bpf: Skip invalid kfunc call in backtrack_insn

Overview

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

bpf: Skip invalid kfunc call in backtrack_insn

The verifier skips invalid kfunc call in check_kfunc_call(), which would be captured in fixup_kfunc_call() if such insn is not eliminated by dead code elimination. However, this can lead to the following warning in backtrack_insn(), also see [1]:

------------[ cut here ]------------ verifier backtracking bug WARNING: CPU: 6 PID: 8646 at kernel/bpf/verifier.c:2756 backtrack_insn kernel/bpf/verifier.c:2756 __mark_chain_precision kernel/bpf/verifier.c:3065 mark_chain_precision kernel/bpf/verifier.c:3165 adjust_reg_min_max_vals kernel/bpf/verifier.c:10715 check_alu_op kernel/bpf/verifier.c:10928 do_check kernel/bpf/verifier.c:13821 [inline] do_check_common kernel/bpf/verifier.c:16289 [...]

So make backtracking conservative with this by returning ENOTSUPP.

[1] https://lore.kernel.org/bpf/CACkBjsaXNceR8ZjkLG=dT3P=4A8SBsg0Z5h5PWLryF5=ghKq=g@mail.gmail.com/

Affected

• Linux >= e6ac2450d6dee3121cd8bbf2907b78a68a8a353d < 6e2fac197de2c4c041bdd8982cffb104689113f1
• Linux >= e6ac2450d6dee3121cd8bbf2907b78a68a8a353d < 74eec8266f37aff609db6a2f2b093e56a11c28c4
• Linux >= e6ac2450d6dee3121cd8bbf2907b78a68a8a353d < d3178e8a434b58678d99257c0387810a24042fb6
• Linux 5.13

Remediation

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