[RFC PATCH 04/11] x86/module: Support FineIBT in modules

joao
2022-04-20T00:43:38Z

From: Joao Moreira <joao@overdrivepizza.com>

Identify direct branch relocations targeting FineIBT hash check
sequence and fix the offset to bypass it.

Invoke objtool with fineibt flag for modules to fix in-module
direct branches too.

Signed-off-by: Joao Moreira <joao@overdrivepizza.com>
Tinkered-from-patches-by: Peter Zijlstra <peterz@infradead.org>
---
arch/x86/kernel/module.c | 45 +++++++++++++++++++++++++++++++++++++---
scripts/Makefile.build | 1 +
2 files changed, 43 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
index b98ffcf4d250..4afe71ae3e56 100644
--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -128,6 +128,41 @@ int apply_relocate(Elf32_Shdr *sechdrs,
return 0;
}
#else /*X86_64*/
+
+// shamelessly reshaped from PeterZ's IBT patches v2
+static inline void fineibt_branch_fix(void *loc, u64 *val)
+{
+#ifdef CONFIG_X86_KERNEL_FINEIBT
+ const void *addr = (void *)(4 + *val);
+ union text_poke_insn text;
+ u32 insn;
+
+ if (get_kernel_nofault(insn, addr) || !is_endbr(insn))
+ return;
+
+ if (get_kernel_nofault(text, addr+4) || text.opcode != SUB_INSN_OPCODE)
+ return;
+
+ if (get_kernel_nofault(text, addr+11) || text.opcode != JE_INSN_OPCODE)
+ return;
+
+ if (get_kernel_nofault(text, addr+13) ||
+ text.opcode != CALL_INSN_OPCODE)
+ return;
+
+ /* validate jmp.d32/call @ loc */
+ if (WARN_ONCE(get_kernel_nofault(text, loc-1) ||
+ (text.opcode != CALL_INSN_OPCODE &&
+ text.opcode != JMP32_INSN_OPCODE),
+ "Unexpected code at: %pS\n", loc))
+ return;
+
+ DEBUGP("FineIBT fixed direct branch: %pS\n", addr);
+
+ *val += 18;
+#endif
+}
+
static int __apply_relocate_add(Elf64_Shdr *sechdrs,
const char *strtab,
unsigned int symindex,
@@ -139,6 +174,7 @@ static int __apply_relocate_add(Elf64_Shdr *sechdrs,
Elf64_Rela *rel = (void *)sechdrs[relsec].sh_addr;
Elf64_Sym *sym;
void *loc;
+ int type;
u64 val;

DEBUGP("Applying relocate section %u to %u\n",
@@ -153,13 +189,14 @@ static int __apply_relocate_add(Elf64_Shdr *sechdrs,
sym = (Elf64_Sym *)sechdrs[symindex].sh_addr
+ ELF64_R_SYM(rel[i].r_info);

+ type = ELF64_R_TYPE(rel[i].r_info);
+
DEBUGP("type %d st_value %Lx r_addend %Lx loc %Lx\n",
- (int)ELF64_R_TYPE(rel[i].r_info),
- sym->st_value, rel[i].r_addend, (u64)loc);
+ type, sym->st_value, rel[i].r_addend, (u64)loc);

val = sym->st_value + rel[i].r_addend;

- switch (ELF64_R_TYPE(rel[i].r_info)) {
+ switch (type) {
case R_X86_64_NONE:
break;
case R_X86_64_64:
@@ -185,6 +222,8 @@ static int __apply_relocate_add(Elf64_Shdr *sechdrs,
case R_X86_64_PLT32:
if (*(u32 *)loc != 0)
goto invalid_relocation;
+ if (type == R_X86_64_PLT32)
+ fineibt_branch_fix(loc, &val);
val -= (u64)loc;
write(loc, &val, 4);
#if 0
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 9717e6f6fb31..d8862673b416 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -230,6 +230,7 @@ objtool_args = \
$(if $(CONFIG_UNWINDER_ORC),orc generate,check) \
$(if $(part-of-module), --module) \
$(if $(CONFIG_X86_KERNEL_IBT), --lto --ibt) \
+ $(if $(CONFIG_X86_KERNEL_FINEIBT), --fineibt) \
$(if $(CONFIG_FRAME_POINTER),, --no-fp) \
$(if $(CONFIG_GCOV_KERNEL)$(CONFIG_LTO_CLANG), --no-unreachable)\
$(if $(CONFIG_RETPOLINE), --retpoline) \
--
2.35.1