linux-kernel - Re: [PATCH] x86/microcode: Change checksum to u32

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Message-ID: <20160227175118.GC5261@pd.tnic>
Date:	Sat, 27 Feb 2016 18:51:18 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Chris Bainbridge <chris.bainbridge@...il.com>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/microcode: Change checksum to u32

On Sat, Feb 27, 2016 at 11:01:47AM +0000, Chris Bainbridge wrote:
> Checksum should be unsigned 32-bit otherwise the calculation overflows
> resulting in undefined behaviour:
> 
> [    0.000000] ================================================================================
> [    0.000000] UBSAN: Undefined behaviour in arch/x86/kernel/cpu/microcode/intel_lib.c:105:12
> [    0.000000] signed integer overflow:
> [    0.000000] -1500151068 + -2125470173 cannot be represented in type 'int'

So I've been staring at this error message for a while now and I'm
having hard time understanding what it is telling me. Let's look at all
three ints:

* sum is computed here:

                sum = orig_sum
                        - (mc_header->sig + mc_header->pf + mc_header->cksum)
                        + (ext_sig->sig + ext_sig->pf + ext_sig->cksum);

and checks mc_header against each extended signature. If it overflows, we abort:

                if (sum) {
                        if (print_err)
                                pr_err("aborting, bad checksum\n");


* orig_sum in the above sum can only be 0:

        if (orig_sum) {
                if (print_err)
                        pr_err("aborting, bad checksum\n");
                return -EINVAL;

and it adds a bunch of integers which can overflow, sure, but if it
overflows, we exit early.

* and

	ext_sigcount = ext_header->count;

so it is some count of extended signatures.

So what is ubsan complaining about?

/me is confused.

-- 
Regards/Gruss,
    Boris.

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