The process of creating reproducer programs for syzkaller bugs is automated, however it's not perfect, so syzkaller provides a few tools for executing and reproducing programs manually.
Crash logs created in manager workdir/crashes dir contain programs executed just before a crash. In parallel execution mode (when procs parameter in manager config is set to value larger than 1), program that caused the crash does not necessary immediately precedes it; the guilty program can be somewhere before. There are two tools that can help you identify and minimize the program that causes a crash: tools/syz-execprog and tools/syz-prog2c.
tools/syz-execprog executes a single syzkaller program or a set of programs in various modes (once or loop indefinitely; in threaded/collide mode (see below), with or without coverage collection). You can start by running all programs in the crash log in a loop to check that at least one of them indeed crashes kernel: ./syz-execprog -executor=./syz-executor -repeat=0 -procs=16 -cover=0 crash-log. Then try to identify the single program that causes the crash, you can test programs with ./syz-execprog -executor=./syz-executor -repeat=0 -procs=16 -cover=0 file-with-a-single-program.
Note: syz-execprog executes programs locally. So you need to copy syz-execprog and syz-executor into a VM with the test kernel and run it there.
Once you have a single program that causes the crash, try to minimize it by removing individual syscalls from the program (you can comment out single lines with # at the beginning of line), and by removing unnecessary data (e.g. replacing &(0x7f0000001000)="73656c6600" syscall argument with &(0x7f0000001000)=nil). You can also try to coalesce all mmap calls into a single mmap call that maps whole required area. Again, test minimization with syz-execprog tool.
Now that you have a minimized program, check if the crash still reproduces with ./syz-execprog -threaded=0 -collide=0 flags. If not, then you will need to do some additional work later.
Now, run syz-prog2c tool on the program. It will give you executable C source. If the crash reproduces with -threaded/collide=0 flags, then this C program should cause the crash as well.
If the crash is not reproducible with -threaded/collide=0 flags, then you need this last step. You can think of threaded mode as if each syscall is executed in its own thread. To model such execution mode, move individual syscalls into separate threads. You can see an example here: https://groups.google.com/d/msg/syzkaller/fHZ42YrQM-Y/Z4Xf-BbUDgAJ.
This process is automated to some degree in the syz-repro utility. You need to give it your manager config and a crash report file. And you can refer to the example config file.
./syz-repro -config my.cfg crash-qemu-1-1455745459265726910
It will try to find the offending program and minimize it. But since there are lots of factors that can affect reproducibility, it does not always work.