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Message-ID: <Y068kIjJr/5kJa23@hirez.programming.kicks-ass.net>
Date: Tue, 18 Oct 2022 16:47:44 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: x86@...nel.org
Cc: Kees Cook <keescook@...omium.org>,
Sami Tolvanen <samitolvanen@...gle.com>,
Joao Moreira <joao@...rdrivepizza.com>,
linux-kernel@...r.kernel.org, Mark Rutland <mark.rutland@....com>,
Josh Poimboeuf <jpoimboe@...hat.com>,
"H.J. Lu" <hjl.tools@...il.com>
Subject: Re: [PATCH] x86/ibt: Implement FineIBT
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;
> +}
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