CVE-2021-47363 nexthop: Fix division by zero while replacing a resilient group

What is CVE-2021-47363?

CVE-2021-47363 is a medium-severity denial of service vulnerability in Linux kernel, published on 21 May 2024. It can only be exploited from the same machine or local network, with no action from the victim.

Official description

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

nexthop: Fix division by zero while replacing a resilient group

The resilient nexthop group torture tests in fib_nexthop.sh exposed a possible division by zero while replacing a resilient group [1]. The division by zero occurs when the data path sees a resilient nexthop group with zero buckets.

The tests replace a resilient nexthop group in a loop while traffic is forwarded through it. The tests do not specify the number of buckets while performing the replacement, resulting in the kernel allocating a stub resilient table (i.e, 'struct nh_res_table') with zero buckets.

This table should never be visible to the data path, but the old nexthop group (i.e., 'oldg') might still be used by the data path when the stub table is assigned to it.

Fix this by only assigning the stub table to the old nexthop group after making sure the group is no longer used by the data path.

Tested with fib_nexthops.sh:

Tests passed: 222 Tests failed: 0

[1] divide error: 0000 [#1] PREEMPT SMP KASAN CPU: 0 PID: 1850 Comm: ping Not tainted 5.14.0-custom-10271-ga86eb53057fe #1107 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-4.fc34 04/01/2014 RIP: 0010:nexthop_select_path+0x2d2/0x1a80 [...] Call Trace: fib_select_multipath+0x79b/0x1530 fib_select_path+0x8fb/0x1c10 ip_route_output_key_hash_rcu+0x1198/0x2da0 ip_route_output_key_hash+0x190/0x340 ip_route_output_flow+0x21/0x120 raw_sendmsg+0x91d/0x2e10 inet_sendmsg+0x9e/0xe0 __sys_sendto+0x23d/0x360 __x64_sys_sendto+0xe1/0x1b0 do_syscall_64+0x35/0x80 entry_SYSCALL_64_after_hwframe+0x44/0xae

Weakness: CWE-369

Which versions are affected by CVE-2021-47363?

It affects 1 product.

Products include Linux kernel.

How do I fix CVE-2021-47363?

Apply the vendor's patch.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2021-47363 being exploited?

There is no known exploitation of CVE-2021-47363 in the wild and no public exploit code.

Other vulnerabilities in Linux kernel

• CVE-2026-53266 Linux Kernel Out-of-Bounds Write Exploited High 8.8
• CVE-2013-6282 Linux Kernel Improper Input Validation Exploited High 8.8
• CVE-2022-0185 Linux Kernel Heap-Based Buffer Overflow Exploited High 8.4
• CVE-2013-2094 Linux Kernel Privilege Escalation Exploited High 8.4
• CVE-2026-53362 Linux Kernel Unspecified Exploited High 7.8

All Linux kernel vulnerabilities

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as DEBIAN-CVE-2021-47363, GHSA-5w2f-hr95-2h6j, UBUNTU-CVE-2021-47363.