linux-kernel - Re: [PATCH 1/2] x86/insn: Fix some potential undefined behavior.

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Message-ID: <20201015080131.GA894367@gmail.com>
Date:   Thu, 15 Oct 2020 10:01:31 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        linux-kernel@...r.kernel.org,
        Adrian Hunter <adrian.hunter@...el.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Numfor Mbiziwo-Tiapo <nums@...gle.com>
Subject: Re: [PATCH 1/2] x86/insn: Fix some potential undefined behavior.


* Ian Rogers <irogers@...gle.com> wrote:

> From: Numfor Mbiziwo-Tiapo <nums@...gle.com>
> 
> If insn_init is given a NULL kaddr and 0 buflen then validate_next will
> perform arithmetic on NULL, add a guard to avoid this.
> 
> Don't perform unaligned loads in __get_next and __peek_nbyte_next as
> these are forms of undefined behavior.

So, 'insn' is a kernel structure, usually allocated on the kernel stack. 
How could these fields ever be unaligned?

> 
> These problems were identified using the undefined behavior sanitizer
> (ubsan) with the tools version of the code and perf test. Part of this
> patch was previously posted here:
> https://lore.kernel.org/lkml/20190724184512.162887-4-nums@google.com/
> 
> Signed-off-by: Ian Rogers <irogers@...gle.com>
> Signed-off-by: Numfor Mbiziwo-Tiapo <nums@...gle.com>
> ---
>  arch/x86/lib/insn.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/lib/insn.c b/arch/x86/lib/insn.c
> index 404279563891..57236940de46 100644
> --- a/arch/x86/lib/insn.c
> +++ b/arch/x86/lib/insn.c
> @@ -17,13 +17,13 @@
>  
>  /* Verify next sizeof(t) bytes can be on the same instruction */
>  #define validate_next(t, insn, n)	\
> -	((insn)->next_byte + sizeof(t) + n <= (insn)->end_kaddr)
> +	((insn)->end_kaddr != 0 && (insn)->next_byte + sizeof(t) + n <= (insn)->end_kaddr)
>  
>  #define __get_next(t, insn)	\
> -	({ t r = *(t*)insn->next_byte; insn->next_byte += sizeof(t); r; })
> +	({ t r; memcpy(&r, insn->next_byte, sizeof(t)); insn->next_byte += sizeof(t); r; })
>  
>  #define __peek_nbyte_next(t, insn, n)	\
> -	({ t r = *(t*)((insn)->next_byte + n); r; })
> +	({ t r; memcpy(&r, (insn)->next_byte + n, sizeof(t)); r; })
>  
>  #define get_next(t, insn)	\
>  	({ if (unlikely(!validate_next(t, insn, 0))) goto err_out; __get_next(t, insn); })

Is there any code generation side effect of this change to the resulting 
code?

Thanks,

	Ingo