linux-kernel - Re: [PATCH] objtool: Detect non-relocated text references

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Message-ID: <20241004082117.GH18071@noisy.programming.kicks-ass.net>
Date: Fri, 4 Oct 2024 10:21:17 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH] objtool: Detect non-relocated text references

On Thu, Oct 03, 2024 at 05:31:10PM -0700, Josh Poimboeuf wrote:
> When kernel IBT is enabled, objtool detects all text references in order
> to determine which functions can be indirectly branched to.
> 
> In text, such references look like one of the following:
> 
>    mov    $0x0,%rax        R_X86_64_32S     .init.text+0x7e0a0
>    lea    0x0(%rip),%rax   R_X86_64_PC32    autoremove_wake_function-0x4
> 
> Either way the function pointer is denoted by a relocation, so objtool
> just reads that.
> 
> However there are some "lea xxx(%rip)" cases which don't use relocations
> because they're referencing code in the same translation unit.  Objtool
> doesn't have visibility to those.
> 
> The only currently known instances of that are a few hand-coded asm text
> references which don't actually need ENDBR.  So it's not actually a
> problem at the moment.
> 
> However if we enable -fpie, the compiler would start generating them and
> there would definitely be bugs in the IBT sealing.
> 
> Detect non-relocated text references and handle them appropriately.
> 
> [ Note: I removed the manual static_call_tramp check -- that should
>   already be handled by the noendbr check. ]
> 
> Reported-by: Ard Biesheuvel <ardb@...nel.org>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> ---

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>