[PATCH] x86/static-call: Remove early_boot_irqs_disabled check to fix Xen PVH dom0

[Andrew Cooper]

__static_call_update_early() has a check for early_boot_irqs_disabled, but
is used before early_boot_irqs_disabled is set up in start_kernel().

Xen PV has always special cased early_boot_irqs_disabled, but Xen PVH does
not and falls over the BUG when booting as dom0.

It is very suspect that early_boot_irqs_disabled starts as 0, becomes 1 for
a time, then becomes 0 again, but as this needs backporting to fix a
breakage in a security fix, dropping the BUG_ON() is the far safer option.

Fixes: 0ef8047b737d ("x86/static-call: provide a way to do very early static-call updates")
Reported-by: Alex Zenla <alex@xxxxxxxxx>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219620
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Tested-by: Alex Zenla <alex@xxxxxxxxx>
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
CC: Jason Baron <jbaron@xxxxxxxxxx>
CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
CC: Ard Biesheuvel <ardb@xxxxxxxxxx>
CC: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Borislav Petkov <bp@xxxxxxxxx>
CC: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CC: x86@xxxxxxxxxx
CC: "H. Peter Anvin" <hpa@xxxxxxxxx>
CC: Juergen Gross <jgross@xxxxxxxx>
CC: linux-kernel@xxxxxxxxxxxxxxx
CC: stable@xxxxxxxxxx

It's not entirely clear why PVH domU is fine but PVH dom0 is not.  It crashes
so early there's no console or useful backtrace.
---
 arch/x86/kernel/static_call.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
index 9eed0c144dad..9e51242ed125 100644
--- a/arch/x86/kernel/static_call.c
+++ b/arch/x86/kernel/static_call.c
@@ -175,7 +175,6 @@ EXPORT_SYMBOL_GPL(arch_static_call_transform);
 noinstr void __static_call_update_early(void *tramp, void *func)
 {
 	BUG_ON(system_state != SYSTEM_BOOTING);
-	BUG_ON(!early_boot_irqs_disabled);
 	BUG_ON(static_call_initialized);
 	__text_gen_insn(tramp, JMP32_INSN_OPCODE, tramp, func, JMP32_INSN_SIZE);
 	sync_core();

base-commit: e84a3bf7f4aa669c05e3884497774148ac111468
-- 
2.39.5