linux-kernel - [Linux-kernel-mentees] [PATCH net] AX.25: Prevent out-of-bounds read in ax25

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Date:   Wed, 22 Jul 2020 12:05:12 -0400
From:   Peilin Ye <yepeilin.cs@...il.com>
To:     Joerg Reuter <jreuter@...na.de>, Ralf Baechle <ralf@...ux-mips.org>
Cc:     Peilin Ye <yepeilin.cs@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        syzkaller-bugs@...glegroups.com,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, linux-hams@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [Linux-kernel-mentees] [PATCH net] AX.25: Prevent out-of-bounds read in ax25_sendmsg()

Checks on `addr_len` and `usax->sax25_ndigis` are insufficient.
ax25_sendmsg() can go out of bounds when `usax->sax25_ndigis` equals to 7
or 8. Fix it.

It is safe to remove `usax->sax25_ndigis > AX25_MAX_DIGIS`, since
`addr_len` is guaranteed to be less than or equal to
`sizeof(struct full_sockaddr_ax25)`

Signed-off-by: Peilin Ye <yepeilin.cs@...il.com>
---
 net/ax25/af_ax25.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
index ef5bf116157a..0862fe49d434 100644
--- a/net/ax25/af_ax25.c
+++ b/net/ax25/af_ax25.c
@@ -1509,7 +1509,8 @@ static int ax25_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 			struct full_sockaddr_ax25 *fsa = (struct full_sockaddr_ax25 *)usax;
 
 			/* Valid number of digipeaters ? */
-			if (usax->sax25_ndigis < 1 || usax->sax25_ndigis > AX25_MAX_DIGIS) {
+			if (usax->sax25_ndigis < 1 || addr_len < sizeof(struct sockaddr_ax25) +
+			    sizeof(ax25_address) * usax->sax25_ndigis) {
 				err = -EINVAL;
 				goto out;
 			}
-- 
2.25.1