Can salicylic acid be synthesized from salicylaldehyde?
There are a large number of reagents out there that will transform aldehydes into the corresponding acids (benzaldehydes and benzoic acids in this case) and $\ce{KMnO4}$ is indeed one of them.

Oxidation of the phenolic hydroxide is only possible if at the same time the aromatic system is destroyed. However, it is rather hard to come up with a good product of this oxidation since a simple quinone isn’t possible. It therefore takes much harsher conditions than the oxidation of an aldehyde.

Things would change if a second hydroxy group were attached to the same aromatic system either ortho or para to the first. Then the oxidation to the corresponding quinone is much easier.