linux-kernel - Re: [PATCH v2] x86/microcode/AMD: check microcode file sanity before loading it

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Date:   Mon, 12 Mar 2018 14:32:30 +0100
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86/microcode/AMD: check microcode file sanity before
 loading it

On 12.03.2018 14:06, Borislav Petkov wrote:
> On Mon, Mar 12, 2018 at 01:56:59PM +0100, Maciej S. Szmigiero wrote:
(..)
>> Since the maximum entry count in current microcode files is 18 the
> 
> Where did you dream up that 18?

"microcode_amd.bin" in linux-firmware.

>> Also, looking in the past, there probably won't be more than 256 AMD CPU
>> types in one CPU family.
> 
> Wrong.

There is no problem raising this value in that (future) case.
As I wrote previously, currently the maximum used count is 18.

> The only limitation on the equivalence table size we have is the 32-bit
> unsigned length field at offset 8 in the equivalence table header.

Not really, since even in the existing code CONTAINER_HDR_SZ (12) gets
added to this size, then the sum is cast to a (signed) int.
If this value is negative then the file get rejected.

>> This limit is an absolute upper cap of a patch size.
> 
> More dreamt up crap.
> 
> See verify_patch_size() for the actual patch sizes.
> 

It can be changed to the current maximum across sizes for particular
families, but then the limit will need to be raised when adding a new
family (if it uses a larger patch).

Maciej