[PATCH RESEND v2 11/18] fs: Ensure the mounter of a filesystem is privileged towards its inodes

[Eric W. Biederman]

Seth Forshee <seth.forshee at canonical.com> writes:

> On Fri, Mar 04, 2016 at 04:43:06PM -0600, Eric W. Biederman wrote:
>> In general this is only an issue if uids and gids on the filesystem
>> do not map into the user namespace.
>> 
>> Therefore the general fix is to limit the logic of checking for
>> capabilities in s_user_ns if we are dealing with INVALID_UID and
>> INVALID_GID.  For proc and kernfs that should never be the case
>> so the problem becomes a non-issue.
>> 
>> Further I would look at limiting that relaxation to just
>> inode_change_ok. 
>
> Finally got around to implementing this today; is the patch below what
> you had in mind?

Pretty much.

For the same reason that capble_wrt_inode_uidgid(inode) had to look
at both inode->i_uid and inode->i_gid I think we need to look at
both inode->i_uid and inode->i_gid in those case.

I am worried about chgrp_ok in cases such as inode->i_uid is valid
but unmapped.  I have a similiar worry about chown_ok where
inode->i_gid is valid but unmapped (although that worry is less
serious).

>> So that we can easily wrap that check per filesystem
>> and deny the relaxation for proc and kernfs.  proc and kernfs already
>> have wrappers for .setattr so denying changes when !uid_vaid and
>> !gid_valid would be a trivial addition, and ensure calamity does
>> not ensure.
>
> I'm confused about this part though. As you say above, proc and kernfs
> will never have inodes with invalid ids, so it's not an issue. Do you
> just mean this to be extra insurance against problems?

I meant two things.
1) As filesystems explicitly have to call inode_change_ok they can
   over ride the default if it is possible.

2) Because being paranoid about backward compatibility matters, it
   almost certainly workth add adding a check:
   "if (!uid_valid(inode->i_uid) ||!gid_valid(inode->i_gid)) return -EPERM"
   To proc and sysfs just before they call inode_change_ok just so we
   don't need to analyze them and confirm that they don't use
   INVALID_UID.

   That just makes the patch more robust.

   The we could leave removing that code for a follow on patch where
   someone takes the time to read through and audit all of the proc and
   sysfs code to ensure that the case does not arise, instead of just
   implicitily assuming it.

   That is the usual pattern when pushing down changes.  Do something
   that is easily guaranteed to work, and leave the careful looking for
   a patch all of it's own.

Eric


> Thanks,
> Seth
>
> ---
>
> diff --git a/fs/attr.c b/fs/attr.c
> index 3cfaaac4a18e..f2bcd3f7dfbb 100644
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -16,6 +16,31 @@
>  #include <linux/evm.h>
>  #include <linux/ima.h>
>  
> +static bool chown_ok(const struct inode *inode, kuid_t uid)
> +{
> +	if (uid_eq(current_fsuid(), inode->i_uid) && uid_eq(uid, inode->i_uid))
> +		return true;
> +	if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +		return true;
> +	if (!uid_valid(inode->i_uid) &&
> +	    ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
> +		return true;
> +	return false;
> +}
> +
> +static bool chgrp_ok(const struct inode *inode, kgid_t gid)
> +{
> +	if (uid_eq(current_fsuid(), inode->i_uid) &&
> +	    (in_group_p(gid) || gid_eq(gid, inode->i_gid)))
> +		return true;
> +	if (capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +		return true;
> +	if (!gid_valid(inode->i_gid) &&
> +	    ns_capable(inode->i_sb->s_user_ns, CAP_CHOWN))
> +		return true;
> +	return false;
> +}
> +
>  /**
>   * inode_change_ok - check if attribute changes to an inode are allowed
>   * @inode:	inode to check
> @@ -58,17 +83,11 @@ int inode_change_ok(const struct inode *inode, struct iattr *attr)
>  		return 0;
>  
>  	/* Make sure a caller can chown. */
> -	if ((ia_valid & ATTR_UID) &&
> -	    (!uid_eq(current_fsuid(), inode->i_uid) ||
> -	     !uid_eq(attr->ia_uid, inode->i_uid)) &&
> -	    !capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +	if ((ia_valid & ATTR_UID) && !chown_ok(inode, attr->ia_uid))
>  		return -EPERM;
>  
>  	/* Make sure caller can chgrp. */
> -	if ((ia_valid & ATTR_GID) &&
> -	    (!uid_eq(current_fsuid(), inode->i_uid) ||
> -	    (!in_group_p(attr->ia_gid) && !gid_eq(attr->ia_gid, inode->i_gid))) &&
> -	    !capable_wrt_inode_uidgid(inode, CAP_CHOWN))
> +	if ((ia_valid & ATTR_GID) && !chgrp_ok(inode, attr->ia_gid))
>  		return -EPERM;
>  
>  	/* Make sure a caller can chmod. */