The tun_get() function has a race condition where tun->dev can become
NULL between checking tun and calling dev_hold(tun->dev).
This occurs when tun_detach_all() runs concurrently:
1. tun_get() calls rcu_dereference(tfile->tun), gets valid tun
2. Another thread calls tun_detach_all() which sets tun->dev to NULL
3. dev_hold(tun->dev) dereferences NULL pointer -> kernel crash
Fix by reading tun->dev into a local variable with READ_ONCE() and
verifying both the pointer and device registration state before
taking the reference.
Crash signature:
Unable to handle kernel NULL pointer dereference at virtual address 00000004
PC is at tun_get+0x16/0x22 [tun]
Alex
fba975192b