CVE-2025-39748

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

bpf: Forget ranges when refining tnum after JSET

Syzbot reported a kernel warning due to a range invariant violation on the following BPF program.

0: call bpf_get_netns_cookie 1: if r0 == 0 goto 2: if r0 & Oxffffffff goto

The issue is on the path where we fall through both jumps.

That path is unreachable at runtime: after insn 1, we know r0 != 0, but with the sign extension on the jset, we would only fallthrough insn 2 if r0 == 0. Unfortunately, is_branch_taken() isnt currently able to figure this out, so the verifier walks all branches. The verifier then refines the register bounds using the second condition and we end up with inconsistent bounds on this unreachable path:

1: if r0 == 0 goto r0: u64=[0x1, 0xffffffffffffffff] var_off=(0, 0xffffffffffffffff) 2: if r0 & 0xffffffff goto r0 before reg_bounds_sync: u64=[0x1, 0xffffffffffffffff] var_off=(0, 0) r0 after reg_bounds_sync: u64=[0x1, 0] var_off=(0, 0)

Improving the range refinement for JSET to cover all cases is tricky. We also dont expect many users to rely on JSET given LLVM doesnt generate those instructions. So instead of improving the range refinement for JSETs, Eduard suggested we forget the ranges whenever were narrowing tnums after a JSET. This patch implements that approach.

Affected Software

References

• https://git.kernel.org/stable/c/2fd0c26bacd90ef26522bd3169000a4715bf151f
• https://git.kernel.org/stable/c/6279846b9b2532e1b04559ef8bd0dec049f29383
• https://git.kernel.org/stable/c/80a6b11862a7cfdf691e8f9faee89cfea219f098
• https://git.kernel.org/stable/c/f01e06930444cab289a8783017af9b64255bd103