netdev - [PATCH] Bluetooth: smp: Fix biased random passkey generation

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Message-Id: <20201207174957.408-1-encrypted.def@gmail.com>
Date:   Tue,  8 Dec 2020 02:49:57 +0900
From:   Mincheol Son <encrypted.def@...il.com>
To:     marcel@...tmann.org
Cc:     johan.hedberg@...il.com, davem@...emloft.net, kuba@...nel.org,
        linux-bluetooth@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Mincheol Son <encrypted.def@...il.com>
Subject: [PATCH] Bluetooth: smp: Fix biased random passkey generation

Since u32 range size is not a multiple of 1,000,000, current passkey generation logic is biased.

Fixed this by adding a routine that selects passkey again if passkey is 4,200,000,000 or more.

Signed-off-by: Mincheol Son <encrypted.def@...il.com>
---
 net/bluetooth/smp.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/bluetooth/smp.c b/net/bluetooth/smp.c
index c659c464f7ca..26ed83e0db34 100644
--- a/net/bluetooth/smp.c
+++ b/net/bluetooth/smp.c
@@ -922,7 +922,9 @@ static int tk_request(struct l2cap_conn *conn, u8 remote_oob, u8 auth,
 	/* Generate random passkey. */
 	if (smp->method == CFM_PASSKEY) {
 		memset(smp->tk, 0, sizeof(smp->tk));
-		get_random_bytes(&passkey, sizeof(passkey));
+		do {
+			get_random_bytes(&passkey, sizeof(passkey));
+		} while (passkey >= (u32)4200000000);
 		passkey %= 1000000;
 		put_unaligned_le32(passkey, smp->tk);
 		BT_DBG("PassKey: %d", passkey);
-- 
2.25.1