[Q] ADB SHELL CHMOD 644 ,nothing is working anymore... topic

Hello everybody
I needed to push a file to /system/app but since I was on stock rom & kernel, that didnt support adb root.
I needed to be able to push the backup file after restart but ADBD Insecure couldn't patch after restart.
my phone is optimus G E975 , fully stock 4.1.2
I found this command which could do the thing BUT :
adb shell
su
chmod 777 /system

since it didnt restore to default after restarted I tried to restore it myself(didn't work) with:
adb shell
su
chmod 644 /system
but after that every thing force stopped & after restart, phone can't boot anymore,just when want t start boot animation, it just cant & goes to recovery every time!:(
even Hard reset didnt work, but as stock recovery supports adb, I wish you guys can help me. but It seems adb shell gives permission denied(13) in recovery. please someone help...
Anyway am I able to flash in download mode????? (for the last solution)
any help to change that CHMOD to 777 in recovery or something else?
any help appreciated so much!