In the Linux kernel, the following vulnerability has been resolved:
bpf: Reject non-scalar bpf_loop iteration counts
bpf_loop() declares its nr_loops argument as ARG_ANYTHING. Privileged
programs may pass pointer values to such arguments, so check_func_arg()
lets a pointer-valued R1 reach the helper-specific checks.
Since commit bb124da69c47 (“bpf: keep track of max number of bpf_loop
callback iterations”), the verifier marks R1 precise and reads its upper
bound to limit callback simulation. Precision backtracking only accepts
scalar registers, so passing a pointer instead triggers the “backtracking
misuse” verifier warning. Kernels with panic_on_warn enabled subsequently
panic.
Introduce ARG_SCALAR for helper arguments that only accept scalar values
and use it for bpf_loop() nr_loops. Generic helper argument validation then
rejects pointers before loop inlining and precision processing.
