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/collide mode as if each syscall is executed in its own thread. To mode 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:
./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.