linux-kernel - Re: Re: Re: Re: Re: [PATCH 1/3] switch_creds: Syscall to switch creds for file server ops

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Message-Id: <201311020107.FDJ87003.SHJFMOOOtLVFQF@I-love.SAKURA.ne.jp>
Date:	Sat, 2 Nov 2013 01:07:59 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	jlieb@...asas.com
Cc:	luto@...capital.net, linux-fsdevel@...r.kernel.org,
	ebiederm@...ssion.com, viro@...iv.linux.org.uk, tytso@....edu,
	linux-kernel@...r.kernel.org, bfields@...hat.com,
	jlayton@...hat.com
Subject: Re: Re: Re: Re: Re: [PATCH 1/3] switch_creds: Syscall to switch creds for file server ops

Jim Lieb wrote:
> On Friday, November 01, 2013 22:24:12 Tetsuo Handa wrote:
> > Jim Lieb wrote:
> > > Subsequent uses look like:
> > > 	use_creds(cached fd);
> > > 
> > > followed by
> > > 
> > > 	open/creat/mknod/write
> > > 
> > > followed by
> > > 
> > > 	use_creds(-1);
> > 
> > Are you aware that calling commit_creds() is prohibitted between
> > override_creds() and revert_creds() ?
> > 
> > If the caller does some operation that calls commit_creds() (like
> > example below), the kernel triggers BUG().
> 
> Yes, I do.  I caught this in an early pass.  I only use override_creds() and 
> revert_creds().  

Excuse me, but even below example will trigger BUG(). You pack
override_creds() + open() + revert_creds() into one system call so that the
caller of this system call shall not do something that calls commit_creds() ?

---------- example module start ----------
#include <linux/module.h>
#include <linux/cred.h>
#include <linux/fs.h>
#include <linux/file.h>

static int __init test_init(void)
{
        const struct cred *orig;
        { /* switch_cred() syscall */
                struct fd f = fdget(0);
                if (!f.file)
                        return -EBADF;
                orig = override_creds(f.file->f_cred);
                fdput(f);
        }
        { /* something that calls commit_creds() */
                struct cred *cred = prepare_creds();
                if (cred)
                        commit_creds(cred);
        }
        { /* restore */
                revert_creds(orig);
        }
        return 0;
}

static void test_exit(void)
{
}

module_init(test_init);
module_exit(test_exit);
MODULE_LICENSE("GPL");
---------- example module end ----------
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