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

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Date:	Fri, 1 Nov 2013 22:24:12 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	jlieb@...asas.com, luto@...capital.net
Cc:	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: [PATCH 1/3] switch_creds: Syscall to switch creds for file server ops

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().

---------- 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)
{
        { /* switch_creds() syscall */
                struct fd f = fdget(0);
                if (!f.file)
                        return -EBADF;
                put_cred(override_creds(f.file->f_cred));
                fdput(f);
        }
        { /* something that calls commit_creds() */
                struct cred *cred = prepare_creds();
                if (cred)
                        commit_creds(cred);
        }
        return 0;
}

static void test_exit(void)
{
}

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

Since nobody can guarantee that the caller of switch_creds() never does
some operation that calls commit_creds(), I don't think switch_creds()
based on override_creds() will work.
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