Re: __vm_enough_memory(), OVERCOMMIT_NEVER, current->mm, kernelthread

From: Alan Cox
Date: Tue Oct 21 2008 - 11:46:23 EST

> In this case, NFSD (kernel thread) calls shmem_file_write() or
> shmem_write_begin() and __vm_enough_memory() is called. But current->mm
> is NULL and the kernel crashes.
> If a user have to set OVERCOMMIT_NEVER, where should we fix?

Calling into the file system code assuming that current->mm is
NULL isn't safe and hasn't been for a very long time since someone added
the 3% hack.

The shmem case is actually a bit special so my thoughts are:

Make security_vm_enough_memory() WARN() if current->mm = NULL
Make security_vm_enough_memory_mm() WARN() if the passed mm = NULL
Add security_vm_enough_memory_fs() which does not do the warning test

All would still call security->ops->vm_enough_memory and then
__vm_enough_memory() would skip the 3% adjustment when the passed mm was

Does that sound sensible ?

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