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

Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.

Affected Software

Extended Description

The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:

In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.

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