Re: [PATCH 4/6] Bluetooth: SMP: clear the aes_cmac_key when done
From: Thomas Huth
Date: Thu Aug 06 2026 - 09:02:11 EST
On 05/08/2026 22.46, Eric Biggers wrote:
On Wed, Aug 05, 2026 at 04:36:07PM +0200, Thomas Huth wrote:Sure, I can add a line to clear tmp[] here, too.
From: Thomas Huth <thuth@xxxxxxxxxx>
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@xxxxxxxxxx>
---
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.)
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