The FRED specification has been changed in v9.0 to state 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 research shows that placing ENDBR at entry points has negative value thus led to this change.
Fixes: 14619d912b65 ("x86/fred: FRED entry/exit and dispatch code") Link: https://lore.kernel.org/linux-hardening/Z60NwR4w%2F28Z7XUa@ubun/ Reviewed-by: H. Peter Anvin (Intel) hpa@zytor.com Reviewed-by: Andrew Cooper andrew.cooper3@citrix.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 Cc: stable@vger.kernel.org # v6.9+ ---
Change in v3: *) Revise the FRED spec change description to clearly indicate that it deviates from previous versions and is based on new research showing that placing ENDBR at entry points has negative value (Andrew Cooper).
Change in v2: *) CC stable and add a fixes tag (PeterZ). --- 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
On 7/15/2025 11:33 PM, Xin Li (Intel) wrote:
The FRED specification has been changed in v9.0 to state 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 research shows that placing ENDBR at entry points has negative value thus led to this change.
Fixes: 14619d912b65 ("x86/fred: FRED entry/exit and dispatch code") Link: https://lore.kernel.org/linux-hardening/Z60NwR4w%2F28Z7XUa@ubun/ Reviewed-by: H. Peter Anvin (Intel) hpa@zytor.com Reviewed-by: Andrew Cooper andrew.cooper3@citrix.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 Cc: stable@vger.kernel.org # v6.9+
Change in v3: *) Revise the FRED spec change description to clearly indicate that it deviates from previous versions and is based on new research showing that placing ENDBR at entry points has negative value (Andrew Cooper).
Change in v2:
*) CC stable and add a fixes tag (PeterZ).
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
Ping :)
On 7/15/25 23:33, Xin Li (Intel) wrote:
The FRED specification has been changed in v9.0 to state 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.
So, the spec is being updated or has been updated to reflect the new architecture, right?
Remind me, are there no FRED systems out in the wild today, so we don't have to worry about breaking anything?
On 2025-08-13 14:07, Dave Hansen wrote:
On 7/15/25 23:33, Xin Li (Intel) wrote:
The FRED specification has been changed in v9.0 to state 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.
So, the spec is being updated or has been updated to reflect the new architecture, right?
Remind me, are there no FRED systems out in the wild today, so we don't have to worry about breaking anything?
Correct. The FRED v9 spec contains this change.
All production hardware and late pre-production hardware will have this.
-hpa
linux-stable-mirror@lists.linaro.org
-
Dave Hansen -
H. Peter Anvin -
Xin Li -
Xin Li (Intel)