UBUNTU-CVE-2026-98065: Medium severity ubuntu/linux-hwe-edge vulnerability

In the Linux kernel, the following vulnerability has been resolved: bpf: Reject key-less BTF for hash maps mapcheckbtf() allows a key-less BTF (btfkeytypeid == 0) only for maps that have a ->mapcheckbtf callback, and leaves the actual decision to that callback. Hash maps used to have no ->mapcheckbtf, so a key-less BTF was rejected outright. That changed when htab and rhtab gained a ->mapcheckbtf to register a dtor - htab in commit 1df97a7453ee ("bpf: Register dtor for freeing special fields") and rhtab in commit 6905f8601298 ("bpf: Allow special fields in resizable hashtab"). Neither looks at the key, so a key-less hash map now passes mapcheckbtf() and gets created. Reading it back through bpffs feeds the key typeid 0 into btftypeseqshow(); btftypebyid() returns the void type, kindops[BTFKINDUNKN] is NULL, and btftypeshow() dereferences it: RIP: 0010:btftypeshow+0x223/0x2e0 kernel/bpf/btf.c:8232 RSP: 0018:ffffc9000399f868 EFLAGS: 00010206 RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000 RDX: 0000000000000005 RSI: 0000000000000000 RDI: 0000000000000028 RBP: 0000000000000000 R08: 0000000000000001 R09: 0000000000000000 R10: ffffc9000399f970 R11: 0000000000000001 R12: ffffffff9b96b140 R13: ffffc9000399f8e0 R14: ffff88803d393c00 R15: 0000000000000003 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 0000200000000000 CR3: 000000003d213000 CR4: 0000000000352ef0 DR0: 0000000039ae8f55 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400 Call Trace: <TASK> btftypeseqshowflags+0xca/0x120 kernel/bpf/btf.c:8250 htabmapseqshowelem+0x12e/0x350 kernel/bpf/hashtab.c:1669 mapseqshow+0x13d/0x1e0 kernel/bpf/inode.c:293 traverse.part.0.constprop.0+0x107/0x650 fs/seqfile.c:112 traverse fs/seqfile.c:99 [inline] seqreaditer+0x93f/0x1270 fs/seqfile.c:196 seqread+0x344/0x4d0 fs/seqfile.c:163 vfsread+0x1e4/0xb40 fs/readwrite.c:572 ksyspread64 fs/readwrite.c:764 [inline] dosyspread64 fs/readwrite.c:772 [inline] sesyspread64 fs/readwrite.c:769 [inline] x64syspread64+0x1eb/0x250 fs/readwrite.c:769 dosyscallx64 arch/x86/entry/syscall64.c:61 [inline] dosyscall64+0x123/0x790 arch/x86/entry/syscall64.c:84 entrySYSCALL64afterhwframe+0x77/0x7f Reject a key-less BTF in htabmapcheckbtf() and rhtabmapcheckbtf(), restoring the previous behavior.