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

Commit-ID: 3da01ffe1aeaa0d427ab5235ba735226670a80d9 Gitweb: https://git.kernel.org/tip/3da01ffe1aeaa0d427ab5235ba735226670a80d9 Author: Xin Li (Intel) xin@zytor.com AuthorDate: Tue, 15 Jul 2025 23:33:20 -07:00 Committer: Dave Hansen dave.hansen@linux.intel.com CommitterDate: Wed, 13 Aug 2025 15:05:32 -07:00

x86/fred: Remove ENDBR64 from FRED entry points

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.

Note: This is obviously an incompatible change to the FRED architecture. But, it's OK because there no FRED systems out in the wild today. All production hardware and late pre-production hardware will follow the FRED v9 spec and be compatible with this approach.

[ dhansen: add note to changelog about incompatibility ]

Fixes: 14619d912b65 ("x86/fred: FRED entry/exit and dispatch code") Signed-off-by: Xin Li (Intel) xin@zytor.com Signed-off-by: Dave Hansen dave.hansen@linux.intel.com Reviewed-by: H. Peter Anvin (Intel) hpa@zytor.com Reviewed-by: Andrew Cooper andrew.cooper3@citrix.com Link: https://lore.kernel.org/linux-hardening/Z60NwR4w%2F28Z7XUa@ubun/ Cc:stable@vger.kernel.org Link: https://lore.kernel.org/all/20250716063320.1337818-1-xin%40zytor.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 29c5c32..907bd23 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