linux-kernel - Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been taken exclusively

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Date:   Sat, 30 Sep 2017 20:33:36 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Dave Chinner <david@...morbit.com>,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-integrity@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Jan Kara <jack@...e.cz>, "Theodore Ts'o" <tytso@....edu>
Subject: Re: [RFC PATCH 3/3] fs: detect that the i_rwsem has already been taken exclusively

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Thu, Sep 28, 2017 at 6:53 PM, Mimi Zohar <zohar@...ux.vnet.ibm.com> wrote:
>>
>> The locking issue isn't with validating the file hash, but with the
>> setxattr, chmod, chown syscalls.  Each of these syscalls takes the
>> i_rwsem exclusively before IMA (or EVM) is called.
>
> Read my email again.
>
>> In setxattr, chmod, chown syscalls, IMA (and EVM) are called after the
>> i_rwsem is already taken.  So the locking would be:
>>
>> lock: i_rwsem
>> lock: iint->mutex
>
> No.
>
> Two locks. One inner, one outer. Only the actual ones that calculates
> the hash would take the outer one. Read my email.

That would require a task_work or another kind of work callback so that
the writes of the xattr are not synchronous with the vfs callback
correct?

Eric