CVE-2021-46955

In the Linux kernel, the following vulnerability has been resolved:
openvswitch: fix stack OOB read while fragmenting IPv4 packets running
openvswitch on kernels built with KASAN, it’s possible to see the following
splat while testing fragmentation of IPv4 packets: BUG: KASAN:
stack-out-of-bounds in ip_do_fragment+0x1b03/0x1f60 Read of size 1 at addr
ffff888112fc713c by task handler2/1367 CPU: 0 PID: 1367 Comm: handler2 Not
tainted 5.12.0-rc6+ #418 Hardware name: Red Hat KVM, BIOS
1.11.1-4.module+el8.1.0+4066+0f1aadab 04/01/2014 Call Trace:
dump_stack+0x92/0xc1 print_address_description.constprop.7+0x1a/0x150
kasan_report.cold.13+0x7f/0x111 ip_do_fragment+0x1b03/0x1f60
ovs_fragment+0x5bf/0x840 [openvswitch] do_execute_actions+0x1bd5/0x2400
[openvswitch] ovs_execute_actions+0xc8/0x3d0 [openvswitch]
ovs_packet_cmd_execute+0xa39/0x1150 [openvswitch]
genl_family_rcv_msg_doit.isra.15+0x227/0x2d0 genl_rcv_msg+0x287/0x490
netlink_rcv_skb+0x120/0x380 genl_rcv+0x24/0x40 netlink_unicast+0x439/0x630
netlink_sendmsg+0x719/0xbf0 sock_sendmsg+0xe2/0x110
____sys_sendmsg+0x5ba/0x890 ___sys_sendmsg+0xe9/0x160
__sys_sendmsg+0xd3/0x170 do_syscall_64+0x33/0x40
entry_SYSCALL_64_after_hwframe+0x44/0xae RIP: 0033:0x7f957079db07 Code: c3
66 90 41 54 41 89 d4 55 48 89 f5 53 89 fb 48 83 ec 10 e8 eb ec ff ff 44 89
e2 48 89 ee 89 df 41 89 c0 b8 2e 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 35
44 89 c7 48 89 44 24 08 e8 24 ed ff ff 48 RSP: 002b:00007f956ce35a50
EFLAGS: 00000293 ORIG_RAX: 000000000000002e RAX: ffffffffffffffda RBX:
0000000000000019 RCX: 00007f957079db07 RDX: 0000000000000000 RSI:
00007f956ce35ae0 RDI: 0000000000000019 RBP: 00007f956ce35ae0 R08:
0000000000000000 R09: 00007f9558006730 R10: 0000000000000000 R11:
0000000000000293 R12: 0000000000000000 R13: 00007f956ce37308 R14:
00007f956ce35f80 R15: 00007f956ce35ae0 The buggy address belongs to the
page: page:00000000af2a1d93 refcount:0 mapcount:0 mapping:0000000000000000
index:0x0 pfn:0x112fc7 flags: 0x17ffffc0000000() raw: 0017ffffc0000000
0000000000000000 dead000000000122 0000000000000000 raw: 0000000000000000
0000000000000000 00000000ffffffff 0000000000000000 page dumped because:
kasan: bad access detected addr ffff888112fc713c is located in stack of
task handler2/1367 at offset 180 in frame: ovs_fragment+0x0/0x840
[openvswitch] this frame has 2 objects: [32, 144) ‘ovs_dst’ [192, 424)
‘ovs_rt’ Memory state around the buggy address: ffff888112fc7000: f3 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 ffff888112fc7080: 00 f1 f1 f1 f1 00
00 00 00 00 00 00 00 00 00 00 >ffff888112fc7100: 00 00 00 f2 f2 f2 f2 f2 f2
00 00 00 00 00 00 00 ^ ffff888112fc7180: 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 ffff888112fc7200: 00 00 00 00 00 00 f2 f2 f2 00 00 00 00 00
00 00 for IPv4 packets, ovs_fragment() uses a temporary struct dst_entry.
Then, in the following call graph: ip_do_fragment() ip_skb_dst_mtu()
ip_dst_mtu_maybe_forward() ip_mtu_locked() the pointer to struct dst_entry
is used as pointer to struct rtable: this turns the access to struct
members like rt_mtu_locked into an OOB read in the stack. Fix this changing
the temporary variable used for IPv4 packets in ovs_fragment(), similarly
to what is done for IPv6 few lines below.