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Date:   Wed, 13 Oct 2021 11:59:37 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     x86@kernel.org, Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org,
        llvm@lists.linux.dev
Subject: Re: [PATCH v5 01/15] objtool: Add CONFIG_CFI_CLANG support
Message-ID: <202110131159.DA32533@keescook>
References: <20211013181658.1020262-1-samitolvanen@google.com>
 <20211013181658.1020262-2-samitolvanen@google.com>
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On Wed, Oct 13, 2021 at 11:16:44AM -0700, Sami Tolvanen wrote:
> The upcoming CONFIG_CFI_CLANG support uses -fsanitize=cfi, the
> non-canonical version of which hijacks function entry by changing
> function relocation references to point to an intermediary jump table.
> 
> For example:
> 
>   Relocation section '.rela.discard.func_stack_frame_non_standard' at offset 0x37e018 contains 6 entries:
>       Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
>   0000000000000000  0002944700000002 R_X86_64_PC32          00000000000023f0 do_suspend_lowlevel + 0
>   0000000000000008  0003c11900000001 R_X86_64_64            0000000000000008 xen_cpuid$e69bc59f4fade3b6f2b579b3934137df.cfi_jt + 0
>   0000000000000010  0003980900000001 R_X86_64_64            0000000000000060 machine_real_restart.cfi_jt + 0
>   0000000000000018  0003962b00000001 R_X86_64_64            0000000000000e18 kretprobe_trampoline.cfi_jt + 0
>   0000000000000020  000028f300000001 R_X86_64_64            0000000000000000 .rodata + 12
>   0000000000000028  000349f400000001 R_X86_64_64            0000000000000018 __crash_kexec.cfi_jt + 0
> 
>   0000000000000060 <machine_real_restart.cfi_jt>:
>     60: e9 00 00 00 00          jmpq   65 <machine_real_restart.cfi_jt+0x5>
>                         61: R_X86_64_PLT32      machine_real_restart-0x4
>     65: cc                      int3
>     66: cc                      int3
>     67: cc                      int3
> 
> This breaks objtool vmlinux validation in many ways, including static
> call site detection and the STACK_FRAME_NON_STANDARD() macro.
> 
> Fix it by converting those relocations' symbol references back to their
> original non-jump-table versions.  Note this doesn't change the actual
> relocations in the object itself, it just changes objtool's view of
> them. This change is based on Josh's initial patch:
> 
> https://lore.kernel.org/r/d743f4b36e120c06506567a9f87a062ae03da47f.1611263462.git.jpoimboe@redhat.com/
> 
> Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
> Suggested-by: Josh Poimboeuf <jpoimboe@redhat.com>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

This looks really clean. Thanks!

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook