[PATCH v4 08/10] x86: BHI stubs

Peter Zijlstra
2025-02-24T12:46:00Z

arch/x86/include/asm/cfi.h | 4 +
arch/x86/lib/Makefile | 3
arch/x86/lib/bhi.S | 146 +++++++++++++++++++++++++++++++++++++++++++++
3 files changed, 152 insertions(+), 1 deletion(-)

--- a/arch/x86/include/asm/cfi.h
+++ b/arch/x86/include/asm/cfi.h
@@ -101,6 +101,10 @@ enum cfi_mode {

extern enum cfi_mode cfi_mode;

+typedef u8 bhi_thunk[32];
+extern bhi_thunk __bhi_args[];
+extern bhi_thunk __bhi_args_end[];
+
struct pt_regs;

#ifdef CONFIG_CFI_CLANG
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -66,5 +66,6 @@ endif
lib-y += clear_page_64.o copy_page_64.o
lib-y += memmove_64.o memset_64.o
lib-y += copy_user_64.o copy_user_uncached_64.o
- lib-y += cmpxchg16b_emu.o
+ lib-y += cmpxchg16b_emu.o
+ lib-y += bhi.o
endif
--- /dev/null
+++ b/arch/x86/lib/bhi.S
@@ -0,0 +1,146 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#include <linux/linkage.h>
+#include <asm/unwind_hints.h>
+#include <asm/nospec-branch.h>
+
+/*
+ * Notably, the FineIBT preamble calling these will have ZF set and r10 zero.
+ *
+ * The very last element is in fact larger than 32 bytes, but since its the
+ * last element, this does not matter,
+ *
+ * There are 2 #UD sites, located between 0,1-2,3 and 4,5-6,7 such that they
+ * can be reached using Jcc.d8, these elements (1 and 5) have sufficiently
+ * big alignment holes for this to not stagger the array.
+ */
+
+.pushsection .noinstr.text, "ax"
+
+ .align 32
+SYM_CODE_START(__bhi_args)
+
+#ifdef CONFIG_FINEIBT_BHI
+
+ .align 32
+SYM_INNER_LABEL(__bhi_args_0, SYM_L_LOCAL)
+ ANNOTATE_NOENDBR
+ UNWIND_HINT_FUNC
+ jne .Lud_1
+ ANNOTATE_UNRET_SAFE
+ ret
+ int3
+
+ .align 32
+SYM_INNER_LABEL(__bhi_args_1, SYM_L_LOCAL)
+ ANNOTATE_NOENDBR
+ UNWIND_HINT_FUNC
+ jne .Lud_1
+ cmovne %r10, %rdi
+ ANNOTATE_UNRET_SAFE
+ ret
+ int3
+
+ .align 8
+ ANNOTATE_REACHABLE
+.Lud_1: ud2
+ ANNOTATE_UNRET_SAFE
+ ret
+ int3
+
+ .align 32
+SYM_INNER_LABEL(__bhi_args_2, SYM_L_LOCAL)
+ ANNOTATE_NOENDBR
+ UNWIND_HINT_FUNC
+ jne .Lud_1
+ cmovne %r10, %rdi
+ cmovne %r10, %rsi
+ ANNOTATE_UNRET_SAFE
+ ret
+ int3
+
+ .align 32
+SYM_INNER_LABEL(__bhi_args_3, SYM_L_LOCAL)
+ ANNOTATE_NOENDBR
+ UNWIND_HINT_FUNC
+ jne .Lud_1
+ cmovne %r10, %rdi
+ cmovne %r10, %rsi
+ cmovne %r10, %rdx
+ ANNOTATE_UNRET_SAFE
+ ret
+ int3
+
+ .align 32
+SYM_INNER_LABEL(__bhi_args_4, SYM_L_LOCAL)
+ ANNOTATE_NOENDBR
+ UNWIND_HINT_FUNC
+ jne .Lud_2
+ cmovne %r10, %rdi
+ cmovne %r10, %rsi
+ cmovne %r10, %rdx
+ cmovne %r10, %rcx
+ ANNOTATE_UNRET_SAFE
+ ret
+ int3
+
+ .align 32
+SYM_INNER_LABEL(__bhi_args_5, SYM_L_LOCAL)
+ ANNOTATE_NOENDBR
+ UNWIND_HINT_FUNC
+ jne .Lud_2
+ cmovne %r10, %rdi
+ cmovne %r10, %rsi
+ cmovne %r10, %rdx
+ cmovne %r10, %rcx
+ cmovne %r10, %r8
+ ANNOTATE_UNRET_SAFE
+ ret
+ int3
+
+ .align 8
+ ANNOTATE_REACHABLE
+.Lud_2: ud2
+ ANNOTATE_UNRET_SAFE
+ ret
+ int3
+
+ .align 32
+SYM_INNER_LABEL(__bhi_args_6, SYM_L_LOCAL)
+ ANNOTATE_NOENDBR
+ UNWIND_HINT_FUNC
+ jne .Lud_2
+ cmovne %r10, %rdi
+ cmovne %r10, %rsi
+ cmovne %r10, %rdx
+ cmovne %r10, %rcx
+ cmovne %r10, %r8
+ cmovne %r10, %r9
+ ANNOTATE_UNRET_SAFE
+ ret
+ int3
+
+ .align 32
+SYM_INNER_LABEL(__bhi_args_7, SYM_L_LOCAL)
+ ANNOTATE_NOENDBR
+ UNWIND_HINT_FUNC
+ jne .Lud_2
+ cmovne %r10, %rdi
+ cmovne %r10, %rsi
+ cmovne %r10, %rdx
+ cmovne %r10, %rcx
+ cmovne %r10, %r8
+ cmovne %r10, %r9
+ cmovne %r10, %rsp
+ ANNOTATE_UNRET_SAFE
+ ret
+ int3
+
+#endif /* CONFIG_FINEIBT_BHI */
+
+ .align 32
+SYM_INNER_LABEL(__bhi_args_end, SYM_L_GLOBAL)
+ ANNOTATE_NOENDBR
+SYM_CODE_END(__bhi_args)
+
+.popsection