linux-kernel - Re: CAAM RSA breaks cfg80211 certificate verification on iMX8QXP

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Message-ID: <51e2de7a-5913-4c53-9637-6d60f875e3d8@kernel.org>
Date: Fri, 28 Nov 2025 11:36:24 +0100
From: Ahmad Fatoum <ahmad@...nel.org>
To: Vitor Soares <ivitro@...il.com>, Ahmad Fatoum <ahmad@...nel.org>,
 linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
 imx@...ts.linux.dev
Cc: horia.geanta@....com, pankaj.gupta@....com, gaurav.jain@....com,
 herbert@...dor.apana.org.au, john.ernberg@...ia.se,
 meenakshi.aggarwal@....com
Subject: Re: CAAM RSA breaks cfg80211 certificate verification on iMX8QXP

Hi Vitor,

On 11/26/25 7:35 PM, Vitor Soares wrote:
> On Wed, 2025-11-26 at 13:59 +0100, Ahmad Fatoum wrote:
>> Is the CAAM cache-coherent on your SoC? If so does the DT specify dma-coherent
>> as it should? On i.MX8M, it's not cache-coherent, but on Layerscape it was and
>> the mismatch with the DT leads to symptoms matching what you are observing.
>>
> 
> Thanks for the suggestion. I tested with dma-coherent added to the CAAM and job
> ring nodes but the issue persists.
> I traced through the DMA path in caampkc.c and confirmed:
> 
> - dma_map_sg() is called in rsa_edesc_alloc() with DMA_FROM_DEVICE
> - dma_unmap_sg() is called in rsa_io_unmap() from rsa_pub_done() before
> completion
> - CAAM returns status err=0x00000000 (success)
> - dst_nents=1 
> 
> Yet the output buffer remains untouched (still contains my 0xAA poison pattern).
> The kernel DMA handling appears correct. CAAM accepts the job and reports
> success, but never writes the RSA result. Given that CAAM reports success but
> does not populate the RSA output buffer, the problem appears to be somewhere in
> the RSA execution flow (possibly in how the result buffer is handled or
> returned), but I don't have enough insight into CAAM's RSA implementation.

Ok.. That was the only thing off the top of my head right now.

>> Off-topic remark: If you have performance comparison between running with and
>> without CAAM RSA acceleration, I'd be interested to hear about them.
>> At least for the hashing algorithms, using the Cortex-A53 (+ CE) CPU was a lot
>> faster than bothering with the CAAM "acceleration".
>>
> 
> I haven't done a kernel-level CAAM vs software RSA comparison, but OpenSSL with
> ARM Crypto Extensions shows ~3100 verify ops/sec and ~80 sign ops/sec for RSA
> 2048 on the Cortex-A35.

I see, thanks.

Cheers,
Ahmad

> 
> Regards,
> Vítor
> 
> 
> 
> 

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