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.
---
xen/arch/x86/setup.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index d041cbd5f6f1..19ee857abfb8 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -908,6 +908,13 @@ static void __init noreturn reinit_bsp_stack(void)
if ( cpu_has_xen_shstk )
{
+ /*
+ * Immediately after enabling CET, SSP is 0 and most interrupts and
+ * exceptions are fatal. Like the SYSCALL/SYSENTER gaps, IST vectors
+ * (including NMI and #MC) are safe owing to IST switching the shstk.
+ */
+ local_irq_disable();
+
wrmsrl(MSR_S_CET, xen_msr_s_cet_value());
/*
@@ -932,6 +939,8 @@ static void __init noreturn reinit_bsp_stack(void)
}
else
asm volatile ( "setssbsy" ::: "memory" );
+
+ local_irq_enable();
}
reset_stack_and_jump(init_done);
base-commit: f4af571dd70bea97d8de82d7aa39c62c530db897
