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

[Roberto Sassu]

On Thu, 2023-04-20 at 10:48 +0200, Roberto Sassu wrote:
> 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.

Actually, after studying the code better, this is
what security_dentry_create_files_as() is useful for.

 * Compute a context for a dentry as the inode is not yet available and set
 * that context in passed in creds so that new files are created using that
 * context. Context is calculated using the passed in creds and not the creds
 * of the caller.

And Smack is doing:

		if (may > 0 && (may & MAY_TRANSMUTE)) {
			ntsp->smk_task = isp->smk_inode;

The new inode will be created with the label of the current task, that
was replaced with the label of the parent directory (see above) in smac
k_dentry_create_files_as().

I think the reason why Mengchi was not obtaining the desired label when
replacing /data/dir1 was because /data is incorrectly labeled.

To ensure that /data has label 'system' and transmute is true, I added
smackfstransmute=system to the mount options.

However, at the beginning, it seemed that it didn't work:

# mount -t overlay overlay -o lowerdir=/data,upperdir=/home/root/data,workdir=/home/root/data_work,smackfstransmute=system /data
# chsmack /data
/data access="system"

I found that the reason for this is that smack_inode_getsecurity()
retrieves metadata from the inode only for SMACK64, and the rest from
xattrs (which would not work for mount options). I just made a patch to
handle SMACK64TRANSMUTE too.

With the patch applied, I correctly get:

# mount -t overlay overlay -o lowerdir=/data,upperdir=/home/root/data,workdir=/home/root/data_work,smackfstransmute=system /data
# chsmack /data
/data access="system" transmute="TRUE"

With the root inode correctly labeled, I get:

# mount -t overlay overlay -o lowerdir=/data,upperdir=/home/root/data,workdir=/home/root/data_work,smackfstransmute=system /data
# rm -Rf /data/dir1
# mkdir /data/dir1
# chsmack /data/dir1
/data/dir1 access="system"

This is partially correct, transmute="TRUE" is missing.

Judging from smk_task, we cannot determine in smack_inode_init_security
() if transmuting was successful in smack_dentry_create_files_as(). We
need an extra information.

Mengchi's solution was to add the new inode as parameter
to security_dentry_create_files_as(), so that SMK_INODE_CHANGED can be
set in smk_flags, and SMACK64TRANSMUTE is set in smack_d_instantiate().

One concern could be that preallocating the inode maybe is overlayfs-
specific? A comment also says that we might not use that one:

	err = ovl_create_or_link(dentry, inode, &attr, false);
	/* Did we end up using the preallocated inode? */
	if (inode != d_inode(dentry))

We could achieve the same goal without adding a new parameter to security_dentry_create_files_as() and, instead, by adding a new field
in the task_smack structure, smk_transmuted, that is set to smk_task
when transmuting is successful.

Then, if smk_task == smk_transmuted, smack_inode_init_security() would
set SMK_INODE_CHANGED. On top of that, I would instead just provide the
second xattr SMACK64TRANSMUTE, in addition to SMACK64.

Will send the patches for upstream first, and then switch to providing 
SMACK64TRANSMUTE in smack_inode_init_security(), in a new version of
the EVM patch set.

Thanks

Roberto

> 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