net: ipv6: keep room for the mac header in dst_dev_overhead()

Overview

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

net: ipv6: keep room for the mac header in dst_dev_overhead()

The seg6, ioam6 and rpl lwtunnels size their skb_cow_head() request as the length they are about to push plus dst_dev_overhead(), then push the new headers and rebuild the mac header below them with skb_mac_header_rebuild(). That rebuild needs skb->mac_len of headroom, but dst_dev_overhead() leaves LL_RESERVED_SPACE() of the egress device, 16 bytes for plain Ethernet.

Where the mac header is longer than that, as it is on ingress through a VLAN device with reorder_hdr off, the rebuild runs out of room: skb_set_mac_header(skb, -skb->mac_len) computes a negative offset, stores it unchecked in the u16 skb->mac_header, and the memmove that follows writes skb->mac_len bytes about 64 KB past skb->head. Forwarding plain ping6 traffic through such a device reproduces it on all five seg6 encapsulation modes and on the rpl and ioam6 inline paths; skb->mac_header comes back as 65534 on a 704-byte head.

Return the larger of the two. The helper already returns skb->mac_len when it has no dst, so this only makes the other branch agree, and it covers every caller rather than each call site in turn.

Affected

• Linux >= dce525185bc92864e5a318040285ee070563fe34 < 36c07127924f11fc5741a780abcaaf808fe9ef13
• Linux >= dce525185bc92864e5a318040285ee070563fe34 < 87cd6b717e4069dca34e0866e57eaeb44d3b173e
• Linux 8471e228448a818e4c27cab4b681b2e758e46cf5
• Linux c432cd18a07b623651ed15ea167bd303467004cc
• Linux 634710a372ba65bee667c2f6e58a97658c639eab
• Linux c447c5a9c47bb4858ab12bf90879fe595d2b7f46
• Linux ab9de9c48316d2622d3bf353916b99971570e2c9
• Linux 2fcc9f3798ef82399564d247458e69abd10e683b
• Linux 371d8dc298b4f59e99b4564ac5d906fce0932c69
• Linux b26f015dfb07b6be12d005a0ed0c5c43b7bedbf3
• Linux c0b11dc50590e10590b39dcda79835cd32f49d00
• Linux 4c12c3c8abd8b888945d31e7bdbb0d8b73032fc5
• Linux 25d8e065c7972dc98fd7a4cac322d5367eaede88
• Linux 4a855c5aec41bf84922224ab7a74e088776e3ac4
• Linux a9f0407414e16176fdbbbd298c9e18d6dfdf96bc
• Linux >= 6.12.16 < 6.13
• Linux >= 6.13.4 < 6.14
• Linux >= 5.15.179 < 5.16
• Linux >= 6.1.130 < 6.2
• Linux >= 6.6.81 < 6.7
• Linux >= 5.10.235 < 5.11
• Linux 6.14

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.