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Subject[PATCH v3] tun: bail out from tun_get_user() if the skb is empty
Date
KMSAN (https://github.com/google/kmsan) reported accessing uninitialized
skb->data[0] in the case the skb is empty (i.e. skb->len is 0):

================================================
BUG: KMSAN: use of uninitialized memory in tun_get_user+0x19ba/0x3770
CPU: 0 PID: 3051 Comm: probe Not tainted 4.13.0+ #3140
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Call Trace:
...
__msan_warning_32+0x66/0xb0 mm/kmsan/kmsan_instr.c:477
tun_get_user+0x19ba/0x3770 drivers/net/tun.c:1301
tun_chr_write_iter+0x19f/0x300 drivers/net/tun.c:1365
call_write_iter ./include/linux/fs.h:1743
new_sync_write fs/read_write.c:457
__vfs_write+0x6c3/0x7f0 fs/read_write.c:470
vfs_write+0x3e4/0x770 fs/read_write.c:518
SYSC_write+0x12f/0x2b0 fs/read_write.c:565
SyS_write+0x55/0x80 fs/read_write.c:557
do_syscall_64+0x242/0x330 arch/x86/entry/common.c:284
entry_SYSCALL64_slow_path+0x25/0x25 arch/x86/entry/entry_64.S:245
...
origin:
...
kmsan_poison_shadow+0x6e/0xc0 mm/kmsan/kmsan.c:211
slab_alloc_node mm/slub.c:2732
__kmalloc_node_track_caller+0x351/0x370 mm/slub.c:4351
__kmalloc_reserve net/core/skbuff.c:138
__alloc_skb+0x26a/0x810 net/core/skbuff.c:231
alloc_skb ./include/linux/skbuff.h:903
alloc_skb_with_frags+0x1d7/0xc80 net/core/skbuff.c:4756
sock_alloc_send_pskb+0xabf/0xfe0 net/core/sock.c:2037
tun_alloc_skb drivers/net/tun.c:1144
tun_get_user+0x9a8/0x3770 drivers/net/tun.c:1274
tun_chr_write_iter+0x19f/0x300 drivers/net/tun.c:1365
call_write_iter ./include/linux/fs.h:1743
new_sync_write fs/read_write.c:457
__vfs_write+0x6c3/0x7f0 fs/read_write.c:470
vfs_write+0x3e4/0x770 fs/read_write.c:518
SYSC_write+0x12f/0x2b0 fs/read_write.c:565
SyS_write+0x55/0x80 fs/read_write.c:557
do_syscall_64+0x242/0x330 arch/x86/entry/common.c:284
return_from_SYSCALL_64+0x0/0x6a arch/x86/entry/entry_64.S:245
================================================

Make sure tun_get_user() doesn't touch skb->data[0] unless there is
actual data.

C reproducer below:
==========================
// autogenerated by syzkaller (http://github.com/google/syzkaller)

#define _GNU_SOURCE

#include <fcntl.h>
#include <linux/if_tun.h>
#include <netinet/ip.h>
#include <net/if.h>
#include <string.h>
#include <sys/ioctl.h>

int main()
{
int sock = socket(PF_INET, SOCK_STREAM, IPPROTO_IP);
int tun_fd = open("/dev/net/tun", O_RDWR);
struct ifreq req;
memset(&req, 0, sizeof(struct ifreq));
strcpy((char*)&req.ifr_name, "gre0");
req.ifr_flags = IFF_UP | IFF_MULTICAST;
ioctl(tun_fd, TUNSETIFF, &req);
ioctl(sock, SIOCSIFFLAGS, "gre0");
write(tun_fd, "hi", 0);
return 0;
}
==========================

Signed-off-by: Alexander Potapenko <glider@google.com>
---
v3: per request by Eric Dumazet introduce ip_version to avoid copy/paste
v2: free the skb
---
drivers/net/tun.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 3c9985f29950..5ce580f413b9 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1496,11 +1496,13 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
switch (tun->flags & TUN_TYPE_MASK) {
case IFF_TUN:
if (tun->flags & IFF_NO_PI) {
- switch (skb->data[0] & 0xf0) {
- case 0x40:
+ u8 ip_version = skb->len ? (skb->data[0] >> 4) : 0;
+
+ switch (ip_version) {
+ case 4:
pi.proto = htons(ETH_P_IP);
break;
- case 0x60:
+ case 6:
pi.proto = htons(ETH_P_IPV6);
break;
default:
--
2.14.2.822.g60be5d43e6-goog
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