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

From: Borislav Petkov
Date: Sat Feb 27 2016 - 12:51:31 EST

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.


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