linux-kernel - Re: __vm_enough_memory(), OVERCOMMIT

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Message-ID: <20081021164619.341f01d9@lxorguk.ukuu.org.uk>
Date:	Tue, 21 Oct 2008 16:46:19 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	hooanon05@...oo.co.jp
Cc:	linux-kernel@...r.kernel.org
Subject: Re: __vm_enough_memory(), OVERCOMMIT_NEVER, current->mm, kernel
 thread

> 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
NULL

Does that sound sensible ?

Alan
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