linux-hardening - Re: [PATCH v5 01/15] objtool: Add CONFIG_CFI

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Date:   Thu, 14 Oct 2021 12:22:26 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     x86@...nel.org, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v5 01/15] objtool: Add CONFIG_CFI_CLANG support

On Wed, Oct 13, 2021 at 11:16:44AM -0700, Sami Tolvanen wrote:
> diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
> index 1f2ae708b223..5fe31523e51f 100644
> --- a/tools/objtool/arch/x86/decode.c
> +++ b/tools/objtool/arch/x86/decode.c
> @@ -63,6 +63,23 @@ bool arch_callee_saved_reg(unsigned char reg)
>  	}
>  }
>  
> +unsigned long arch_cfi_section_reloc_offset(struct reloc *reloc)
> +{
> +	if (!reloc->addend)
> +		return 0;
> +
> +	if (reloc->type == R_X86_64_PC32 || reloc->type == R_X86_64_PLT32)
> +		return reloc->addend + 4;
> +
> +	return reloc->addend;
> +}
> +
> +unsigned long arch_cfi_jump_reloc_offset(unsigned long offset)
> +{
> +	/* offset to the relocation in a jmp instruction */
> +	return offset + 1;
> +}
> +
>  unsigned long arch_dest_reloc_offset(int addend)
>  {
>  	return addend + 4;
> diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
> index b18f0055b50b..cd09c93c34fb 100644
> --- a/tools/objtool/elf.c
> +++ b/tools/objtool/elf.c

> @@ -575,6 +580,49 @@ static int read_rela_reloc(struct section *sec, int i, struct reloc *reloc, unsi
>  	return 0;
>  }
>  
> +/*
> + * CONFIG_CFI_CLANG replaces function relocations to refer to an intermediate
> + * jump table. Undo the conversion so objtool can make sense of things.
> + */
> +static int fix_cfi_relocs(const struct elf *elf)
> +{
> +	struct section *sec;
> +	struct reloc *reloc;
> +
> +	list_for_each_entry(sec, &elf->sections, list) {
> +		list_for_each_entry(reloc, &sec->reloc_list, list) {
> +			struct reloc *cfi_reloc;
> +			unsigned long offset;
> +
> +			if (!reloc->sym->sec->cfi_jt)
> +				continue;
> +
> +			if (reloc->sym->type == STT_SECTION)
> +				offset = arch_cfi_section_reloc_offset(reloc);
> +			else
> +				offset = reloc->sym->offset;
> +
> +			/*
> +			 * The jump table immediately jumps to the actual function,
> +			 * so look up the relocation there.
> +			 */
> +			offset = arch_cfi_jump_reloc_offset(offset);
> +			cfi_reloc = find_reloc_by_dest(elf, reloc->sym->sec, offset);
> +
> +			if (!cfi_reloc || !cfi_reloc->sym) {
> +				WARN("can't find a CFI jump table relocation at %s+0x%lx",
> +					reloc->sym->sec->name, offset);
> +				return -1;
> +			}
> +
> +			reloc->sym = cfi_reloc->sym;
> +			reloc->addend = 0;
> +		}
> +	}
> +
> +	return 0;
> +}

If that section ever gets modified and we end up running
elf_rebuild_reloc_section() on it, we're in trouble, right?

Do we want a fatal error in elf_rebuild_reloc_section() when ->cfi_jt is
set?