powerpc64/bpf: Limit 'ldbrx' to processors compliant with ISA v2.06

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

powerpc64/bpf: Limit 'ldbrx' to processors compliant with ISA v2.06

Johan reported the below crash with test_bpf on ppc64 e5500:

test_bpf: #296 ALU_END_FROM_LE 64: 0x0123456789abcdef -> 0x67452301 jited:1

Oops: Exception in kernel mode, sig: 4 [#1]

BE PAGE_SIZE=4K SMP NR_CPUS=24 QEMU e500

Modules linked in: test_bpf(+)

CPU: 0 PID: 76 Comm: insmod Not tainted 5.14.0-03771-g98c2059e008a-dirty #1

NIP: 8000000000061c3c LR: 80000000006dea64 CTR: 8000000000061c18

REGS: c0000000032d3420 TRAP: 0700 Not tainted (5.14.0-03771-g98c2059e008a-dirty)

MSR: 0000000080089000 <EE,ME> CR: 88002822 XER: 20000000 IRQMASK: 0

<...>

NIP [8000000000061c3c] 0x8000000000061c3c

LR [80000000006dea64] .__run_one+0x104/0x17c [test_bpf]

Call Trace:

.__run_one+0x60/0x17c [test_bpf] (unreliable)

.test_bpf_init+0x6a8/0xdc8 [test_bpf]

.do_one_initcall+0x6c/0x28c

.do_init_module+0x68/0x28c

.load_module+0x2460/0x2abc

.__do_sys_init_module+0x120/0x18c

.system_call_exception+0x110/0x1b8

system_call_common+0xf0/0x210

--- interrupt: c00 at 0x101d0acc

<...>

---[ end trace 47b2bf19090bb3d0 ]---

Illegal instruction

The illegal instruction turned out to be 'ldbrx' emitted for

BPF_FROM_[L|B]E, which was only introduced in ISA v2.06. Guard use of

the same and implement an alternative approach for older processors.