Re: [PATCH v3 2/2] crypto: caam - Use bounce buffer for unaligned RSA destination buffers

[Herbert Xu <herbert@gondor.apana.org.au>]

On Tue, Oct 20, 2026 at 01:54:26PM +1100, Changwei Zou wrote:
> The rsa-caam driver directly DMA-maps the destination buffer supplied by
> the caller via req->dst without checking whether it meets the cacheline
> alignment requirements of DMA-incoherent hardware such as i.MX8.
> 
> On CPUs with non-coherent DMA caches, if the destination buffer shares a
> cacheline with other data (i.e. it is not cacheline-aligned), cache
> writeback/invalidation during DMA can corrupt adjacent memory or cause
> stale data to be read back. This manifests as intermittent -EKEYREJECTED
> errors when loading signed kernel modules.
> 
> When any segment of req->dst is not cacheline-aligned in either its
> start offset or length, allocate a single contiguous aligned bounce
> buffer covering the full dst_len rounded up to a cacheline multiple,
> redirect the operation to it, and copy the result back to the original
> destination once the hardware has completed successfully.
> 
> Introduce a helper sg_is_dma_aligned() that checks both sg->offset and
> sg->length for cacheline alignment. Checking sg->offset suffices for
> the start address since physical pages are always page-aligned and
> PAGE_SIZE is a multiple of dma_get_cache_alignment(). sg->length is
> also checked to ensure the buffer end does not share a cacheline with
> adjacent memory.
> 
> The intermittent error 'Key was rejected by service' on i.MX8 with CAAM
> can be triggered when loading signed kernel modules:
> 
>     for i in $(seq 1 100); do
>         sudo modprobe xfs 2>&1 && echo "SUCCESS on attempt $i" \
>         && sudo rmmod xfs || echo "FAILED on attempt $i"
>     done
> 
> Reported-by: Martin Kepplinger-Novakovic <Martin.Kepplinger-Novakovic@ginzinger.com>
> Closes: https://lore.kernel.org/r/6029acc0f0ddfe25e2537c2866d54fd7f54bc182.camel@ginzinger.com
> Signed-off-by: Changwei Zou <changwei.zou@canonical.com>
> Assisted-by: OpenCode:claude-sonnet-4.6
> Tested-by: Martin Kepplinger-Novakovic <martin.kepplinger-novakovic@ginzinger.com>
> Tested-by: Stephane Debusne <stephane.debusne@cynove.com>
> ---
>  drivers/crypto/caam/caampkc.c | 87 ++++++++++++++++++++++++++++++++++-
>  drivers/crypto/caam/caampkc.h |  6 +++
>  2 files changed, 92 insertions(+), 1 deletion(-)

Please respond to the Sashiko comments:

https://sashiko.dev/#/patchset/20261020025426.955531-1-changwei.zou%40canonical.com

I'm also disappointed with the lack of response from the caam
maintainers.  Unless active maintainence can be demonstrated
for this driver, I'm going to have it removed again.

Thanks,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt