Re: [PATCH v10 2/4] security: Allow all LSMs to provide xattrs for inode_init_security hook

[Roberto Sassu]

On Tue, 2023-04-11 at 03:22 -0400, Mimi Zohar wrote:
> Hi Roberto,
> 
> Sorry for the delay in responding...

Hi Mimi

no worries!

> The patch description reads as though support for per LSM multiple
> xattrs is being added in this patch, though lsm_get_xattr_slot() only
> ever is incremented once for each LSM.  To simplify review, it would be
> nice to mention that lsm_get_xattr_slot() would be called multiple
> times per LSM xattr.

Ok, I will mention it.

> On Fri, 2023-03-31 at 14:32 +0200, 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_count 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.
> > 
> > Dynamically allocate the new_xattrs array to be populated by LSMs with the
> > inode_init_security hook, and pass it to the latter instead of the
> > name/value/len triple. Unify the !initxattrs and initxattrs case, simply
> > don't allocate the new_xattrs array in the former.
> > 
> > Also, pass to the hook the number of xattrs filled by each LSM, so that
> > there are no gaps when the next LSM fills the array. Gaps might occur
> > because an LSM can legitimately request xattrs to the LSM infrastructure,
> > but not fill the reserved slots, if it was not initialized.
> 
> The number of security xattrs permitted per LSM was discussed in the
> second paragraph.  The first line of this paragraph needs to be updated
> to reflect the current number of security xattrs used, though that is
> more related to the new lsm_get_xattr_slot().  Or perhaps the entire
> paragraph is unnecessary, a remnant from
> security_check_compact_filled_xattrs(), and should be removed.  

I would probably say in that paragraph that the number specified for
the lbs_xattr_count field determines how many times an LSM can call
lsm_get_xattr_slot().

> > Update the documentation of security_inode_init_security() to reflect the
> > changes, and fix the description of the xattr name, as it is not allocated
> > anymore.
> > 
> > Finally, adapt both SELinux and Smack to use the new definition of the
> > inode_init_security hook, and to fill the reserved slots in the xattr
> > array. Introduce the lsm_get_xattr_slot() helper to retrieve an available
> > slot to fill, and to increment the number of filled slots.
> > 
> > Move the xattr->name assignment after the xattr->value one, so that it is
> > done only in case of successful memory allocation. For Smack, also reserve
> > space for the other defined xattrs although they are not set yet in
> > smack_inode_init_security().
> 
> This Smack comment should be moved to the previous paragraph and even
> expanded explaining that lsm_get_xattr_slot() will be called for each
> additional security xattr.

>From previous Paul's and Casey's comments, Smack will have just two
xattrs, assuming that security.SMACK_TRASMUTE64 can be set in
smack_inode_init_security(). I will change this part accordingly once
Casey can have a look at the function.

> > Reported-by: Nicolas Bouchinet <nicolas.bouchinet@xxxxxxxxxxx> (EVM crash)
> > Link: https://lore.kernel.org/linux-integrity/Y1FTSIo+1x+4X0LS@archlinux/
> > Signed-off-by: Roberto Sassu <roberto.sassu@xxxxxxxxxx>
> > ---
> > diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> > index c2be66c669a..9eb9b686493 100644
> > --- a/include/linux/lsm_hooks.h
> > +++ b/include/linux/lsm_hooks.h
> > @@ -28,6 +28,7 @@
> >  #include <linux/security.h>
> >  #include <linux/init.h>
> >  #include <linux/rculist.h>
> > +#include <linux/xattr.h>
> >  
> >  union security_list_options {
> >  	#define LSM_HOOK(RET, DEFAULT, NAME, ...) RET (*NAME)(__VA_ARGS__);
> > @@ -63,8 +64,27 @@ struct lsm_blob_sizes {
> >  	int	lbs_ipc;
> >  	int	lbs_msg_msg;
> >  	int	lbs_task;
> > +	int	lbs_xattr_count; /* number of xattr slots in new_xattrs array */
> >  };
> >  
> > +/**
> > + * lsm_get_xattr_slot - Return the next available slot and increment the index
> > + * @xattrs: array storing LSM-provided xattrs
> > + * @xattr_count: number of already stored xattrs (updated)
> > + *
> > + * Retrieve the first available slot in the @xattrs array to fill with an xattr,
> > + * and increment @xattr_count.
> > + *
> > + * Return: The slot to fill in @xattrs if non-NULL, NULL otherwise.
> > + */
> > +static inline struct xattr *lsm_get_xattr_slot(struct xattr *xattrs,
> > +					       int *xattr_count)
> > +{
> > +	if (unlikely(!xattrs))
> > +		return NULL;
> > +	return xattrs + (*xattr_count)++;
> 
> At some point, since lsm_get_xattr_slot() could be called multiple
> times from the same LSM, shouldn't there be some sort of bounds
> checking?

>From previous Paul's comments, I understood that he prefers to avoid
extra checks. It will be up to LSM developers to ensure that the API is
used correctly.

Thanks

Roberto