

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.05.2026 10:43, Andrew Cooper wrote:
> Gitlab CI reported a crash on boot on Alder Lake hardware.  The bug is years
> old, making it an incredibly rare occurance:
> 
>   (XEN) *** DOUBLE FAULT ***
>   (XEN) ----[ Xen-4.22-unstable  x86_64  debug=y ubsan=y  Not tainted ]----
>   (XEN) CPU:    0
>   (XEN) RIP:    e008:[<ffff82d04077bbc4>] 
> arch/x86/setup.c#reinit_bsp_stack+0xfa/0x160
>   (XEN) RFLAGS: 0000000000010202   CONTEXT: hypervisor
>   (XEN) rax: 0000000000000007   rbx: ffff83049a4b0000   rcx: 00000000000006a2
>   (XEN) rdx: 0000000000000000   rsi: 0000000000000000   rdi: 0000000000000000
>   (XEN) rbp: ffff83049a4b7f00   rsp: ffff83049a4b7ef8   r8:  ffff830497e47000
>   (XEN) r9:  00000000ffffffff   r10: 00000000900c2121   r11: 000000009a392956
>   (XEN) r12: ffff830497e47000   r13: ffff830497e49f40   r14: 0000000000000000
>   (XEN) r15: ffff82d0407dad10   cr0: 0000000080050033   cr4: 0000000000f526e0
>   (XEN) cr3: 0000000043c16000   cr2: fffffffffffffffc
>   (XEN) fsb: 0000000000000000   gsb: 0000000000000000   gss: 0000000000000000
>   (XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: 0000   cs: e008
>   (XEN) Xen code around <ffff82d04077bbc4> 
> (arch/x86/setup.c#reinit_bsp_stack+0xfa/0x160):
>   (XEN)  00 b9 a2 06 00 00 0f 30 <80> 3d 71 26 f1 ff 00 74 3e 48 8d 93 f8 5f 
> 00 00
>   (XEN) Valid stack range: ffff83049a4b6000-ffff83049a4b8000, 
> sp=ffff83049a4b7ef8, tss.rsp0=ffff83049a4b7fb0
>   (XEN) No stack overflow detected. Skipping stack trace.
>   (XEN)
>   (XEN) ****************************************
>   (XEN) Panic on CPU 0:
>   (XEN) DOUBLE FAULT -- system shutdown
>   (XEN) ****************************************
> 
> This is on the instruction boundary after enabling CET (writing MSR_S_CET) and
> prior to establishing SSP.  Despite identifying this as a critical window
> where any fault was deadly (the CPU tries to push a shadow stack frame at 0,
> hence the CR2 value wrapping around to the top of the address space), I
> clearly forgot that this meant interrupts too, which are enabled.
> 
> Disable interrupts during the critical period.
> 
> Fixes: b60ab42db2f0 ("x86/shstk: Activate Supervisor Shadow Stacks")
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <jbeulich@xxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Teddy Astie <teddy.astie@xxxxxxxxxx>
> 
> v2:
>  * Only disable regular interrupts.  NMIs are fine.

Much neater a fix as a result:
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan