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

[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@edera.dev>
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219620
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Tested-by: Alex Zenla <alex@edera.dev>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Peter Zijlstra <peterz@infradead.org>
CC: Josh Poimboeuf <jpoimboe@kernel.org>
CC: Jason Baron <jbaron@akamai.com>
CC: Steven Rostedt <rostedt@goodmis.org>
CC: Ard Biesheuvel <ardb@kernel.org>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Ingo Molnar <mingo@redhat.com>
CC: Borislav Petkov <bp@alien8.de>
CC: Dave Hansen <dave.hansen@linux.intel.com>
CC: x86@kernel.org
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Juergen Gross <jgross@suse.com>
CC: linux-kernel@vger.kernel.org
CC: stable@kernel.org

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

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

[Peter Zijlstra]

On Sat, Dec 21, 2024 at 09:10:46PM +0000, Andrew Cooper wrote:
> __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.

Agreed.

> Fixes: 0ef8047b737d ("x86/static-call: provide a way to do very early static-call updates")
> Reported-by: Alex Zenla <alex@edera.dev>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219620
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Tested-by: Alex Zenla <alex@edera.dev>
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

> ---
>  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
> 

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

[Jürgen Groß]

[-- Attachment #1.1.1: Type: text/plain, Size: 1903 bytes --]

On 21.12.24 22:10, Andrew Cooper wrote:
> __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@edera.dev>
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219620
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Tested-by: Alex Zenla <alex@edera.dev>
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
> CC: Peter Zijlstra <peterz@infradead.org>
> CC: Josh Poimboeuf <jpoimboe@kernel.org>
> CC: Jason Baron <jbaron@akamai.com>
> CC: Steven Rostedt <rostedt@goodmis.org>
> CC: Ard Biesheuvel <ardb@kernel.org>
> CC: Thomas Gleixner <tglx@linutronix.de>
> CC: Ingo Molnar <mingo@redhat.com>
> CC: Borislav Petkov <bp@alien8.de>
> CC: Dave Hansen <dave.hansen@linux.intel.com>
> CC: x86@kernel.org
> CC: "H. Peter Anvin" <hpa@zytor.com>
> CC: Juergen Gross <jgross@suse.com>
> CC: linux-kernel@vger.kernel.org
> CC: stable@kernel.org

Reviewed-by: Juergen Gross <jgross@suse.com>

> 
> 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.

I suspect that the Xen hypervisor doesn't supply a memory map for PVH dom0 via
the start_info data, while the Xen tools do so for PVH guests. This requires
dom0 to issue a hypercall very early in order to obtain the memory map.


Juergen

[-- Attachment #1.1.2: OpenPGP public key --]
[-- Type: application/pgp-keys, Size: 3743 bytes --]

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 495 bytes --]

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

[tip-bot2 for Andrew Cooper]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     5cc2db37124bb33914996d6fdbb2ddb3811f2945
Gitweb:        https://git.kernel.org/tip/5cc2db37124bb33914996d6fdbb2ddb3811f2945
Author:        Andrew Cooper <andrew.cooper3@citrix.com>
AuthorDate:    Sat, 21 Dec 2024 21:10:46 
Committer:     Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Thu, 02 Jan 2025 17:11:29 +01:00

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

__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")
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219620
Reported-by: Alex Zenla <alex@edera.dev>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Alex Zenla <alex@edera.dev>
Link: https://lore.kernel.org/r/20241221211046.6475-1-andrew.cooper3@citrix.com
---
 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 9eed0c1..9e51242 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();