CVE-2021-46954

In the Linux kernel, the following vulnerability has been resolved:

net/sched: sch_frag: fix stack OOB read while fragmenting IPv4 packets

when act_mirred tries to fragment IPv4 packets that had been previously re-assembled using act_ct, splats like the following can be observed on kernels built with KASAN:

BUG: KASAN: stack-out-of-bounds in ip_do_fragment+0x1b03/0x1f60 Read of size 1 at addr ffff888147009574 by task ping/947

CPU: 0 PID: 947 Comm: ping 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 sch_fragment+0x4bf/0xe40 tcf_mirred_act+0xc3d/0x11a0 [act_mirred] tcf_action_exec+0x104/0x3e0 fl_classify+0x49a/0x5e0 [cls_flower] tcf_classify_ingress+0x18a/0x820 __netif_receive_skb_core+0xae7/0x3340 __netif_receive_skb_one_core+0xb6/0x1b0 process_backlog+0x1ef/0x6c0 __napi_poll+0xaa/0x500 net_rx_action+0x702/0xac0 __do_softirq+0x1e4/0x97f do_softirq+0x71/0x90 __local_bh_enable_ip+0xdb/0xf0 ip_finish_output2+0x760/0x2120 ip_do_fragment+0x15a5/0x1f60 __ip_finish_output+0x4c2/0xea0 ip_output+0x1ca/0x4d0 ip_send_skb+0x37/0xa0 raw_sendmsg+0x1c4b/0x2d00 sock_sendmsg+0xdb/0x110 __sys_sendto+0x1d7/0x2b0 __x64_sys_sendto+0xdd/0x1b0 do_syscall_64+0x33/0x40 entry_SYSCALL_64_after_hwframe+0x44/0xae RIP: 0033:0x7f82e13853eb Code: 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 f3 0f 1e fa 48 8d 05 75 42 2c 00 41 89 ca 8b 00 85 c0 75 14 b8 2c 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 75 c3 0f 1f 40 00 41 57 4d 89 c7 41 56 41 89 RSP: 002b:00007ffe01fad888 EFLAGS: 00000246 ORIG_RAX: 000000000000002c RAX: ffffffffffffffda RBX: 00005571aac13700 RCX: 00007f82e13853eb RDX: 0000000000002330 RSI: 00005571aac13700 RDI: 0000000000000003 RBP: 0000000000002330 R08: 00005571aac10500 R09: 0000000000000010 R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffe01faefb0 R13: 00007ffe01fad890 R14: 00007ffe01fad980 R15: 00005571aac0f0a0

The buggy address belongs to the page: page:000000001dff2e03 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x147009 flags: 0x17ffffc0001000(reserved) raw: 0017ffffc0001000 ffffea00051c0248 ffffea00051c0248 0000000000000000 raw: 0000000000000000 0000000000000000 00000001ffffffff 0000000000000000 page dumped because: kasan: bad access detected

Memory state around the buggy address: ffff888147009400: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ffff888147009480: f1 f1 f1 f1 04 f2 f2 f2 f2 f2 f2 f2 00 00 00 00

ffff888147009500: 00 00 00 00 00 00 00 00 00 00 f2 f2 f2 f2 f2 f2 ^ ffff888147009580: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ffff888147009600: 00 00 00 00 00 00 00 00 00 00 00 00 00 f2 f2 f2

for IPv4 packets, sch_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 sch_fragment(), similarly to what is done for IPv6 few lines below.

Weakness

The product reads data past the end, or before the beginning, of the intended buffer.

Affected Software

Potential Mitigations

• Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
• When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
• Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
• To reduce the likelihood of introducing an out-of-bounds read, ensure that you validate and ensure correct calculations for any length argument, buffer size calculation, or offset. Be especially careful of relying on a sentinel (i.e. special character such as NUL) in untrusted inputs.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/540.html

References

• https://git.kernel.org/stable/c/018bb8da5b5888e19585f9b802f036afe643fcef
• https://git.kernel.org/stable/c/31fe34a0118e0acc958c802e830ad5d37ef6b1d3
• https://git.kernel.org/stable/c/8e6dfb7beeb6489ac1365b8a71052e737f5da76e