[PATCH v1 1/1] x86/fred: Remove ENDBR64 from FRED entry points

[PATCH v1 1/1] x86/fred: Remove ENDBR64 from FRED entry points

[Xin Li (Intel)]

The FRED specification v9.0 states that there is no need for FRED
event handlers to begin with ENDBR64, because in the presence of
supervisor indirect branch tracking, FRED event delivery does not
enter the WAIT_FOR_ENDBRANCH state.

As a result, remove ENDBR64 from FRED entry points.

Then add ANNOTATE_NOENDBR to indicate that FRED entry points will
never be used for indirect calls to suppress an objtool warning.

This change implies that any indirect CALL/JMP to FRED entry points
causes #CP in the presence of supervisor indirect branch tracking.

Credit goes to Jennifer Miller <jmill@asu.edu> and other contributors
from Arizona State University whose work led to this change.

Link: https://lore.kernel.org/linux-hardening/Z60NwR4w%2F28Z7XUa@ubun/
Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Signed-off-by: Xin Li (Intel) <xin@zytor.com>
Cc: Jennifer Miller <jmill@asu.edu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: H. Peter Anvin <hpa@zytor.com>
---
 arch/x86/entry/entry_64_fred.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S
index 29c5c32c16c3..907bd233c6c1 100644
--- a/arch/x86/entry/entry_64_fred.S
+++ b/arch/x86/entry/entry_64_fred.S
@@ -16,7 +16,7 @@
 
 .macro FRED_ENTER
 	UNWIND_HINT_END_OF_STACK
-	ENDBR
+	ANNOTATE_NOENDBR
 	PUSH_AND_CLEAR_REGS
 	movq	%rsp, %rdi	/* %rdi -> pt_regs */
 .endm
-- 
2.50.1

Re: [PATCH v1 1/1] x86/fred: Remove ENDBR64 from FRED entry points

[Xin Li]

On 7/14/2025 11:44 PM, Xin Li (Intel) wrote:
> The FRED specification v9.0 states that there is no need for FRED
> event handlers to begin with ENDBR64, because in the presence of
> supervisor indirect branch tracking, FRED event delivery does not
> enter the WAIT_FOR_ENDBRANCH state.
> 
> As a result, remove ENDBR64 from FRED entry points.
> 
> Then add ANNOTATE_NOENDBR to indicate that FRED entry points will
> never be used for indirect calls to suppress an objtool warning.
> 
> This change implies that any indirect CALL/JMP to FRED entry points
> causes #CP in the presence of supervisor indirect branch tracking.
> 
> Credit goes to Jennifer Miller <jmill@asu.edu> and other contributors
> from Arizona State University whose work led to this change.
> 
> Link: https://lore.kernel.org/linux-hardening/Z60NwR4w%2F28Z7XUa@ubun/
> Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
> Signed-off-by: Xin Li (Intel) <xin@zytor.com>
> Cc: Jennifer Miller <jmill@asu.edu>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: H. Peter Anvin <hpa@zytor.com>

Sorry, forgot to put CC stable.

> ---
>   arch/x86/entry/entry_64_fred.S | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S
> index 29c5c32c16c3..907bd233c6c1 100644
> --- a/arch/x86/entry/entry_64_fred.S
> +++ b/arch/x86/entry/entry_64_fred.S
> @@ -16,7 +16,7 @@
>   
>   .macro FRED_ENTER
>   	UNWIND_HINT_END_OF_STACK
> -	ENDBR
> +	ANNOTATE_NOENDBR
>   	PUSH_AND_CLEAR_REGS
>   	movq	%rsp, %rdi	/* %rdi -> pt_regs */
>   .endm

Re: [PATCH v1 1/1] x86/fred: Remove ENDBR64 from FRED entry points

[Peter Zijlstra]

On Mon, Jul 14, 2025 at 11:50:57PM -0700, Xin Li wrote:
> On 7/14/2025 11:44 PM, Xin Li (Intel) wrote:
> > The FRED specification v9.0 states that there is no need for FRED
> > event handlers to begin with ENDBR64, because in the presence of
> > supervisor indirect branch tracking, FRED event delivery does not
> > enter the WAIT_FOR_ENDBRANCH state.
> > 
> > As a result, remove ENDBR64 from FRED entry points.
> > 
> > Then add ANNOTATE_NOENDBR to indicate that FRED entry points will
> > never be used for indirect calls to suppress an objtool warning.
> > 
> > This change implies that any indirect CALL/JMP to FRED entry points
> > causes #CP in the presence of supervisor indirect branch tracking.
> > 
> > Credit goes to Jennifer Miller <jmill@asu.edu> and other contributors
> > from Arizona State University whose work led to this change.
> > 
> > Link: https://lore.kernel.org/linux-hardening/Z60NwR4w%2F28Z7XUa@ubun/
> > Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
> > Signed-off-by: Xin Li (Intel) <xin@zytor.com>
> > Cc: Jennifer Miller <jmill@asu.edu>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> > Cc: H. Peter Anvin <hpa@zytor.com>
> 
> Sorry, forgot to put CC stable.

Fixes: 14619d912b65 ("x86/fred: FRED entry/exit and dispatch code")

Right?

> 
> > ---
> >   arch/x86/entry/entry_64_fred.S | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/entry/entry_64_fred.S b/arch/x86/entry/entry_64_fred.S
> > index 29c5c32c16c3..907bd233c6c1 100644
> > --- a/arch/x86/entry/entry_64_fred.S
> > +++ b/arch/x86/entry/entry_64_fred.S
> > @@ -16,7 +16,7 @@
> >   .macro FRED_ENTER
> >   	UNWIND_HINT_END_OF_STACK
> > -	ENDBR
> > +	ANNOTATE_NOENDBR
> >   	PUSH_AND_CLEAR_REGS
> >   	movq	%rsp, %rdi	/* %rdi -> pt_regs */
> >   .endm
> 

Re: [PATCH v1 1/1] x86/fred: Remove ENDBR64 from FRED entry points

[Xin Li]

On 7/15/2025 1:40 AM, Peter Zijlstra wrote:
> On Mon, Jul 14, 2025 at 11:50:57PM -0700, Xin Li wrote:
>> On 7/14/2025 11:44 PM, Xin Li (Intel) wrote:
>>> The FRED specification v9.0 states that there is no need for FRED
>>> event handlers to begin with ENDBR64, because in the presence of
>>> supervisor indirect branch tracking, FRED event delivery does not
>>> enter the WAIT_FOR_ENDBRANCH state.
>>>
>>> As a result, remove ENDBR64 from FRED entry points.
>>>
>>> Then add ANNOTATE_NOENDBR to indicate that FRED entry points will
>>> never be used for indirect calls to suppress an objtool warning.
>>>
>>> This change implies that any indirect CALL/JMP to FRED entry points
>>> causes #CP in the presence of supervisor indirect branch tracking.
>>>
>>> Credit goes to Jennifer Miller<jmill@asu.edu> and other contributors
>>> from Arizona State University whose work led to this change.
>>>
>>> Link:https://lore.kernel.org/linux-hardening/Z60NwR4w%2F28Z7XUa@ubun/
>>> Reviewed-by: H. Peter Anvin (Intel)<hpa@zytor.com>
>>> Signed-off-by: Xin Li (Intel)<xin@zytor.com>
>>> Cc: Jennifer Miller<jmill@asu.edu>
>>> Cc: Peter Zijlstra<peterz@infradead.org>
>>> Cc: Andrew Cooper<andrew.cooper3@citrix.com>
>>> Cc: H. Peter Anvin<hpa@zytor.com>
>> Sorry, forgot to put CC stable.
> Fixes: 14619d912b65 ("x86/fred: FRED entry/exit and dispatch code")
> 
> Right?

You bet!

Will send v2 to address it soon.

Re: [PATCH v1 1/1] x86/fred: Remove ENDBR64 from FRED entry points

[Andrew Cooper]

On 15/07/2025 7:44 am, Xin Li (Intel) wrote:
> The FRED specification v9.0 states that there is no need for FRED
> event handlers to begin with ENDBR64, because in the presence of
> supervisor indirect branch tracking, FRED event delivery does not
> enter the WAIT_FOR_ENDBRANCH state.

I would suggest phrasing this as "The FRED specification has been
changed in v9 to..."

Simply "v9 states" can be read as "we mistook what v8 said and did it
wrong".

After all, the change here is specifically as a result of new research
showing ENDBR on the entrypoints to be of negative value.

>
> As a result, remove ENDBR64 from FRED entry points.
>
> Then add ANNOTATE_NOENDBR to indicate that FRED entry points will
> never be used for indirect calls to suppress an objtool warning.
>
> This change implies that any indirect CALL/JMP to FRED entry points
> causes #CP in the presence of supervisor indirect branch tracking.
>
> Credit goes to Jennifer Miller <jmill@asu.edu> and other contributors
> from Arizona State University whose work led to this change.
>
> Link: https://lore.kernel.org/linux-hardening/Z60NwR4w%2F28Z7XUa@ubun/
> Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
> Signed-off-by: Xin Li (Intel) <xin@zytor.com>
> Cc: Jennifer Miller <jmill@asu.edu>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: H. Peter Anvin <hpa@zytor.com>

Preferably with an adjusted commit message, Reviewed-by Andrew Cooper
<andrew.cooper3@citrix.com>

Any idea when an updated SIMICS will be available?

~Andrew

Re: [PATCH v1 1/1] x86/fred: Remove ENDBR64 from FRED entry points

[Xin Li]

On 7/15/2025 8:49 AM, Andrew Cooper wrote:
> On 15/07/2025 7:44 am, Xin Li (Intel) wrote:
>> The FRED specification v9.0 states that there is no need for FRED
>> event handlers to begin with ENDBR64, because in the presence of
>> supervisor indirect branch tracking, FRED event delivery does not
>> enter the WAIT_FOR_ENDBRANCH state.
> 
> I would suggest phrasing this as "The FRED specification has been
> changed in v9 to..."
> 
> Simply "v9 states" can be read as "we mistook what v8 said and did it
> wrong".
> 
> After all, the change here is specifically as a result of new research
> showing ENDBR on the entrypoints to be of negative value.

Sure, that makes it more like a story ;)

> 
>>
>> As a result, remove ENDBR64 from FRED entry points.
>>
>> Then add ANNOTATE_NOENDBR to indicate that FRED entry points will
>> never be used for indirect calls to suppress an objtool warning.
>>
>> This change implies that any indirect CALL/JMP to FRED entry points
>> causes #CP in the presence of supervisor indirect branch tracking.
>>
>> Credit goes to Jennifer Miller <jmill@asu.edu> and other contributors
>> from Arizona State University whose work led to this change.
>>
>> Link: https://lore.kernel.org/linux-hardening/Z60NwR4w%2F28Z7XUa@ubun/
>> Reviewed-by: H. Peter Anvin (Intel) <hpa@zytor.com>
>> Signed-off-by: Xin Li (Intel) <xin@zytor.com>
>> Cc: Jennifer Miller <jmill@asu.edu>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
>> Cc: H. Peter Anvin <hpa@zytor.com>
> 
> Preferably with an adjusted commit message, Reviewed-by Andrew Cooper
> <andrew.cooper3@citrix.com>

Thanks, will add in v3 later today (Sent out v2 just now).

> 
> Any idea when an updated SIMICS will be available?

The FRED testing is more focused on hardware now :).

Anyway, let me check here.