bpf: Mark bpf prog stack with kmsan_unposion_memory in interpreter mode

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

bpf: Mark bpf prog stack with kmsan_unposion_memory in interpreter mode

syzbot reported uninit memory usages during map_{lookup,delete}_elem.

==========

BUG: KMSAN: uninit-value in __dev_map_lookup_elem kernel/bpf/devmap.c:441 [inline]

BUG: KMSAN: uninit-value in dev_map_lookup_elem+0xf3/0x170 kernel/bpf/devmap.c:796

__dev_map_lookup_elem kernel/bpf/devmap.c:441 [inline]

dev_map_lookup_elem+0xf3/0x170 kernel/bpf/devmap.c:796

____bpf_map_lookup_elem kernel/bpf/helpers.c:42 [inline]

bpf_map_lookup_elem+0x5c/0x80 kernel/bpf/helpers.c:38

___bpf_prog_run+0x13fe/0xe0f0 kernel/bpf/core.c:1997

__bpf_prog_run256+0xb5/0xe0 kernel/bpf/core.c:2237

==========

The reproducer should be in the interpreter mode.

The C reproducer is trying to run the following bpf prog:

0: (18) r0 = 0x0

2: (18) r1 = map[id:49]

4: (b7) r8 = 16777216

5: (7b) *(u64 *)(r10 -8) = r8

6: (bf) r2 = r10

7: (07) r2 += -229

^^^^^^^^^^

8: (b7) r3 = 8

9: (b7) r4 = 0

10: (85) call dev_map_lookup_elem#1543472

11: (95) exit

It is due to the "void *key" (r2) passed to the helper. bpf allows uninit

stack memory access for bpf prog with the right privileges. This patch

uses kmsan_unpoison_memory() to mark the stack as initialized.

This should address different syzbot reports on the uninit "void *key"

argument during map_{lookup,delete}_elem.