[PATCH v2] net/packet: initialize val in packet_getsockopt()

[Alexander Potapenko]

In the case getsockopt() is called with PACKET_HDRLEN and optlen < 4
|val| remains uninitialized and the syscall may behave differently
depending on its value. This doesn't have security consequences (as the
uninit bytes aren't copied back), but it's still cleaner to initialize
|val| and ensure optlen is not less than sizeof(int).

This bug has been detected with KMSAN.

Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
---
v2: - if len < sizeof(int), make it 0

KMSAN report below:

==================================================================
BUG: KMSAN: use of unitialized memory in packet_getsockopt+0xb9b/0xbe0
inter: 0
CPU: 0 PID: 1036 Comm: probe Tainted: G    B           4.11.0-rc5+ #2444
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:16
 dump_stack+0x143/0x1b0 lib/dump_stack.c:52
 kmsan_report+0x16b/0x1e0 mm/kmsan/kmsan.c:1078
 __kmsan_warning_32+0x5c/0xa0 mm/kmsan/kmsan_instr.c:510
 packet_getsockopt+0xb9b/0xbe0 net/packet/af_packet.c:3839
 SYSC_getsockopt+0x495/0x540 net/socket.c:1829
 SyS_getsockopt+0xb0/0xd0 net/socket.c:1811
 entry_SYSCALL_64_fastpath+0x13/0x94 arch/x86/entry/entry_64.S:204
RIP: 0033:0x436d8a
RSP: 002b:00007ffce54e52c8 EFLAGS: 00000203 ORIG_RAX: 0000000000000037
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000436d8a
RDX: 000000000000000b RSI: 0000000000000107 RDI: 0000000000000003
RBP: 00007ffce54e52b0 R08: 00007ffce54e52d8 R09: 0000000000000004
R10: 00007ffce54e52d4 R11: 0000000000000203 R12: 00007ffce54e53c8
R13: 00007ffce54e53d8 R14: 0000000000000002 R15: 0000000000000000
origin description: ----val@packet_getsockopt (origin=00000000f6600052)
local variable created at:
 packet_getsockopt+0xcd/0xbe0 net/packet/af_packet.c:3789
 SYSC_getsockopt+0x495/0x540 net/socket.c:1829
==================================================================
---
 net/packet/af_packet.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 8489beff5c25..dfc762df5da9 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -3787,7 +3787,7 @@ static int packet_getsockopt(struct socket *sock, int level, int optname,
 			     char __user *optval, int __user *optlen)
 {
 	int len;
-	int val, lv = sizeof(val);
+	int val = 0, lv = sizeof(val);
 	struct sock *sk = sock->sk;
 	struct packet_sock *po = pkt_sk(sk);
 	void *data = &val;
@@ -3836,6 +3836,8 @@ static int packet_getsockopt(struct socket *sock, int level, int optname,
 	case PACKET_HDRLEN:
 		if (len > sizeof(int))
 			len = sizeof(int);
+		if (len < sizeof(int))
+			len = 0;
 		if (copy_from_user(&val, optval, len))
 			return -EFAULT;
 		switch (val) {
-- 
2.12.2.816.g2cccc81164-goog