linux-kernel - Re: [PATCH v5 4/6] x86/microcode/AMD: Check microcode container data in the late loader

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Date:   Tue, 1 May 2018 18:19:56 +0200
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 4/6] x86/microcode/AMD: Check microcode container data
 in the late loader

On 01.05.2018 10:43, Borislav Petkov wrote:
> On Tue, May 01, 2018 at 12:27:51AM +0200, Maciej S. Szmigiero wrote:
>> 1) -EINVAL maps to a valid return value of 4294967274 bytes.
>> We have a different behavior for invalid data in the container file
>> (including too large lengths) than for grave errors like a failed memory
>> allocation.
> 
> WTF?

Could you please elaborate this comment?

-EINVAL cast to unsigned int is 4294967274 and this value is also
a valid count of bytes to skip that this function can return.

The "grave errors" behavior comes from the existing code, a comment
in code above verify_and_add_patch() says:
"a grave error like a memory allocation has failed and the driver cannot
continue functioning normally. In such cases, we tear down everything
we've used up so far and exit."

>> 2) This function single caller (__load_microcode_amd()) normalized any
>> error that verify_and_add_patch() returned to UCODE_ERROR anyway,
> 
> So?

This means that there is no loss of information here.

The function these three points are about (verify_and_add_patch()) is
declared as "static", so it cannot be called from any other kernel code.

>> 3) The existing code uses a convention that zero return value means
>> 'terminate processing' for the parse_container() function in the early
>> loader which normally returns a 'bytes consumed' value, as this function
>> does.
> 
> parse_container() could very well change its convention to return
> negative on error and positive value if the loop is supposed to skip
> bytes.
> 

Yes, but then the problem from the point 1) above will be introduced
also to parse_container().

Maciej