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Xin Li (Intel) acc2b118c5 x86/fred: Remove ENDBR64 from FRED entry points 
commit 3da01ffe1a upstream.

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: 14619d912b ("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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2025-10-19 16:37:36 +02:00
..
syscalls fs: introduce file_getattr and file_setattr syscalls 2025-07-02 17:05:17 +02:00
vdso stackleak: Split KSTACK_ERASE_CFLAGS from GCC_PLUGINS_CFLAGS 2025-07-21 21:40:57 -07:00
vsyscall x86/mm/64: Make 5-level paging support unconditional 2025-05-17 10:38:16 +02:00
calling.h stackleak: Rename STACKLEAK to KSTACK_ERASE 2025-07-21 21:35:01 -07:00
entry.S x86/bugs: Rename MDS machinery to something more generic 2025-06-16 18:45:18 +02:00
entry_32.S x86/percpu: Move top_of_stack to percpu hot section 2025-03-04 20:30:33 +01:00
entry_64.S x86/its: Align RETs in BHB clear sequence to avoid thunking 2025-05-09 13:22:05 -07:00
entry_64_compat.S x86/percpu: Move top_of_stack to percpu hot section 2025-03-04 20:30:33 +01:00
entry_64_fred.S x86/fred: Remove ENDBR64 from FRED entry points 2025-10-19 16:37:36 +02:00
entry_fred.c x86/irq: Install posted MSI notification handler 2024-04-30 00:54:42 +02:00
Makefile x86/syscall: Move sys_ni_syscall() 2025-03-19 11:19:17 +01:00
syscall_32.c x86/syscall/32: Add comment to conditional 2025-03-19 11:19:20 +01:00
syscall_64.c x86/syscall: Remove stray semicolons 2025-03-19 11:19:18 +01:00
thunk.S x86/entry: Merge thunk_64.S and thunk_32.S into thunk.S 2024-04-09 09:57:56 +02:00