CVE-2021-47162

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

tipc: skb_linearize the head skb when reassembling msgs

Its not a good idea to append the frag skb to a skbs frag_list if the frag_list already has skbs from elsewhere, such as this skb was created by pskb_copy() where the frag_list was cloned (all the skbs in it were skb_geted) and shared by multiple skbs.

However, the new appended frag skb should have been only seen by the current skb. Otherwise, it will cause use after free crashes as this appended frag skb are seen by multiple skbs but it only got skb_get called once.

The same thing happens with a skb updated by pskb_may_pull() with a skb_cloned skb. Li Shuang has reported quite a few crashes caused by this when doing testing over macvlan devices:

[] kernel BUG at net/core/skbuff.c:1970! [] Call Trace: [] skb_clone+0x4d/0xb0 [] macvlan_broadcast+0xd8/0x160 [macvlan] [] macvlan_process_broadcast+0x148/0x150 [macvlan] [] process_one_work+0x1a7/0x360 [] worker_thread+0x30/0x390

[] kernel BUG at mm/usercopy.c:102! [] Call Trace: [] __check_heap_object+0xd3/0x100 [] __check_object_size+0xff/0x16b [] simple_copy_to_iter+0x1c/0x30 [] __skb_datagram_iter+0x7d/0x310 [] __skb_datagram_iter+0x2a5/0x310 [] skb_copy_datagram_iter+0x3b/0x90 [] tipc_recvmsg+0x14a/0x3a0 [tipc] [] ____sys_recvmsg+0x91/0x150 [] ___sys_recvmsg+0x7b/0xc0

[] kernel BUG at mm/slub.c:305! [] Call Trace: [] [] kmem_cache_free+0x3ff/0x400 [] __netif_receive_skb_core+0x12c/0xc40 [] ? kmem_cache_alloc+0x12e/0x270 [] netif_receive_skb_internal+0x3d/0xb0 [] ? get_rx_page_info+0x8e/0xa0 [be2net] [] be_poll+0x6ef/0xd00 [be2net] [] ? irq_exit+0x4f/0x100 [] net_rx_action+0x149/0x3b0

…

This patch is to fix it by linearizing the head skb if it has frag_list set in tipc_buf_append(). Note that we choose to do this before calling skb_unshare(), as __skb_linearize() will avoid skb_copy(). Also, we can not just drop the frag_list either as the early time.

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 Any operations using the original pointer are no longer valid because the memory “belongs” to the code that operates on the new pointer.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/436d650d374329a591c30339a91fa5078052ed1e
• https://git.kernel.org/stable/c/4b1761898861117c97066aea6c58f68a7787f0bf
• https://git.kernel.org/stable/c/5489f30bb78ff0dafb4229a69632afc2ba20765c
• https://git.kernel.org/stable/c/64d17ec9f1ded042c4b188d15734f33486ed9966
• https://git.kernel.org/stable/c/6da24cfc83ba4f97ea44fc7ae9999a006101755c
• https://git.kernel.org/stable/c/ace300eecbccaa698e2b472843c74a5f33f7dce8
• https://git.kernel.org/stable/c/b2c8d28c34b3070407cb1741f9ba3f15d0284b8b
• https://git.kernel.org/stable/c/b7df21cf1b79ab7026f545e7bf837bd5750ac026
• https://git.kernel.org/stable/c/436d650d374329a591c30339a91fa5078052ed1e
• https://git.kernel.org/stable/c/4b1761898861117c97066aea6c58f68a7787f0bf
• https://git.kernel.org/stable/c/5489f30bb78ff0dafb4229a69632afc2ba20765c
• https://git.kernel.org/stable/c/64d17ec9f1ded042c4b188d15734f33486ed9966
• https://git.kernel.org/stable/c/6da24cfc83ba4f97ea44fc7ae9999a006101755c
• https://git.kernel.org/stable/c/ace300eecbccaa698e2b472843c74a5f33f7dce8
• https://git.kernel.org/stable/c/b2c8d28c34b3070407cb1741f9ba3f15d0284b8b
• https://git.kernel.org/stable/c/b7df21cf1b79ab7026f545e7bf837bd5750ac026