Bug 2426225 (CVE-2022-50816)

Summary: CVE-2022-50816 kernel: ipv6: ensure sane device mtu in tunnels
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedKeywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in the Linux kernel’s IPv6 networking code affecting the handling of IPv6 GRE tunnels. Under certain conditions, an IPv6 tunnel configuration could result in an invalid MTU (Maximum Transmission Unit) value being written to a network device without sanitization. Because the MTU value was not validated before being published to device state, concurrent or lockless readers in the IPv6 multicast code path could observe malformed MTU values, potentially leading to a kernel BUG or panic and resulting in loss of network connectivity or system instability. A local user with privileges to create or configure IPv6 GRE tunnels may trigger this condition
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Description OSIDB Bzimport 2025-12-30 13:17:43 UTC 
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 ip6gre_tnl_link_config_route(),
ip6_tnl_link_config() and ipip6_tunnel_bind_dev()

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: skb_over_panic: 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 mld_ifc_work
pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
pc : skb_panic+0x4c/0x50 net/core/skbuff.c:116
lr : skb_panic+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:
skb_panic+0x4c/0x50 net/core/skbuff.c:116
skb_over_panic net/core/skbuff.c:125 [inline]
skb_put+0xd4/0xdc net/core/skbuff.c:2049
ip6_mc_hdr net/ipv6/mcast.c:1714 [inline]
mld_newpack+0x14c/0x270 net/ipv6/mcast.c:1765
add_grhead net/ipv6/mcast.c:1851 [inline]
add_grec+0xa20/0xae0 net/ipv6/mcast.c:1989
mld_send_cr+0x438/0x5a8 net/ipv6/mcast.c:2115
mld_ifc_work+0x38/0x290 net/ipv6/mcast.c:2653
process_one_work+0x2d8/0x504 kernel/workqueue.c:2289
worker_thread+0x340/0x610 kernel/workqueue.c:2436
kthread+0x12c/0x158 kernel/kthread.c:376
ret_from_fork+0x10/0x20 arch/arm64/kernel/entry.S:860
Code: 91011400 aa0803e1 a90027ea 94373093 (d4210000)
Comment 1 Alexander B 2025-12-31 00:03:40 UTC 
Upstream advisory:
https://lore.kernel.org/linux-cve-announce/2025123014-CVE-2022-50816-3fda@gregkh/T