stackdepot: fix stack_depot_save_flags() in NMI context

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: stackdepot: fix stack_depot_save_flags() in NMI context Per documentation, stack_depot_save_flags() was meant to be usable from NMI context if STACK_DEPOT_FLAG_CAN_ALLOC is unset. However, it still would try to take the pool_lock in an attempt to save a stack trace in the current pool (if space is available). This could result in deadlock if an NMI is handled while pool_lock is already held. To avoid deadlock, only try to take the lock in NMI context and give up if unsuccessful. The documentation is fixed to clearly convey this.

References

• https://git.kernel.org/stable/c/9bfeeeff2c92b9dd261198b601b45bde4c529841
• https://git.kernel.org/stable/c/031e04bdc834cda3b054ef6b698503b2b97e8186