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Re: [PATCH v2] x86/boot: Disable interrupts when establishing SSP
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 5 May 2026 10:51:51 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 05 May 2026 09:52:06 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 05/05/2026 10:00 am, Jan Beulich wrote:
> 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>
Thanks, and yes; I'm rather embarrassed at how long it took to realise
it was exactly the same as the SYSCALL gap, and therefore the same
safety reasoning applied.
~Andrew
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