Re: [PATCH v4 3/5] security: Allow all LSMs to provide xattrs for inode_init_security hook

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On Thu, 2022-11-17 at 09:40 -0800, Casey Schaufler wrote:
> On 11/17/2022 9:24 AM, Mimi Zohar wrote:
> > On Thu, 2022-11-17 at 09:18 -0800, Casey Schaufler wrote:
> >> On 11/17/2022 8:05 AM, Mimi Zohar wrote:
> >>> hOn Thu, 2022-11-10 at 10:46 +0100, Roberto Sassu wrote:
> >>>> From: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> >>>>
> >>>> Currently, security_inode_init_security() supports only one LSM providing
> >>>> an xattr and EVM calculating the HMAC on that xattr, plus other inode
> >>>> metadata.
> >>>>
> >>>> Allow all LSMs to provide one or multiple xattrs, by extending the security
> >>>> blob reservation mechanism. Introduce the new lbs_xattr field of the
> >>>> lsm_blob_sizes structure, so that each LSM can specify how many xattrs it
> >>>> needs, and the LSM infrastructure knows how many xattr slots it should
> >>>> allocate.
> >>> Perhaps supporting per LSM multiple xattrs is a nice idea, but EVM
> >>> doesn't currently support it.  The LSM xattrs are hard coded in
> >>> evm_config_default_xattrnames[],  based on whether the LSM is
> >>> configured.  Additional security xattrs may be included in the
> >>> security.evm calculation, by extending the list via
> >>> security/integrity/evm/evm_xattrs.
> >> Smack uses multiple xattrs. All file system objects have a SMACK64
> >> attribute, which is used for access control. A program file may have
> >> a SMACK64EXEC attribute, which is the label the program will run with.
> >> A library may have a SMACK64MMAP attribute to restrict loading. A
> >> directory may have a SMACK64TRANSMUTE attribute, which modifies the
> >> new object creation behavior.
> >>
> >> The point being that it may be more than a "nice idea" to support
> >> multiple xattrs. It's not a hypothetical situation.
> > And each of these addiitonal Smack xattrs are already defined in 
> > evm_config_default_xattrnames[].
> Then I'm confused by the statement that "EVM doesn't currently support it".

My mistake.  As you pointed out, Smack is defining multiple security


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