Re: [PATCH 4/6] Bluetooth: SMP: clear the aes_cmac_key when done

[Thomas Huth <thuth@redhat.com>]

On 05/08/2026 22.46, Eric Biggers wrote:
> On Wed, Aug 05, 2026 at 04:36:07PM +0200, Thomas Huth wrote:
>> From: Thomas Huth <thuth@redhat.com>
>>
>> Clear the local aes_cmac_key structure via __cleanup() function
>> when we're done with it to avoid that sensitive data could leak on
>> the stack.
>>
>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>> ---
>>   net/bluetooth/smp.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/net/bluetooth/smp.c b/net/bluetooth/smp.c
>> index 031d3022cb1e5..ed30ca0d773f1 100644
>> --- a/net/bluetooth/smp.c
>> +++ b/net/bluetooth/smp.c
>> @@ -164,7 +164,7 @@ static inline void swap_buf(const u8 *src, u8 *dst, size_t len)
>>   static int smp_aes_cmac(const u8 k[16], const u8 *m, size_t len, u8 mac[16])
>>   {
>>   	uint8_t tmp[16], mac_msb[16], msg_msb[CMAC_MSG_MAX];
>> -	struct aes_cmac_key key;
>> +	struct aes_cmac_key key __cleanup(aes_cmac_zeroize_key);
>>   	int err;
>>   
>>   	if (len > CMAC_MSG_MAX)
> 
> Well, the reason I didn't add a memzero_explicit() here when converting
> the code to use the AES-CMAC library is because this same function
> already puts the raw key on the stack without zeroizing it.
> 
> I guess the __cleanup trick makes zeroizing the struct trivial enough
> that we should just do it anyway.  But it is always a bit awkward to be
> "fixing" something when the same problem is still present.
> 
> Perhaps you'd like to expand this patch a bit to zeroize the other data
> too?  (There is the 'tmp' array in this same function of course, but
> there may be other places that need "fixing" too.)
Sure, I can add a line to clear tmp[] here, too.

And I agree, there are other spots in this file that likely need fixing, 
e.g. smp_e() already clears struct aes_enckey aes, but misses to zeroize its 
tmp[] array, too, that again contains key material, I think?

But I think I'll rather tackle those in a separate patch series, since this 
series here is clearly aimed at aes_cmac_key & aes_cmac_ctx.

  Thomas