ipv4: stop exception dump when nexthop group shrinks

Overview

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

ipv4: stop exception dump when nexthop group shrinks

Commit 4ce5dc9316de ("inet: switch inet_dump_fib() to RCU protection") allowed IPv4 FIB dumps to run concurrently with nexthop replacement.

fib_dump_info_fnhe() uses fib_info_num_path() as the loop bound, then fib_info_nhc() independently reloads nh->nh_grp. If RTM_NEWNEXTHOP replaces a group with fewer paths between those loads, fib_info_nhc() returns NULL and the dump dereferences it through nhc_flags.

Stop the walk when the indexed path is absent. Nexthop groups are dense, so the current snapshot has no later path.

Affected

• Linux >= 4ce5dc9316de50e0da84beafe55d1344f829cece < f91e472821e32f58b03eb631e20a67157e2b928b
• Linux >= 4ce5dc9316de50e0da84beafe55d1344f829cece < 21cba134c30c17ae55f468b5809336437df5d7e6
• Linux >= 4ce5dc9316de50e0da84beafe55d1344f829cece < b9a30bb7b831e8c5a47fadbdf1760918aaf009cc
• Linux >= 4ce5dc9316de50e0da84beafe55d1344f829cece < 4f4afc07dcb2edd4f9afa36595373f8cdb9b58c2
• Linux 6.9

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.