CVE-2026-98044

Summary

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

bpf: Reject legacy packet loads from callbacks

check_ld_abs() models a failed BPF_LD_ABS or BPF_LD_IND in a subprogram as an implicit return with R0 set to zero. It calls prepare_func_exit() to explore this synthesized path.

When the load is reached directly from a synchronous callback, prepare_func_exit() enforces the callback return contract and marks R0 precise. R0 is not derived from a real instruction on this path, so precision backtracking reaches the callback call with R0 still requested and triggers the "callback unexpected regs" verifier bug. A privileged program loader can therefore cause a verifier warning and an -EFAULT BPF_PROG_LOAD.

These legacy packet-load instructions are deprecated. Reject them from callbacks rather than complicating their implicit-return model. Check all active frames before constructing the implicit return so nested static subprograms cannot hide the callback context.

Global functions are verified independently with a fresh frame zero, so an active-frame check cannot identify a global function called from a callback. Also check the complete subprogram call graph during stack-depth validation and reject a function containing a legacy load when any caller is a callback. This covers global and static descendants without making has_ld_abs transitive, preserving its per-function BTF return-type check. Ordinary uses outside callbacks remain supported.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/3484a99303912db62428494a9061212049027e57
• https://git.kernel.org/stable/c/bc489c0c9b8c86bd7fac42cfd1bb152f042fca56
• https://git.kernel.org/stable/c/e7d28823c662128caae63f14e16bd394916c139b

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