Are there any good attack models of HMAC? Like how it can be attacked? If there are any possible attacks of HMAC I would be happy to know about it.
Beyond brute force and plain key leak, the only plausible attacks against HMAC are attacks on implementations.
One is: Sonia Belaïd, Luk Bettale, Emmanuelle Dottax, Laurie Genelle, Franck Rondepierre: Differential Power Analysis of HMAC SHA-2 in the Hamming Weight Model, in proceedings of SECRYPT 2013.
Another is a generic key search using fault injection, which finds an $n$-bit MAC key in $n+1$ queries (regardless of algorithm), assuming a device computes and outputs the (H)MAC of a fixed message; that device copies the key from long term storage (e.g. Flash) to RAM sequentially at each MAC computation; and (that's the difficult point) an attacker can replace a bit of their choice(s) with zero during that transfer (e.g. by laser illumination of the right location at the right instant in the execution). To find a key bit, the attacker compares the MAC with and without the replacement.
HMAC as message authentication code should be resistant to existential forgery under chosen message attack, this is known as EUF-CMA.
HMAC as a pseudo-random function should be resistant to distinguishing attack under, again, chosen message attack, which is known as IND-CMA.
I'm quite certain about the MAC part, but I'm not absolutely confident about PRF part. I don't know good references on this, feel free to add to here.