Re: [PATCH] x86/ibt: Implement FineIBT

From: Peter Zijlstra
Date: Tue Oct 18 2022 - 10:47:58 EST


On Tue, Oct 18, 2022 at 03:35:50PM +0200, Peter Zijlstra wrote:
> +asm( ".pushsection .rodata \n"
> + "fineibt_caller_start: \n"
> + " movl $0x12345678, %r10d \n"
> + " sub $16, %r11 \n"
> + ASM_NOP4
> + "fineibt_caller_end: \n"
> + ".popsection \n"
> +);

Note: this hard relies on the indirection using %r11 and %r11
being clobbered by the call-abi. That is, there is no expectation on the
value of %r11 after this.

If GCC were to grow kCFI support this needs additional changes; one
option would be to use the 4 byte nop to rewrite it into something like:

movl $0x12345678, %r10d
movl %[reg], %r11
sub $16, %r11

> +static int cfi_rewrite_callers(s32 *start, s32 *end)
> +{
> + s32 *s;
> +
> + for (s = start; s < end; s++) {
> + void *addr = (void *)s + *s;
> + u32 hash;
> +
> + addr -= fineibt_caller_size;
> + hash = decode_caller_hash(addr);
> + if (hash) {
> + text_poke_early(addr, fineibt_caller_start, fineibt_caller_size);
> + WARN_ON(*(u32 *)(addr + fineibt_caller_hash) != 0x12345678);
> + text_poke_early(addr + fineibt_caller_hash, &hash, 4);
> + }
> + /* rely on apply_retpolines() */
> + }
> +
> + return 0;
> +}