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x86/ibt: Optimize the FineIBT instruction sequence 

Scott notes that non-taken branches are faster. Abuse overlapping code
that traps instead of explicit UD2 instructions.

And LEA does not modify flags and will have less dependencies.

Suggested-by: Scott Constable <scott.d.constable@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Link: https://lore.kernel.org/r/20250224124200.371942555@infradead.org
This commit is contained in:
Peter Zijlstra 2025-02-24 13:37:08 +01:00 committed by Ingo Molnar
parent e33d805a10
commit 06926c6cdb
2 changed files with 42 additions and 24 deletions
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61
arch/x86/kernel/alternative.c
View File

@ -1057,9 +1057,9 @@ early_param("cfi", cfi_parse_cmdline);
* __cfi_\func: __cfi_\func:
* movl $0x12345678,%eax // 5 endbr64 // 4
* nop subl $0x12345678,%r10d // 7
* nop jz 1f // 2
* nop ud2 // 2
* nop 1: nop // 1
* nop jne __cfi_\func+6 // 2
* nop nop3 // 3
* nop
* nop
* nop
* nop
@ -1071,37 +1071,50 @@ early_param("cfi", cfi_parse_cmdline);
*
* caller: caller:
* movl $(-0x12345678),%r10d // 6 movl $0x12345678,%r10d // 6
* addl $-15(%r11),%r10d // 4 sub $16,%r11 // 4
* addl $-15(%r11),%r10d // 4 lea -0x10(%r11),%r11 // 4
* je 1f // 2 nop4 // 4
* ud2 // 2
* 1: call __x86_indirect_thunk_r11 // 5 call *%r11; nop2; // 5
* 1: cs call __x86_indirect_thunk_r11 // 6 call *%r11; nop3; // 6
*
*/
asm( ".pushsection .rodata \n"
"fineibt_preamble_start: \n"
" endbr64 \n"
" subl $0x12345678, %r10d \n"
" je fineibt_preamble_end \n"
"fineibt_preamble_ud2: \n"
" ud2 \n"
" nop \n"
"fineibt_preamble_end: \n"
/*
* <fineibt_preamble_start>:
* 0: f3 0f 1e fa endbr64
* 4: 41 81 <ea> 78 56 34 12 sub $0x12345678, %r10d
* b: 75 f9 jne 6 <fineibt_preamble_start+0x6>
* d: 0f 1f 00 nopl (%rax)
*
* Note that the JNE target is the 0xEA byte inside the SUB, this decodes as
* (bad) on x86_64 and raises #UD.
*/
asm( ".pushsection .rodata \n"
"fineibt_preamble_start: \n"
" endbr64 \n"
" subl $0x12345678, %r10d \n"
" jne fineibt_preamble_start+6 \n"
ASM_NOP3
"fineibt_preamble_end: \n"
".popsection\n"
);
extern u8 fineibt_preamble_start[];
extern u8 fineibt_preamble_ud2[];
extern u8 fineibt_preamble_end[];
#define fineibt_preamble_size (fineibt_preamble_end - fineibt_preamble_start)
#define fineibt_preamble_ud2 (fineibt_preamble_ud2 - fineibt_preamble_start)
#define fineibt_preamble_ud 6
#define fineibt_preamble_hash 7
/*
* <fineibt_caller_start>:
* 0: 41 ba 78 56 34 12 mov $0x12345678, %r10d
* 6: 4d 8d 5b f0 lea -0x10(%r11), %r11
* a: 0f 1f 40 00 nopl 0x0(%rax)
*/
asm( ".pushsection .rodata \n"
"fineibt_caller_start: \n"
" movl $0x12345678, %r10d \n"
" sub $16, %r11 \n"
" lea -0x10(%r11), %r11 \n"
ASM_NOP4
"fineibt_caller_end: \n"
".popsection \n"
@ -1432,15 +1445,15 @@ static void poison_cfi(void *addr)
}
/*
* regs->ip points to a UD2 instruction, return true and fill out target and
* type when this UD2 is from a FineIBT preamble.
* When regs->ip points to a 0xEA byte in the FineIBT preamble,
* return true and fill out target and type.
*
* We check the preamble by checking for the ENDBR instruction relative to the
* UD2 instruction.
* 0xEA instruction.
*/
bool decode_fineibt_insn(struct pt_regs *regs, unsigned long *target, u32 *type)
{
unsigned long addr = regs->ip - fineibt_preamble_ud2;
unsigned long addr = regs->ip - fineibt_preamble_ud;
u32 hash;
if (!exact_endbr((void *)addr))
@ -1451,6 +1464,12 @@ bool decode_fineibt_insn(struct pt_regs *regs, unsigned long *target, u32 *type)
__get_kernel_nofault(&hash, addr + fineibt_preamble_hash, u32, Efault);
*type = (u32)regs->r10 + hash;
/*
* Since regs->ip points to the middle of an instruction; it cannot
* continue with the normal fixup.
*/
regs->ip = *target;
return true;
Efault:

5
arch/x86/net/bpf_jit_comp.c
View File

@ -417,9 +417,8 @@ static void emit_fineibt(u8 **pprog, u32 hash)
EMIT_ENDBR();
EMIT3_off32(0x41, 0x81, 0xea, hash); /* subl $hash, %r10d */
EMIT2(0x74, 0x07); /* jz.d8 +7 */
EMIT2(0x0f, 0x0b); /* ud2 */
EMIT1(0x90); /* nop */
EMIT2(0x75, 0xf9); /* jne.d8 .-7 */
EMIT3(0x0f, 0x1f, 0x00); /* nop3 */
EMIT_ENDBR_POISON();
*pprog = prog;