CVE-2021-47162

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

tipc: skb_linearize the head skb when reassembling msgs

It's not a good idea to append the frag skb to a skb's fraglist if the fraglist already has skbs from elsewhere, such as this skb was created by pskbcopy() where the fraglist was cloned (all the skbs in it were skb_get'ed) 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 pskbmaypull() 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: [] skbclone+0x4d/0xb0 [] macvlanbroadcast+0xd8/0x160 [macvlan] [] macvlanprocessbroadcast+0x148/0x150 [macvlan] [] processonework+0x1a7/0x360 [] worker_thread+0x30/0x390

[] kernel BUG at mm/usercopy.c:102! [] Call Trace: [] checkheapobject+0xd3/0x100 [] _checkobjectsize+0xff/0x16b [] simplecopytoiter+0x1c/0x30 [] _skbdatagramiter+0x7d/0x310 [] _skbdatagramiter+0x2a5/0x310 [] skbcopydatagramiter+0x3b/0x90 [] tipcrecvmsg+0x14a/0x3a0 [tipc] [] sysrecvmsg+0x91/0x150 [] _sysrecvmsg+0x7b/0xc0

[] kernel BUG at mm/slub.c:305! [] Call Trace: [] <IRQ> [] kmemcachefree+0x3ff/0x400 [] _netifreceiveskbcore+0x12c/0xc40 [] ? kmemcachealloc+0x12e/0x270 [] netifreceiveskbinternal+0x3d/0xb0 [] ? getrxpageinfo+0x8e/0xa0 [be2net] [] bepoll+0x6ef/0xd00 [be2net] [] ? irqexit+0x4f/0x100 [] netrxaction+0x149/0x3b0

...

This patch is to fix it by linearizing the head skb if it has fraglist set in tipcbufappend(). Note that we choose to do this before calling skbunshare(), as _skblinearize() will avoid skbcopy(). Also, we can not just drop the fraglist either as the early time.