Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1345053

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

Show all headers | View raw

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.

ECO tip #101: Trim your mails when you reply.

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

[PATCH] x86/microcode: Change checksum to u32 Chris Bainbridge <chris.bainbridge@gmail.com> - 2016-02-27 12:10 +0100
  Re: [PATCH] x86/microcode: Change checksum to u32 Borislav Petkov <bp@alien8.de> - 2016-02-27 19:00 +0100
    Re: [PATCH] x86/microcode: Change checksum to u32 Chris Bainbridge <chris.bainbridge@gmail.com> - 2016-02-27 19:30 +0100
      Re: [PATCH] x86/microcode: Change checksum to u32 Henrique de Moraes Holschuh <hmh@hmh.eng.br> - 2016-02-27 20:50 +0100
      Re: [PATCH] x86/microcode: Change checksum to u32 Borislav Petkov <bp@alien8.de> - 2016-02-27 23:30 +0100
    Re: [PATCH] x86/microcode: Change checksum to u32 Henrique de Moraes Holschuh <hmh@hmh.eng.br> - 2016-02-27 21:20 +0100

csiph-web