bpf: Fix overflow of jump offset in constant blinding

Overview

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

bpf: Fix overflow of jump offset in constant blinding

bpf_jit_blind_insn() rewrites 'if rX op imm goto off' into:

AX = imm ^ rnd AX ^= rnd if rX op AX goto off

and subtracts 2 from off when the jump is backward. off is s16, so -32767 and -32768 wrap and the jump goes 32767 insns forward, past the end of the prog. The jump is inside the patch, hence bpf_adj_branches() doesn't check it. The kernel crashes when such prog runs with bpf_jit_harden=2.

Return -ERANGE in such case. bpf_jit_blind_constants() fails the same way when blinding of other insns moves a jump out of range.

Affected

• Linux >= 4f3446bb809f20ad56cadf712e6006815ae7a8f9 < 443328237113ee0ea515e506497334d871dbc2aa
• Linux >= 4f3446bb809f20ad56cadf712e6006815ae7a8f9 < ad139384ecddb6f3e7e3c3c12765aafae0e39670
• Linux 4.7

Remediation

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