Hi,
I already reported the issue to security@kernel.org and
linux-distros@vs.openwall.org and CVE-2020-14386 was assigned.

The report is as follows: ( a proposed patch and a reproducer are attached)

I discovered a bug which leads to a memory corruption in
(net/packet/af_packet.c). It can be exploited to gain root privileges
from unprivileged processes.

To create AF_PACKET sockets you need CAP_NET_RAW in your network
namespace, which can be acquired by unprivileged processes on systems
where unprivileged namespaces are enabled (Ubuntu, Fedora, etc).

I discovered the vulnerability while auditing the 5.7 kernel sources.

The bug occurs in tpacket_rcv function, when calculating the netoff
variable (unsigned short), po->tp_reserve (unsigned int) is added to
it which can overflow netoff so it gets a small value.

macoff is calculated using: "macoff = netoff - maclen", we can control
macoff so it will receive a small value (specifically, smaller then
sizeof(struct virtio_net_hdr)).

Later, when running the following code:
...
if (do_vnet &&
   virtio_net_hdr_from_skb(skb, h.raw + macoff -
sizeof(struct virtio_net_hdr),
...

If do_vnet is set, and because macoff < sizeof(struct virtio_net_hdr)
a pointer to a memory area before the h.raw buffer will be sent to
virtio_net_hdr_from_skb. This can lead to an out-of-bounds write of
1-10 bytes, controlled by the user.

The h.raw buffer is allocated in alloc_pg_vec and it's size is
controlled by the user.

The stack trace is as follows at the time of the crash: ( linux v5.7 )

#0  memset_erms () at arch/x86/lib/memset_64.S:66
#1  0xffffffff831934a6 in virtio_net_hdr_from_skb
(little_endian=<optimized out>, has_data_valid=<optimized out>,
    vlan_hlen=<optimized out>, hdr=<optimized out>, skb=<optimized
out>) at ./include/linux/virtio_net.h:134
#2  tpacket_rcv (skb=0xffff8881ef539940, dev=0xffff8881de534000,
pt=<optimized out>, orig_dev=<optimized out>)
        at net/packet/af_packet.c:2287
#3  0xffffffff82c52e47 in dev_queue_xmit_nit (skb=0xffff8881ef5391c0,
dev=<optimized out>) at net/core/dev.c:2276
#4  0xffffffff82c5e3d4 in xmit_one (more=<optimized out>,
txq=<optimized out>, dev=<optimized out>,
            skb=<optimized out>) at net/core/dev.c:3473
#5  dev_hard_start_xmit (first=0xffffc900001c0ff6, dev=0x0
<fixed_percpu_data>, txq=0xa <fixed_percpu_data+10>,
    ret=<optimized out>) at net/core/dev.c:3493
#6  0xffffffff82c5fc7e in __dev_queue_xmit (skb=0xffff8881ef5391c0,
sb_dev=<optimized out>) at net/core/dev.c:4052
#7  0xffffffff831982d3 in packet_snd (len=65536, msg=<optimized out>,
sock=<optimized out>) 0001-net-packet-fix-overflow-in-tpacket_rcv
at net/packet/af_packet.c:2979
#8  packet_sendmsg (sock=<optimized out>, msg=<optimized out>,
len=65536) at net/packet/af_packet.c:3004
#9  0xffffffff82be09ed in sock_sendmsg_nosec (msg=<optimized out>,
sock=<optimized out>) at net/socket.c:652
#10 sock_sendmsg (sock=0xffff8881e8ff56c0, msg=0xffff8881de56fd88) at
net/socket.c:672

Files attached:
A proposed patch - 0001-net-packet-fix-overflow-in-tpacket_rcv.patch
A reproducer for the bug - trigger_bug.c

We are currently working on an exploit for getting root privileges
from unprivileged context using this bug.

If there is a problem with the patch please let me know and I will fix it.

Or Cohen
Palo Alto Networks