bpf: fix check for attempt to corrupt spilled pointer

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: bpf: fix check for attempt to corrupt spilled pointer When register is spilled onto a stack as a 1/2/4-byte register, we set slot_type[BPF_REG_SIZE - 1] (plus potentially few more below it, depending on actual spill size). So to check if some stack slot has spilled register we need to consult slot_type[7], not slot_type[0]. To avoid the need to remember and double-check this in the future, just use is_spilled_reg() helper.

References

• https://git.kernel.org/stable/c/2757f17972d87773b3677777f5682510f13c66ef
• https://git.kernel.org/stable/c/67e6707f07354ed1acb4e65552e97c60cf9d69cf
• https://git.kernel.org/stable/c/fc3e3c50a0a4cac1463967c110686189e4a59104
• https://git.kernel.org/stable/c/8dc15b0670594543c356567a1a45b0182ec63174
• https://git.kernel.org/stable/c/40617d45ea05535105e202a8a819e388a2b1f036
• https://git.kernel.org/stable/c/ab125ed3ec1c10ccc36bc98c7a4256ad114a3dae