CVE-2021-47394

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

netfilter: nf_tables: unlink table before deleting it

syzbot reports following UAF: BUG: KASAN: use-after-free in memcmp+0x18f/0x1c0 lib/string.c:955 nla_strcmp+0xf2/0x130 lib/nlattr.c:836 nft_table_lookup.part.0+0x1a2/0x460 net/netfilter/nf_tables_api.c:570 nft_table_lookup net/netfilter/nf_tables_api.c:4064 [inline] nf_tables_getset+0x1b3/0x860 net/netfilter/nf_tables_api.c:4064 nfnetlink_rcv_msg+0x659/0x13f0 net/netfilter/nfnetlink.c:285 netlink_rcv_skb+0x153/0x420 net/netlink/af_netlink.c:2504

Problem is that all get operations are lockless, so the commit_mutex held by nft_rcv_nl_event() isnt enough to stop a parallel GET request from doing read-accesses to the table object even after synchronize_rcu().

To avoid this, unlink the table first and store the table objects in on-stack scratch space.

Weakness

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.

Affected Software

Potential Mitigations

References

• https://git.kernel.org/stable/c/a499b03bf36b0c2e3b958a381d828678ab0ffc5e
• https://git.kernel.org/stable/c/f65c73d3aabb87d4353e0bc4a718b5ae8c43fd04
• https://git.kernel.org/stable/c/a499b03bf36b0c2e3b958a381d828678ab0ffc5e
• https://git.kernel.org/stable/c/f65c73d3aabb87d4353e0bc4a718b5ae8c43fd04