Short answer. All the owners, pro rata. Article 490 of the Civil Code says that if the titles do not specify contribution terms and there is no agreement, the entrance floor, front door, common yard, and sanitary works common to all shall be maintained at the expense of all the story owners pro rata.
What the law says
if the titles of ownership do not specify the terms under which they should contribute to the necessary expenses and there exists no agreement on the subject, the following rules shall be observed
Civil Code, Article 490 — Storeys Owned Separately. Read the full provision →
What the law says
the floor of the entrance, front door, common yard and sanitary works common to all, shall be maintained at the expense of all the owners pro rata
Civil Code, Article 490 — Storeys Owned Separately. Read the full provision →
When Article 490's default rules even apply
Article 490 is a fallback provision, not a rule that overrides what the owners have already agreed. If the titles of ownership do not specify the terms under which they should contribute to the necessary expenses and there exists no agreement on the subject, the following rules shall be observed. Its rules on who pays for what only kick in when the titles are silent and no separate agreement exists.
The rule for the entrance hallway and sanitary works
Assuming that default situation applies, Article 490 answers your question directly: the floor of the entrance, front door, common yard and sanitary works common to all, shall be maintained at the expense of all the owners pro rata. The shared entrance hallway and sanitary pipes common to everyone fall under this pro rata sharing rule, with each owner contributing a proportionate share of the upkeep cost.
How this differs from each owner's own floor
Article 490 separately makes each owner responsible for maintaining the floor of their own story, distinct from the shared spaces. The entrance, front door, common yard, and sanitary works are treated differently precisely because they serve everyone in the building, not just one owner's individual unit, which is why the cost of upkeep is spread pro rata rather than assigned entirely to a single owner.
Why 'pro rata' matters, and what else the article covers
The article uses pro rata specifically for the entrance-related items, a different formula from the one it applies to main and party walls, roofs, and other jointly used elements, which are shared in proportion to the value of each owner's story instead. Article 490 also has separate rules for stairs, apportioned differently among owners depending on which stories the stairs actually serve, so it is worth reading the full article rather than assuming one formula covers every shared element in the building.