Bug in atmel-ecc driver

[Tudor.Ambarus at microchip.com]

On 5/17/22 13:24, Uwe Kleine-König wrote:
> Hello,
> 
Hi, Uwe,

> [added Ard to Cc as he last touched that driver]
> 
> On Fri, May 13, 2022 at 03:59:54PM +0200, Uwe Kleine-König wrote:
>> TL;DR: when a device bound to the drivers/crypto/atmel-ecc.c driver is
>> unbound while tfm_count isn't zero, this probably results in a
>> use-after-free.
>>
>> The .remove function has:
>>
>> 	if (atomic_read(&i2c_priv->tfm_count)) {
>>                 dev_err(&client->dev, "Device is busy\n");
>>                 return -EBUSY;
>>         }
>>
>> before actually calling the cleanup stuff. If this branch is hit the
>> result is likely:
>>
>>  - "Device is busy" from drivers/crypto/atmel-ecc.c
>>  - "remove failed (EBUSY), will be ignored" from the i2c core
>>  - the devm cleanup callbacks are called, including the one kfreeing
>>    *i2c_priv
>>  - at a later time atmel_ecc_i2c_client_free() is called which does
>>    atomic_dec(&i2c_priv->tfm_count);
>>  - *boom*
>>
>> I think to fix that you need to call get_device for the i2c device
>> before increasing tfm_count (and a matching put_device when decreasing
>> it). Having said that the architecture of this driver looks strange to
>> me, so there might be nicer fixes (probably with more effort).
> I tried to understand the architecture a bit, what I found is
> irritating. So the atmel-ecc driver provides a static struct kpp_alg
> atmel_ecdh_nist_p256 which embeds a struct crypto_alg (.base). During
> .probe() it calls crypto_register_kpp on that global kpp_alg. That is,
> if there are two or more devices bound to this driver, the same kpp_alg
> structure is registered repeatedly.  This involves (among others)
> 
>  - refcount_set(&atmel_ecdh_nist_p256.base.cra_refcount)
>    in crypto_check_alg()
>  - INIT_LIST_HEAD(&atmel_ecdh_nist_p256.base.cra_users)
>    in __crypto_register_alg()
> 
> and then a check about registering the same alg twice which makes the
> call crypto_register_alg() return -EEXIST. So if a second device is
> bound, it probably corrupts the first device and then fails to probe.
> 
> So there can always be (at most) only one bound device which somehow
> makes the whole logic in atmel_ecdh_init_tfm ->
> atmel_ecc_i2c_client_alloc to select the least used(?) i2c client among
> all the bound devices ridiculous.

It's been a while since I last worked with ateccx08, but as far as I remember
it contains 3 crypto IPs (ecdh, ecdsa, sha) that communicate over the same
i2c address. So if someone adds support for all algs and plug in multiple
ateccx08 devices, then the distribution of tfms across the i2c clients may work.
Anyway, if you feel that the complexity is superfluous as the code is now, we
can get rid of the i2c_client_alloc logic and add it later on when/if needed.

Cheers,
ta
> 
> Is there someone still caring for this driver? Does this justify 
> 
> diff --git a/drivers/crypto/Kconfig b/drivers/crypto/Kconfig
> index 7b2d138bc83e..b3242fba82aa 100644
> --- a/drivers/crypto/Kconfig
> +++ b/drivers/crypto/Kconfig
> @@ -573,6 +573,7 @@ config CRYPTO_DEV_ATMEL_I2C
>  
>  config CRYPTO_DEV_ATMEL_ECC
>  	tristate "Support for Microchip / Atmel ECC hw accelerator"
> +	depends on BROKEN
>  	depends on I2C
>  	select CRYPTO_DEV_ATMEL_I2C
>  	select CRYPTO_ECDH
> 
> ?
> 
> Best regards
> Uwe