bpf: Don't promote bogus looking registers after null check.

If we ever get to a point again where we convert a bogus looking <ptr>_or_null
typed register containing a non-zero fixed or variable offset, then lets not
reset these bounds to zero since they are not and also don't promote the register
to a <ptr> type, but instead leave it as <ptr>_or_null. Converting to a unknown
register could be an avenue as well, but then if we run into this case it would
allow to leak a kernel pointer this way.

Fixes: f1174f77 ("bpf/verifier: rework value tracking")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -9079,15 +9079,15 @@ static void mark_ptr_or_null_reg(struct bpf_func_state *state,
 {
 	if (type_may_be_null(reg->type) && reg->id == id &&
 	    !WARN_ON_ONCE(!reg->id)) {
-		/* Old offset (both fixed and variable parts) should
-		 * have been known-zero, because we don't allow pointer
-		 * arithmetic on pointers that might be NULL.
-		 */
 		if (WARN_ON_ONCE(reg->smin_value || reg->smax_value ||
 				 !tnum_equals_const(reg->var_off, 0) ||
 				 reg->off)) {
-			__mark_reg_known_zero(reg);
-			reg->off = 0;
+			/* Old offset (both fixed and variable parts) should
+			 * have been known-zero, because we don't allow pointer
+			 * arithmetic on pointers that might be NULL. If we
+			 * see this happening, don't convert the register.
+			 */
+			return;
 		}
 		if (is_null) {
 			reg->type = SCALAR_VALUE;