[PATCH 5.4 103/320] ppp: ensure minimum packet size in ppp_write()

Greg Kroah-Hartman
2022-01-24T19:33:22Z

From: Eric Dumazet <edumazet@google.com>

[ Upstream commit 44073187990d5629804ce0627525f6ea5cfef171 ]

It seems pretty clear ppp layer assumed user space
would always be kind to provide enough data
in their write() to a ppp device.

This patch makes sure user provides at least
2 bytes.

It adds PPP_PROTO_LEN macro that could replace
in net-next many occurrences of hard-coded 2 value.

I replaced only one occurrence to ease backports
to stable kernels.

The bug manifests in the following report:

BUG: KMSAN: uninit-value in ppp_send_frame+0x28d/0x27c0 drivers/net/ppp/ppp_generic.c:1740
ppp_send_frame+0x28d/0x27c0 drivers/net/ppp/ppp_generic.c:1740
__ppp_xmit_process+0x23e/0x4b0 drivers/net/ppp/ppp_generic.c:1640
ppp_xmit_process+0x1fe/0x480 drivers/net/ppp/ppp_generic.c:1661
ppp_write+0x5cb/0x5e0 drivers/net/ppp/ppp_generic.c:513
do_iter_write+0xb0c/0x1500 fs/read_write.c:853
vfs_writev fs/read_write.c:924 [inline]
do_writev+0x645/0xe00 fs/read_write.c:967
__do_sys_writev fs/read_write.c:1040 [inline]
__se_sys_writev fs/read_write.c:1037 [inline]
__x64_sys_writev+0xe5/0x120 fs/read_write.c:1037
do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x54/0xd0 arch/x86/entry/common.c:82
entry_SYSCALL_64_after_hwframe+0x44/0xae

Uninit was created at:
slab_post_alloc_hook mm/slab.h:524 [inline]
slab_alloc_node mm/slub.c:3251 [inline]
__kmalloc_node_track_caller+0xe0c/0x1510 mm/slub.c:4974
kmalloc_reserve net/core/skbuff.c:354 [inline]
__alloc_skb+0x545/0xf90 net/core/skbuff.c:426
alloc_skb include/linux/skbuff.h:1126 [inline]
ppp_write+0x11d/0x5e0 drivers/net/ppp/ppp_generic.c:501
do_iter_write+0xb0c/0x1500 fs/read_write.c:853
vfs_writev fs/read_write.c:924 [inline]
do_writev+0x645/0xe00 fs/read_write.c:967
__do_sys_writev fs/read_write.c:1040 [inline]
__se_sys_writev fs/read_write.c:1037 [inline]
__x64_sys_writev+0xe5/0x120 fs/read_write.c:1037
do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x54/0xd0 arch/x86/entry/common.c:82
entry_SYSCALL_64_after_hwframe+0x44/0xae

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: linux-ppp@vger.kernel.org
Reported-by: syzbot <syzkaller@googlegroups.com>
Acked-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/net/ppp/ppp_generic.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index c6c41a7836c93..a085213dc2eaa 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -69,6 +69,8 @@
#define MPHDRLEN 6 /* multilink protocol header length */
#define MPHDRLEN_SSN 4 /* ditto with short sequence numbers */

+#define PPP_PROTO_LEN 2
+
/*
* An instance of /dev/ppp can be associated with either a ppp
* interface unit or a ppp channel. In both cases, file->private_data
@@ -498,6 +500,9 @@ static ssize_t ppp_write(struct file *file, const char __user *buf,

if (!pf)
return -ENXIO;
+ /* All PPP packets should start with the 2-byte protocol */
+ if (count < PPP_PROTO_LEN)
+ return -EINVAL;
ret = -ENOMEM;
skb = alloc_skb(count + pf->hdrlen, GFP_KERNEL);
if (!skb)
@@ -1544,7 +1549,7 @@ ppp_send_frame(struct ppp *ppp, struct sk_buff *skb)
}

++ppp->stats64.tx_packets;
- ppp->stats64.tx_bytes += skb->len - 2;
+ ppp->stats64.tx_bytes += skb->len - PPP_PROTO_LEN;

switch (proto) {
case PPP_IP:
--
2.34.1