CVE-2022-50816

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

ipv6: ensure sane device mtu in tunnels

Another syzbot report [1] with no reproducer hints at a bug in ip6_gre tunnel (dev:ip6gretap0)

Since ipv6 mcast code makes sure to read dev->mtu once and applies a sanity check on it (see commit b9b312a7a451 "ipv6: mcast: better catch silly mtu values"), a remaining possibility is that a layer is able to set dev->mtu to an underflowed value (high order bit set).

This could happen indeed in ip6gretnllinkconfigroute(), ip6tnllinkconfig() and ipip6tunnelbinddev()

Make sure to sanitize mtu value in a local variable before it is written once on dev->mtu, as lockless readers could catch wrong temporary value.

[1] skbuff: skboverpanic: text:ffff80000b7a2f38 len:40 put:40 head:ffff000149dcf200 data:ffff000149dcf2b0 tail:0xd8 end:0xc0 dev:ip6gretap0 ------------[ cut here ]------------ kernel BUG at net/core/skbuff.c:120 Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP Modules linked in: CPU: 1 PID: 10241 Comm: kworker/1:1 Not tainted 6.0.0-rc7-syzkaller-18095-gbbed346d5a96 #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/30/2022 Workqueue: mld mldifcwork pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--) pc : skbpanic+0x4c/0x50 net/core/skbuff.c:116 lr : skbpanic+0x4c/0x50 net/core/skbuff.c:116 sp : ffff800020dd3b60 x29: ffff800020dd3b70 x28: 0000000000000000 x27: ffff00010df2a800 x26: 00000000000000c0 x25: 00000000000000b0 x24: ffff000149dcf200 x23: 00000000000000c0 x22: 00000000000000d8 x21: ffff80000b7a2f38 x20: ffff00014c2f7800 x19: 0000000000000028 x18: 00000000000001a9 x17: 0000000000000000 x16: ffff80000db49158 x15: ffff000113bf1a80 x14: 0000000000000000 x13: 00000000ffffffff x12: ffff000113bf1a80 x11: ff808000081c0d5c x10: 0000000000000000 x9 : 73f125dc5c63ba00 x8 : 73f125dc5c63ba00 x7 : ffff800008161d1c x6 : 0000000000000000 x5 : 0000000000000080 x4 : 0000000000000001 x3 : 0000000000000000 x2 : ffff0001fefddcd0 x1 : 0000000100000000 x0 : 0000000000000089 Call trace: skbpanic+0x4c/0x50 net/core/skbuff.c:116 skboverpanic net/core/skbuff.c:125 [inline] skbput+0xd4/0xdc net/core/skbuff.c:2049 ip6mchdr net/ipv6/mcast.c:1714 [inline] mldnewpack+0x14c/0x270 net/ipv6/mcast.c:1765 addgrhead net/ipv6/mcast.c:1851 [inline] addgrec+0xa20/0xae0 net/ipv6/mcast.c:1989 mldsendcr+0x438/0x5a8 net/ipv6/mcast.c:2115 mldifcwork+0x38/0x290 net/ipv6/mcast.c:2653 processonework+0x2d8/0x504 kernel/workqueue.c:2289 workerthread+0x340/0x610 kernel/workqueue.c:2436 kthread+0x12c/0x158 kernel/kthread.c:376 retfromfork+0x10/0x20 arch/arm64/kernel/entry.S:860 Code: 91011400 aa0803e1 a90027ea 94373093 (d4210000)