ppp: Fix KMSAN uninit-value warning with bpf

ppp: Fix KMSAN uninit-value warning with bpf

Syzbot caught an "KMSAN: uninit-value" warning [1], which is caused by the
ppp driver not initializing a 2-byte header when using socket filter.

The following code can generate a PPP filter BPF program:
'''
struct bpf_program fp;
pcap_t *handle;
handle = pcap_open_dead(DLT_PPP_PPPD, 65535);
pcap_compile(handle, &fp, "ip and outbound", 0, 0);
bpf_dump(&fp, 1);
'''
Its output is:
'''
(000) ldh [2]
(001) jeq #0x21 jt 2 jf 5
(002) ldb [0]
(003) jeq #0x1 jt 4 jf 5
(004) ret #65535
(005) ret #0
'''
Wen can find similar code at the following link:
https://github.com/ppp-project/ppp/blob/master/pppd/options.c#L1680
The maintainer of this code repository is also the original maintainer
of the ppp driver.

As you can see the BPF program skips 2 bytes of data and then reads the
'Protocol' field to determine if it's an IP packet. Then it read the first
byte of the first 2 bytes to determine the direction.

The issue is that only the first byte indicating direction is initialized
in current ppp driver code while the second byte is not initialized.

For normal BPF programs generated by libpcap, uninitialized data won't be
used, so it's not a problem. However, for carefully crafted BPF programs,
such as those generated by syzkaller [2], which start reading from offset
0, the uninitialized data will be used and caught by KMSAN.

[1] https://syzkaller.appspot.com/bug?extid=853242d9c9917165d791
[2] https://syzkaller.appspot.com/text?tag=ReproC&x=11994913980000

Cc: Paul Mackerras <paulus@samba.org>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: syzbot+853242d9c9917165d791@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/bpf/000000000000dea025060d6bc3bc@google.com/
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250228141408.393864-1-jiayuan.chen@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>