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.