[PATCH] selinux: Perform both commoncap and selinux xattr checks

[Stephen Smalley]

On Mon, 2017-10-02 at 09:38 -0500, Eric W. Biederman wrote:
> When selinux is loaded the relax permission checks for writing
> security.capable are not honored.  Which keeps file capabilities
> from being used in user namespaces.
> 
> Stephen Smalley <sds at tycho.nsa.gov> writes:
> > Originally SELinux called the cap functions directly since there
> > was no
> > stacking support in the infrastructure and one had to manually
> > stack a
> > secondary module internally.  inode_setxattr and inode_removexattr
> > however were special cases because the cap functions would check
> > CAP_SYS_ADMIN for any non-capability attributes in the security.*
> > namespace, and we don't want to impose that requirement on setting
> > security.selinux.  Thus, we inlined the capabilities logic into the
> > selinux hook functions and adapted it appropriately.
> 
> Now that the permission checks in commoncap have evolved this
> inlining of their contents has become a problem.  So restructure
> selinux_inode_removexattr, and selinux_inode_setxattr to call
> both the corresponding cap_inode_ function and dentry_has_perm
> when the attribute is not a selinux security xattr.   This ensures
> the policies of both commoncap and selinux are enforced.
> 
> This results in smack and selinux having the same basic structure
> for setxattr and removexattr.  Performing their own special
> permission
> checks when it is their modules xattr being written to, and deferring
> to commoncap when that is not the case.  Then finally performing
> their
> generic module policy on all xattr writes.
> 
> This structure is fine when you only consider stacking with the
> commoncap lsm, but it becomes a problem if two lsms that don't want
> the commoncap security checks on their own attributes need to be
> stack.  This means there will need to be updates in the future as lsm
> stacking is improved, but at least now the structure between smack
> and
> selinux is common making the code easier to refactor.
> 
> This change also has the effect that selinux_linux_setotherxattr
> becomes
> unnecessary so it is removed.
> 
> Fixes: 8db6c34f1dbc ("Introduce v3 namespaced file capabilities")
> Fixes: 7bbf0e052b76 ("[PATCH] selinux merge")
> Historical Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx
> /history.git
> Signed-off-by: "Eric W. Biederman" <ebiederm at xmission.com>

Acked-by: Stephen Smalley <sds at tycho.nsa.gov>

> ---
> 
> While this fixes some things this isn't a regression so it should be
> able to wait until the next merge window to be merged.   Would you
> like
> to take this through the selinux tree?  Or shall I take it through
> mine?

Deferring to Paul Moore on this, since he maintains the selinux tree.

> 
> security/selinux/hooks.c | 43 ++++++++++++++++++---------------------
> ----
>  1 file changed, 18 insertions(+), 25 deletions(-)
> 
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index f5d304736852..c78dbec627f6 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -3124,27 +3124,6 @@ static int selinux_inode_getattr(const struct
> path *path)
>  	return path_has_perm(current_cred(), path, FILE__GETATTR);
>  }
>  
> -static int selinux_inode_setotherxattr(struct dentry *dentry, const
> char *name)
> -{
> -	const struct cred *cred = current_cred();
> -
> -	if (!strncmp(name, XATTR_SECURITY_PREFIX,
> -		     sizeof XATTR_SECURITY_PREFIX - 1)) {
> -		if (!strcmp(name, XATTR_NAME_CAPS)) {
> -			if (!capable(CAP_SETFCAP))
> -				return -EPERM;
> -		} else if (!capable(CAP_SYS_ADMIN)) {
> -			/* A different attribute in the security
> namespace.
> -			   Restrict to administrator. */
> -			return -EPERM;
> -		}
> -	}
> -
> -	/* Not an attribute we recognize, so just check the
> -	   ordinary setattr permission. */
> -	return dentry_has_perm(cred, dentry, FILE__SETATTR);
> -}
> -
>  static bool has_cap_mac_admin(bool audit)
>  {
>  	const struct cred *cred = current_cred();
> @@ -3167,8 +3146,15 @@ static int selinux_inode_setxattr(struct
> dentry *dentry, const char *name,
>  	u32 newsid, sid = current_sid();
>  	int rc = 0;
>  
> -	if (strcmp(name, XATTR_NAME_SELINUX))
> -		return selinux_inode_setotherxattr(dentry, name);
> +	if (strcmp(name, XATTR_NAME_SELINUX)) {
> +		rc = cap_inode_setxattr(dentry, name, value, size,
> flags);
> +		if (rc)
> +			return rc;
> +
> +		/* Not an attribute we recognize, so just check the
> +		   ordinary setattr permission. */
> +		return dentry_has_perm(current_cred(), dentry,
> FILE__SETATTR);
> +	}
>  
>  	sbsec = inode->i_sb->s_security;
>  	if (!(sbsec->flags & SBLABEL_MNT))
> @@ -3282,8 +3268,15 @@ static int selinux_inode_listxattr(struct
> dentry *dentry)
>  
>  static int selinux_inode_removexattr(struct dentry *dentry, const
> char *name)
>  {
> -	if (strcmp(name, XATTR_NAME_SELINUX))
> -		return selinux_inode_setotherxattr(dentry, name);
> +	if (strcmp(name, XATTR_NAME_SELINUX)) {
> +		int rc = cap_inode_removexattr(dentry, name);
> +		if (rc)
> +			return rc;
> +
> +		/* Not an attribute we recognize, so just check the
> +		   ordinary setattr permission. */
> +		return dentry_has_perm(current_cred(), dentry,
> FILE__SETATTR);
> +	}
>  
>  	/* No one is allowed to remove a SELinux security label.
>  	   You can change the label, but all data must be labeled.
> */
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