bpf: Fix out of bounds access from invalid *_or_null type verification

[ no upstream commit given implicitly fixed through the larger refactoring
  in c25b2ae1 ]

While auditing some other code, I noticed missing checks inside the pointer
arithmetic simulation, more specifically, adjust_ptr_min_max_vals(). Several
*_OR_NULL types are not rejected whereas they are _required_ to be rejected
given the expectation is that they get promoted into a 'real' pointer type
for the success case, that is, after an explicit != NULL check.

One case which stands out and is accessible from unprivileged (iff enabled
given disabled by default) is BPF ring buffer. From crafting a PoC, the NULL
check can be bypassed through an offset, and its id marking will then lead
to promotion of mem_or_null to a mem type.

bpf_ringbuf_reserve() helper can trigger this case through passing of reserved
flags, for example.

  func#0 @0
  0: R1=ctx(id=0,off=0,imm=0) R10=fp0
  0: (7a) *(u64 *)(r10 -8) = 0
  1: R1=ctx(id=0,off=0,imm=0) R10=fp0 fp-8_w=mmmmmmmm
  1: (18) r1 = 0x0
  3: R1_w=map_ptr(id=0,off=0,ks=0,vs=0,imm=0) R10=fp0 fp-8_w=mmmmmmmm
  3: (b7) r2 = 8
  4: R1_w=map_ptr(id=0,off=0,ks=0,vs=0,imm=0) R2_w=invP8 R10=fp0 fp-8_w=mmmmmmmm
  4: (b7) r3 = 0
  5: R1_w=map_ptr(id=0,off=0,ks=0,vs=0,imm=0) R2_w=invP8 R3_w=invP0 R10=fp0 fp-8_w=mmmmmmmm
  5: (85) call bpf_ringbuf_reserve#131
  6: R0_w=mem_or_null(id=2,ref_obj_id=2,off=0,imm=0) R10=fp0 fp-8_w=mmmmmmmm refs=2
  6: (bf) r6 = r0
  7: R0_w=mem_or_null(id=2,ref_obj_id=2,off=0,imm=0) R6_w=mem_or_null(id=2,ref_obj_id=2,off=0,imm=0) R10=fp0 fp-8_w=mmmmmmmm refs=2
  7: (07) r0 += 1
  8: R0_w=mem_or_null(id=2,ref_obj_id=2,off=1,imm=0) R6_w=mem_or_null(id=2,ref_obj_id=2,off=0,imm=0) R10=fp0 fp-8_w=mmmmmmmm refs=2
  8: (15) if r0 == 0x0 goto pc+4
   R0_w=mem(id=0,ref_obj_id=0,off=0,imm=0) R6_w=mem(id=0,ref_obj_id=2,off=0,imm=0) R10=fp0 fp-8_w=mmmmmmmm refs=2
  9: R0_w=mem(id=0,ref_obj_id=0,off=0,imm=0) R6_w=mem(id=0,ref_obj_id=2,off=0,imm=0) R10=fp0 fp-8_w=mmmmmmmm refs=2
  9: (62) *(u32 *)(r6 +0) = 0
   R0_w=mem(id=0,ref_obj_id=0,off=0,imm=0) R6_w=mem(id=0,ref_obj_id=2,off=0,imm=0) R10=fp0 fp-8_w=mmmmmmmm refs=2
  10: R0_w=mem(id=0,ref_obj_id=0,off=0,imm=0) R6_w=mem(id=0,ref_obj_id=2,off=0,imm=0) R10=fp0 fp-8_w=mmmmmmmm refs=2
  10: (bf) r1 = r6
  11: R0_w=mem(id=0,ref_obj_id=0,off=0,imm=0) R1_w=mem(id=0,ref_obj_id=2,off=0,imm=0) R6_w=mem(id=0,ref_obj_id=2,off=0,imm=0) R10=fp0 fp-8_w=mmmmmmmm refs=2
  11: (b7) r2 = 0
  12: R0_w=mem(id=0,ref_obj_id=0,off=0,imm=0) R1_w=mem(id=0,ref_obj_id=2,off=0,imm=0) R2_w=invP0 R6_w=mem(id=0,ref_obj_id=2,off=0,imm=0) R10=fp0 fp-8_w=mmmmmmmm refs=2
  12: (85) call bpf_ringbuf_submit#132
  13: R6=invP(id=0) R10=fp0 fp-8=mmmmmmmm
  13: (b7) r0 = 0
  14: R0_w=invP0 R6=invP(id=0) R10=fp0 fp-8=mmmmmmmm
  14: (95) exit

  from 8 to 13: safe
  processed 15 insns (limit 1000000) max_states_per_insn 0 total_states 1 peak_states 1 mark_read 0
  OK

All three commits, that is b121b341 ("bpf: Add PTR_TO_BTF_ID_OR_NULL support"),
457f4436 ("bpf: Implement BPF ring buffer and verifier support for it"), and the
afbf21dc ("bpf: Support readonly/readwrite buffers in verifier") suffer the same
cause and their *_OR_NULL type pendants must be rejected in adjust_ptr_min_max_vals().

Make the test more robust by reusing reg_type_may_be_null() helper such that we catch
all *_OR_NULL types we have today and in future.

Note that pointer arithmetic on PTR_TO_BTF_ID, PTR_TO_RDONLY_BUF, and PTR_TO_RDWR_BUF
is generally allowed.

Fixes: b121b341 ("bpf: Add PTR_TO_BTF_ID_OR_NULL support")
Fixes: 457f4436 ("bpf: Implement BPF ring buffer and verifier support for it")
Fixes: afbf21dc ("bpf: Support readonly/readwrite buffers in verifier")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>