Re: [PATCH] Smack modifications for: security: Allow all LSMs to provide xattrs for inode_init_security hook

[Roberto Sassu]

On Wed, 2023-04-19 at 12:25 -0700, Mengchi Cheng wrote:
> > I got some errors during xattr removal, so not sure if my patch was
> > working properly or not (it happened also without it, didn't
> > investigate more).
> > 
> > However, I saw another discussion related to transmute:
> > 
> > https://lore.kernel.org/linux-security-module/20230419002338.566487-1-mengcc@xxxxxxxxxx/
> > 
> > I add the people in CC.
> > 
> > The steps described were so easy to understand and executed, I tried
> > without and with overlayfs.
> > 
> > Without:
> > 
> > # echo "_ system rwxatl" > /sys/fs/smackfs/load2
> > # mkdir /data
> > # chsmack -a "system" /data
> > # chsmack -t /data
> > # mkdir -p /data/dir1/dir2
> > # chsmack /data/dir1
> > /data/dir1 access="system" transmute="TRUE"
> > # chsmack /data/dir1/dir2
> > /data/dir1/dir2 access="system" transmute="TRUE"
> > 
> > It seems to work, right?
> > 
> > With overlay fs it didn't work, same result as the one Mengchi
> > reported. Since Mengchi's solution was to set SMK_INODE_CHANGED, and I
> > want to get rid of it, I thought to investigate more.
> > 
> > Looking at smack_dentry_create_files_as(), I see that the label of the
> > process is overwritten with the label of the transmuting directory.
> > 
> > That causes smack_inode_init_security() to lookup the transmuting rule
> > on the overridden credential, and not on the original one.
> > 
> > In the example above, it means that, when overlayfs is creating the new
> > inode, the label of the process is system, not _. So no transmute
> > permission, and also the xattr will not be added, as observed by
> > Mengchi.
> > 
> > Hopefully I undertood the code, so in this particular case we would not
> > need to override the label of the process in smack_dentry_create_files_
> > as().
> > 
> > If you see smack_inode_init_security():
> > 
> > 	struct smack_known *skp = smk_of_current();
> > 	struct smack_known *isp = smk_of_inode(inode);
> > 	struct smack_known *dsp = smk_of_inode(dir);
> > 
> > [...]
> > 
> > 		if (may > 0 && ((may & MAY_TRANSMUTE) != 0) &&
> > 		    smk_inode_transmutable(dir)) {
> > 			isp = dsp;
> > [...]
> > 
> > 		xattr->value = kstrdup(isp->smk_known, GFP_NOFS);
> > 
> > This code is telling, if there is a transmute rule, and the directory
> > is transmuting, set the label of the new inode to the label of the
> > directory. That should be already the result that we wanted to obtain.
> > 
> > The current code should have been doing it by overriding the label of
> > the process in smack_dentry_create_files_as() with the label of the
> > parent directory, and letting the inode being created with the
> > overridden label of the process. The transmute xattr is not set due to
> > the problem described above.
> > 
> > So, as a quick test, I kept this patch with the change to xattr2->name, 
> > and skipped the label override in smack_dentry_create_files_as(). It
> > worked, I get the same result as without overlayfs. Wondering if the
> > process label override is necessary in other cases.
> 
> If I understand correctly, removing the if block below is what you suggested.

Yes, more or less is what I did.

> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index cfcbb748da25..a867288e9de9 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -4769,8 +4769,8 @@ static int smack_dentry_create_files_as(struct dentry *dentry, int mode,
>                  * providing access is transmuting use the containing
>                  * directory label instead of the process label.
>                  */
> -               if (may > 0 && (may & MAY_TRANSMUTE))
> -                       ntsp->smk_task = isp->smk_inode;
> +//             if (may > 0 && (may & MAY_TRANSMUTE))
> +//                     ntsp->smk_task = isp->smk_inode;
>         }
>         return 0;
>  }
> 
> This way will have issue in the following situation on the vanila kernel.
> data in the lowerdir has "_" label before overlay and dir1 is already
> created in the lowerdir.
> # chsmack /data
> /data access="_"
> # chsmack /data/dir1
> /data/dir1 access="system" transmute="TRUE"
> Apply overlay on data directory and set the smack rule in the same way.
> data has the same smack label.
> # chsmack /data
> /data access="system" transmute="TRUE"

I'm using an older kernel, but I get _ instead of system.

> After that, remove dir1 and mkdir dir1 again. dir1 did not get the correct
> label.
> # rm -r /data/dir1
> # mkdir -p /data/dir1
> # chsmack /data/dir1
> /data/dir1 access="_"

Unfortunately, it cannot work:

Thread 3 hit Breakpoint 1, smack_inode_init_security (...) at security/smack/smack_lsm.c:959
959	{
(gdb) p dir->i_ino
$12 = 9169116
(gdb) p dsp
$13 = (struct smack_known *) 0xffffffff831fc0a0 <smack_known_floor>


ls -i /home/root/data_work/
9169116 work

So, transmuting is decided on the working directory.

If I do:

# chsmack -a system -t /home/root/data_work/work/
# mkdir /data/dir1
# chsmack /data/dir1
/data/dir1 access="system" transmute="TRUE"

I obtain the expected result. However, this problem is due to how overlayfs works:

static int ovl_create_over_whiteout(struct dentry *dentry, struct inode *inode,
				    struct ovl_cattr *cattr)
{

[...]

	newdentry = ovl_create_temp(ofs, workdir, cattr);
	err = PTR_ERR(newdentry);
	if (IS_ERR(newdentry))
		goto out_dput;


The good news seems to be that, once you set the label to the correct
directory, transmuting works with the changes I proposed.

Roberto

> Since I am not very familiar your change. Could you help check with your
> patch will this issue also happen? 
> 
> 
> Best,
> Mengchi
> 
> >  
> > Roberto