linux-kernel - Re: [PATCH 13/24] objtool: Implement base jump

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Date:   Fri, 26 Jan 2018 10:45:34 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Dave Hansen <dave.hansen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Jason Baron <jbaron@...mai.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 13/24] objtool: Implement base jump_assert support

On Tue, 2018-01-23 at 16:25 +0100, Peter Zijlstra wrote:
> Implement a jump_label assertion that asserts that the code location
> is indeed only reachable through a static_branch. Because if GCC is
> absolutely retaded it could generate code like:
> 
>         xor rax,rax
>         NOP/JMP 1f
>         mov $1, rax
> 1:
>         test rax,rax
>         jz 2f
>         
> 2:
> 
> instead of the sensible:
> 
>         NOP/JMP 1f
>         
> 1:
> 
> This implements objtool infrastructure for ensuring the code ends up
> sane, since we'll rely on that for correctness and security.
> 
> We tag the instructions after the static branch with static_jump_dest=true;
> that is the instruction after the NOP and the instruction at the
> JMP+disp site.
> 
> Then, when we read the .discard.jump_assert section, we assert that
> each entry points to an instruction that has static_jump_dest set.
> 
> With this we can assert that the code emitted for the if statement
> ends up at the static jump location and nothing untowards happened.
> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Josh Poimboeuf <jpoimboe@...hat.com>
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

Thank you for pandering to my paranoia. I suspect that misspelling the
word 'retarded' isn't going to be sufficient to stop people from
objecting to the use of that word, but other than that,

Reviewed-by: David Woodhouse <dwmw@...zon.co.uk>

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