Is it possible for an SU-25 to fly high enough to shoot down a Boeing 777?
There is an excellent answer at http://scottlocklin.wordpress.com/2014/07/21/can-the-su-25-intercept-and-shoot-down-a-777/


The SU-25 is a ground attack fighter unsuited to intercepting and/or shooting down 777s. If the Ukrainian air force wanted to do so, they have better aircraft available. 
The SU-25 max speed is slower than a cruising 777.
The SU-25 has an operational ceiling of 16000 ft with max weapons to 23000 ft clean, MH17 was at 33000 ft.
It is an unpressurised aircraft.
As a ground attack aircraft it probably only rarely carries air to air missiles. It can carry the R-60 infra-red air-to-air missile which has a 6 lb warhead effective against small aircraft. 


An R-60 once hit and destroyed one engine of a small BAE 125 business jet at 35000 ft but failed to bring it down, apparently the missile failed to explode.
KAL902, a Boeing 707, was hit by an R-60 and made an emergency landing 40 minutes later.

The R-60 is a short-range missile (5 miles). The Russians claim the SU-25 came within two or three miles of MH17.


On the whole this makes the SU-25 a poor choice for shooting down a cruising 777. 

It seems unlikely that a 777 would be shot down in this way. Not impossible, but far less likely than other proposed scenarios such as those involving BUK (SA-11) ground to air missiles.


Results of an R-60 hitting a jet engine on a small business jet.


Results of an R-60 hitting a Boeing 707 ... two R-60s were launched, one missed.

These R-60s were fired by supersonic interceptors, not by SU-25 ground-attack aircraft.