linux-kernel - Re: [RFC 1/1] xattr: provide integrity. namespace to read real values

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Date:	Mon, 25 Feb 2013 13:25:04 +0100
From:	"Kasatkin, Dmitry" <dmitry.kasatkin@...el.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org, casey@...aufler-ca.com,
	zohar@...ux.vnet.ibm.com, viro@...iv.linux.org.uk,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RFC 1/1] xattr: provide integrity. namespace to read real values

On Thu, Feb 14, 2013 at 9:05 PM, Vivek Goyal <vgoyal@...hat.com> wrote:
> On Wed, Feb 13, 2013 at 11:07:49AM +0200, Dmitry Kasatkin wrote:
>> User space tools use getxattr() system call to read values of extended
>> attributes. getxattr() system call uses vfs_getattr(), which for "security."
>> namespace might get a value of the xattr indirectly from LSM via calling
>> xattr_getsecurity(). For that reason value set by setxattr and read by getxattr
>> might differ.
>>
>> Here is an example of SMACK label, which shows that set and read values are
>> different:
>>
>>   setfattr -n security.SMACK64 -v "hello world" foo
>>   getfattr -n security.SMACK64 foo
>>   # file: foo
>>   security.SMACK64="hello"
>>
>> EVM uses vfs_getxattr_alloc(), which directly reads xattr values from the file
>> system. When performing the file system labeling with digital signatures, it is
>> necessary to read real xattr values in order to generate the correct signatures.
>>
>> This patch adds the virtual "integrity." name space, which allows to bypass
>> calling LSM and read real extended attribute values.
>>
>>   getfattr -e text -n integrity.SMACK64 foo
>>   # file: foo
>>   integrity.SMACK64="hello world"
>
> Without knowing anything about xattr or LSM, to me it is odd that I
> write an xattr using name "security.SMACK64" and read back the same
> attribute using different name "integrity.SMACK64".
>

It might sound like that, but writing and reading security attributes,
might give different results in security. namespace.
We cannot break userspace and change semantics of calls.
This is a trivial workaround which:
(1) does not to break userspace and
(2) does not require user space modifications.

Please suggest anything else?

- Dmitry

> Thanks
> Vivek
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