bpf: Check load-acquire src ptr type before the load

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

bpf: Check load-acquire src ptr type before the load

check_atomic_load() calls check_load_mem() before atomic_ptr_type_ok().
For a load-acquire that fetches into its own source register (dst_reg ==
src_reg), check_load_mem() overwrites src_reg's type with the type of the
loaded value, so the subsequent atomic_ptr_type_ok() no longer sees the
source pointer and fails to reject the disallowed types (ctx, pkt,
flow_keys, sock).

Since bpf_convert_ctx_accesses() does not rewrite atomic loads, the raw
access to the underlying kernel object is left in place. The destination
type is taken from the ctx access itself, so a load-acquire of the sk
field of struct __sk_buff for example leaves the register typed as
PTR_TO_SOCK_COMMON_OR_NULL, which type_is_sk_pointer() does not match
either, while it actually holds unconverted struct sk_buff bytes. Once
the NULL check has passed this is a type confusion, not just a leak of
kernel data.

Validate src_reg with check_reg_arg() and check the source pointer type
with atomic_ptr_type_ok() before the load again, mirroring
check_atomic_rmw(). Out-of-range register numbers are already rejected
earlier by check_and_resolve_insns() (commit 503d21ef8eac ("bpf: Do
register range validation early")), and the only exemption there,
is_stack_arg_ldx(), requires BPF_LDX | BPF_MEM | BPF_DW and thus never
matches a BPF_ATOMIC insn. atomic_ptr_type_ok() can therefore not
dereference register state out of bounds, that is, the out-of-bounds
read addressed by the Fixes commit below does not reappear (as proven
also via selftest).