In the Linux kernel, the following vulnerability has been resolved:
inet: frags: invalidate queues before flushing them
fqdir_pre_exit() flushes the skbs from incomplete queues without
changing their completion state. A fragment which found a queue before
high_thresh was cleared can then acquire the queue lock and reuse stale
reassembly metadata. A queue concurrently killed after fqdir->dead is
set can instead become INET_FRAG_COMPLETE|INET_FRAG_HASH_DEAD while
still holding its old skbs; skipping it because it is complete leaves
those references behind until asynchronous fqdir teardown.
For IPv6, stale metadata can make ip6_frag_reasm() use the old
nhoffset with a new skb and access memory out of bounds. The resulting
heap corruption can be leveraged for local privilege escalation when
unprivileged network namespaces are available. Unflushed fragments can
also keep conntrack references alive after the conntrack per-net
cleanup point.
Kill each incomplete queue, then flush every queue still owned by the
dying rhashtable. HASH_DEAD identifies that ownership, while complete
queues without it are already owned by another destroy path and must be
left alone. Releasing a timer reference removed by inet_frag_kill() is
deferred to inet_frag_putn(), after the queue lock is dropped.
KASAN report:
BUG: KASAN: slab-out-of-bounds in ipv6_frag_rcv (net/ipv6/reassembly.c:289 (discriminator 2) net/ipv6/reassembly.c:229 (discriminator 2) net/ipv6/reassembly.c:391 (discriminator 2))
Write of size 1 at addr ff110001039c6e00 by task poc/771
Call Trace:
? ipv6_frag_rcv (net/ipv6/reassembly.c:289 (discriminator 2) net/ipv6/reassembly.c:229 (discriminator 2) net/ipv6/reassembly.c:391 (discriminator 2))
ipv6_frag_rcv (net/ipv6/reassembly.c:289 (discriminator 2) net/ipv6/reassembly.c:229 (discriminator 2) net/ipv6/reassembly.c:391 (discriminator 2))
ip6_protocol_deliver_rcu (net/ipv6/ip6_input.c:479 (discriminator 5))
ip6_input_finish (net/ipv6/ip6_input.c:534)
ipv6_rcv (include/net/dst.h:480 (discriminator 3) net/ipv6/ip6_input.c:119 (discriminator 3) net/ipv6/ip6_input.c:109 (discriminator 3) include/linux/netfilter.h:325 (discriminator 3) include/linux/netfilter.h:319 (discriminator 3) net/ipv6/ip6_input.c:351 (discriminator 3))
packet_sendmsg (net/packet/af_packet.c:3110 net/packet/af_packet.c:3142)
__x64_sys_sendmmsg (net/socket.c:2883 net/socket.c:2880 net/socket.c:2880)
The buggy address belongs to the object at ff110001039c6b40
which belongs to the cache skbuff_small_head of size 704
The buggy address is located 0 bytes to the right of
allocated 704-byte region [ff110001039c6b40, ff110001039c6e00)
BUG: KASAN: slab-out-of-bounds in ip6_protocol_deliver_rcu (net/ipv6/ip6_input.c:423 (discriminator 1))
Read of size 1 at addr ff110001039c6e08 by task poc/771
Call Trace:
? ip6_protocol_deliver_rcu (net/ipv6/ip6_input.c:423 (discriminator 1))
ip6_protocol_deliver_rcu (net/ipv6/ip6_input.c:423 (discriminator 1))
ip6_input_finish (net/ipv6/ip6_input.c:534)
ipv6_rcv (include/net/dst.h:480 (discriminator 3) net/ipv6/ip6_input.c:119 (discriminator 3) net/ipv6/ip6_input.c:109 (discriminator 3) include/linux/netfilter.h:325 (discriminator 3) include/linux/netfilter.h:319 (discriminator 3) net/ipv6/ip6_input.c:351 (discriminator 3))
packet_sendmsg (net/packet/af_packet.c:3110 net/packet/af_packet.c:3142)
__x64_sys_sendmmsg (net/socket.c:2883 net/socket.c:2880 net/socket.c:2880)
packet_sendmsg (net/packet/af_packet.c:2959 net/packet/af_packet.c:3053 net/packet/af_packet.c:3142)
__x64_sys_sendmmsg (net/socket.c:2883 net/socket.c:2880 net/socket.c:2880)
The buggy address belongs to the object at ff110001039c6b40
which belongs to the cache skbuff_small_head of size 704
The buggy address is located 8 bytes to the right of
allocated 704-byte region [ff110001039c6b40, ff110001039c6e00)
CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
CVSS Score: 7.8
AV:L - fqdir_pre_exit() only runs from the pernet pre_exit hooks (ipv6_frags_pre_exit_net, ipv4_frags_pre_exit_net, nf_ct_net_pre_exit) while a network namespace is being torn down. A remote sender cannot tear down a netns. The reporter's KASAN trace injects the fragments locally with packet_sendmsg into ipv6_rcv inside the attacker's own namespace.
AC:L - The attacker controls both sides of the race. They send IPv6 fragments into their own netns from threads, then destroy that netns by dropping its last reference. A fragment that found the queue in inet_frag_find() before high_thresh was zeroed then takes fq->lock after the flush, so the attacker can retry this at will.
PR:L - Creating a network namespace and opening an AF_PACKET socket in it both need CAP_NET_RAW or CAP_NET_ADMIN, which unshare -Urn grants to an unprivileged user. The fix commit itself says this path is exploitable when unprivileged network namespaces are available.
UI:N - The attacker creates the namespace, sends the fragments and triggers the teardown. No other user has to do anything.
S:U - This is kernel heap corruption that leads to local privilege escalation inside the same kernel. No VM, IOMMU or other authority boundary is crossed.
C:H - ip6_frag_reasm() reuses a stale fq->nhoffset with a new skb. KASAN shows a slab out-of-bounds read past a 704-byte skbuff_small_head object, and this heap corruption can be shaped into memory disclosure.
I:H - skb_network_header(skb)[nhoff] = ... in ip6_frag_reasm() writes at the stale nhoffset. KASAN shows it writing past the end of an skbuff_small_head allocation, and the fix commit states this out-of-bounds write can be leveraged for privilege escalation.
A:H - The out-of-bounds write corrupts the kernel heap and can crash the host. Unflushed fragments also keep conntrack references alive past the per-net cleanup point, so the whole machine can go down.
| Attack Vector |
Local |
Scope |
Unchanged |
| Attack Complexity |
Low |
Confidentiality Impact |
High |
| Privileges Required |
Low |
Integrity Impact |
High |
| User Interaction |
None |
Availability Impact |
High |
AV:L - fqdir_pre_exit() only runs from the pernet pre_exit hooks (ipv6_frags_pre_exit_net, ipv4_frags_pre_exit_net, nf_ct_net_pre_exit) while a network namespace is being torn down. A remote sender cannot tear down a netns. The reporter's KASAN trace injects the fragments locally with packet_sendmsg into ipv6_rcv inside the attacker's own namespace.
AC:L - The attacker controls both sides of the race. They send IPv6 fragments into their own netns from threads, then destroy that netns by dropping its last reference. A fragment that found the queue in inet_frag_find() before high_thresh was zeroed then takes fq->lock after the flush, so the attacker can retry this at will.
PR:L - Creating a network namespace and opening an AF_PACKET socket in it both need CAP_NET_RAW or CAP_NET_ADMIN, which unshare -Urn grants to an unprivileged user. The fix commit itself says this path is exploitable when unprivileged network namespaces are available.
UI:N - The attacker creates the namespace, sends the fragments and triggers the teardown. No other user has to do anything.
S:U - This is kernel heap corruption that leads to local privilege escalation inside the same kernel. No VM, IOMMU or other authority boundary is crossed.
C:H - ip6_frag_reasm() reuses a stale fq->nhoffset with a new skb. KASAN shows a slab out-of-bounds read past a 704-byte skbuff_small_head object, and this heap corruption can be shaped into memory disclosure.
I:H - skb_network_header(skb)[nhoff] = ... in ip6_frag_reasm() writes at the stale nhoffset. KASAN shows it writing past the end of an skbuff_small_head allocation, and the fix commit states this out-of-bounds write can be leveraged for privilege escalation.
A:H - The out-of-bounds write corrupts the kernel heap and can crash the host. Unflushed fragments also keep conntrack references alive past the per-net cleanup point, so the whole machine can go down.
CVSS 3.1