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

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Date:	Thu, 14 Feb 2013 15:05:57 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Dmitry Kasatkin <dmitry.kasatkin@...el.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 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".

Thanks
Vivek
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