Message-ID: <0fccab67e496f10f4ee7bf2220e70a655013935f.camel@huaweicloud.com>
Date:   Wed, 19 Apr 2023 15:46:53 +0200
From:   Roberto Sassu <roberto.sassu@...weicloud.com>
To:     Casey Schaufler <casey@...aufler-ca.com>, zohar@...ux.ibm.com,
        dmitry.kasatkin@...il.com, paul@...l-moore.com, jmorris@...ei.org,
        serge@...lyn.com, stephen.smalley.work@...il.com,
        eparis@...isplace.org
Cc:     reiserfs-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org, selinux@...r.kernel.org,
        bpf@...r.kernel.org, kpsingh@...nel.org, keescook@...omium.org,
        nicolas.bouchinet@...p-os.org,
        Roberto Sassu <roberto.sassu@...wei.com>,
        Mengchi Cheng <mengcc@...zon.com>, miklos@...redi.hu,
        linux-unionfs@...r.kernel.org, kamatam@...zon.com,
        yoonjaeh@...zon.com
Subject: Re: [PATCH] Smack modifications for: security: Allow all LSMs to
 provide xattrs for inode_init_security hook

On Tue, 2023-04-18 at 09:02 -0700, Casey Schaufler wrote:
> On 4/18/2023 12:05 AM, Roberto Sassu wrote:
> > On Mon, 2023-04-17 at 09:41 -0700, Casey Schaufler wrote:
> > > On 4/13/2023 12:11 AM, Roberto Sassu wrote:
> > > > On Wed, 2023-04-12 at 13:29 -0700, Casey Schaufler wrote:
> > > > > On 4/12/2023 12:22 AM, Roberto Sassu wrote:
> > > > > > On Tue, 2023-04-11 at 10:54 -0700, Casey Schaufler wrote:
> > > > > > > On 4/11/2023 10:23 AM, Roberto Sassu wrote:
> > > > > > > > From: Roberto Sassu <roberto.sassu@...wei.com>
> > > > > > > > 
> > > > > > > > Very very quick modification. Not tested.
> > > > > > > > 
> > > > > > > > Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
> > > > > > > > ---
> > > > > > > >  security/smack/smack.h     |  2 +-
> > > > > > > >  security/smack/smack_lsm.c | 42 ++++++++++++++++++++------------------
> > > > > > > >  2 files changed, 23 insertions(+), 21 deletions(-)
> > > > > > > > 
> > > > > > > > diff --git a/security/smack/smack.h b/security/smack/smack.h
> > > > > > > > index e2239be7bd6..f00c8498c60 100644
> > > > > > > > --- a/security/smack/smack.h
> > > > > > > > +++ b/security/smack/smack.h
> > > > > > > > @@ -127,7 +127,7 @@ struct task_smack {
> > > > > > > >  
> > > > > > > >  #define	SMK_INODE_INSTANT	0x01	/* inode is instantiated */
> > > > > > > >  #define	SMK_INODE_TRANSMUTE	0x02	/* directory is transmuting */
> > > > > > > > -#define	SMK_INODE_CHANGED	0x04	/* smack was transmuted */
> > > > > > > > +#define	SMK_INODE_CHANGED	0x04	/* smack was transmuted (unused) */
> > > > > > > See below ...
> > > > > > > 
> > > > > > > >  #define	SMK_INODE_IMPURE	0x08	/* involved in an impure transaction */
> > > > > > > >  
> > > > > > > >  /*
> > > > > > > > diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> > > > > > > > index 8392983334b..b43820bdbd0 100644
> > > > > > > > --- a/security/smack/smack_lsm.c
> > > > > > > > +++ b/security/smack/smack_lsm.c
> > > > > > > > @@ -54,12 +54,12 @@
> > > > > > > >  
> > > > > > > >  /*
> > > > > > > >   * Smack uses multiple xattrs.
> > > > > > > > - * SMACK64 - for access control, SMACK64EXEC - label for the program,
> > > > > > > > - * SMACK64MMAP - controls library loading,
> > > > > > > > + * SMACK64 - for access control,
> > > > > > > >   * SMACK64TRANSMUTE - label initialization,
> > > > > > > > - * Not saved on files - SMACK64IPIN and SMACK64IPOUT
> > > > > > > > + * Not saved on files - SMACK64IPIN and SMACK64IPOUT,
> > > > > > > > + * Must be set explicitly - SMACK64EXEC and SMACK64MMAP
> > > > > > > >   */
> > > > > > > > -#define SMACK_INODE_INIT_XATTRS 4
> > > > > > > > +#define SMACK_INODE_INIT_XATTRS 2
> > > > > > > >  
> > > > > > > >  #ifdef SMACK_IPV6_PORT_LABELING
> > > > > > > >  static DEFINE_MUTEX(smack_ipv6_lock);
> > > > > > > > @@ -957,11 +957,11 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
> > > > > > > >  				     const struct qstr *qstr,
> > > > > > > >  				     struct xattr *xattrs, int *xattr_count)
> > > > > > > >  {
> > > > > > > > -	struct inode_smack *issp = smack_inode(inode);
> > > > > > > >  	struct smack_known *skp = smk_of_current();
> > > > > > > >  	struct smack_known *isp = smk_of_inode(inode);
> > > > > > > >  	struct smack_known *dsp = smk_of_inode(dir);
> > > > > > > >  	struct xattr *xattr = lsm_get_xattr_slot(xattrs, xattr_count);
> > > > > > > > +	struct xattr *xattr2;
> > > > > > > I'm going to channel Paul and suggest this be xattr_transmute instead of xattr2.
> > > > > > > It also looks like it could move to be declared in the if clause.
> > > > > > > 
> > > > > > > >  	int may;
> > > > > > > >  
> > > > > > > >  	if (xattr) {
> > > > > > > > @@ -979,7 +979,17 @@ static int smack_inode_init_security(struct inode *inode, struct inode *dir,
> > > > > > > >  		if (may > 0 && ((may & MAY_TRANSMUTE) != 0) &&
> > > > > > > >  		    smk_inode_transmutable(dir)) {
> > > > > > > >  			isp = dsp;
> > > > > > > > -			issp->smk_flags |= SMK_INODE_CHANGED;
> > > > > > > I think you need to keep this. More below.
> > > > > > > 
> > > > > > > > +			xattr2 = lsm_get_xattr_slot(xattrs, xattr_count);
> > > > > > > > +			if (xattr2) {
> > > > > > > > +				xattr2->value = kmemdup(TRANS_TRUE,
> > > > > > > > +							TRANS_TRUE_SIZE,
> > > > > > > > +							GFP_NOFS);
> > > > > > > > +				if (xattr2->value == NULL)
> > > > > > > > +					return -ENOMEM;
> > > > > > > > +
> > > > > > > > +				xattr2->value_len = TRANS_TRUE_SIZE;
> > > > > > > > +				xattr2->name = XATTR_NAME_SMACKTRANSMUTE;
> > > > > > > > +			}
> > > > > > > >  		}
> > > > > > > >  
> > > > > > > >  		xattr->value = kstrdup(isp->smk_known, GFP_NOFS);
> > > > > > > > @@ -3512,20 +3522,12 @@ static void smack_d_instantiate(struct dentry *opt_dentry, struct inode *inode)
> > > > > > > >  			 * If there is a transmute attribute on the
> > > > > > > >  			 * directory mark the inode.
> > > > > > > >  			 */
> > > > > > > > -			if (isp->smk_flags & SMK_INODE_CHANGED) {
> > > > > > > > -				isp->smk_flags &= ~SMK_INODE_CHANGED;
> > > > > > > > -				rc = __vfs_setxattr(&nop_mnt_idmap, dp, inode,
> > > > > > > > -					XATTR_NAME_SMACKTRANSMUTE,
> > > > > > > > -					TRANS_TRUE, TRANS_TRUE_SIZE,
> > > > > > > > -					0);
> > > > > > > > -			} else {
> > > > > > > > -				rc = __vfs_getxattr(dp, inode,
> > > > > > > > -					XATTR_NAME_SMACKTRANSMUTE, trattr,
> > > > > > > > -					TRANS_TRUE_SIZE);
> > > > > > > > -				if (rc >= 0 && strncmp(trattr, TRANS_TRUE,
> > > > > > > > -						       TRANS_TRUE_SIZE) != 0)
> > > > > > > > -					rc = -EINVAL;
> > > > > > > > -			}
> > > > > > > > +			rc = __vfs_getxattr(dp, inode,
> > > > > > > > +					    XATTR_NAME_SMACKTRANSMUTE, trattr,
> > > > > > > > +					    TRANS_TRUE_SIZE);
> > > > > > > > +			if (rc >= 0 && strncmp(trattr, TRANS_TRUE,
> > > > > > > > +					       TRANS_TRUE_SIZE) != 0)
> > > > > > > > +				rc = -EINVAL;
> > > > > > > Where is the SMACK64_TRANSMUTE attribute going to get set on the file?
> > > > > > > It's not going to get set in smack_init_inode_security(). The inode will
> > > > > > Isn't that the purpose of the inode_init_security hook?
> > > > > No. It initializes the in-memory inode. 
> > > > I hope I'm not mistaken here...
> > > > 
> > > > I make a small example. Filesystems call
> > > > security_inode_init_security(). Ext4 does:
> > > > 
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/ext4/xattr_security.c?h=v6.3-rc6#n54
> > > > 
> > > > security_inode_init_security() allocates new_xattrs. Each LSM fills
> > > > new_xattrs. At the end of the loop, if there is at least one xattr
> > > > filled, the initxattrs() callback passed by the caller of
> > > > security_inode_init_security() is called.
> > > > 
> > > > The ext4 initxattrs() callback is:
> > > > 
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/ext4/xattr_security.c?h=v6.3-rc6#n35
> > > > 
> > > > which scans the xattr array and, for each xattr,
> > > > calls ext4_xattr_set_handle().
> > > > 
> > > > Maybe I'm overlooking it, but ext4_xattr_set_handle() is setting xattrs
> > > > on the disk. Am I wrong?
> > > Yes, you're wrong. I tried your change, and the SMACK64_TRANSMUTE isn't
> > > set on the sub-directory when it's created. The __vfs_setxattr() call really
> > > is necessary. 
> > Could you please also check if there is any change with this fix:
> > 
> > Replace:
> > 
> > 	xattr2->name = XATTR_NAME_SMACKTRANSMUTE;
> > 
> > with:
> > 
> > 	xattr2->name = XATTR_SMACK_TRANSMUTE;
> > 
> > Thanks
> 
> Unless I'm missing something really obvious there's no way that any
> of the evm/ima changes would obviate the need for the __vfs_setxattr() call.
> It's real easy to verify correct behavior, see the attached script.
> (you'll want to change the "notroot" value to a user on your system)

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@amazon.com/

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.

Roberto

> > Roberto
> > 
> > > > Thanks
> > > > 
> > > > Roberto
> > > > 
> > > > > > After all LSMs provide one or multiple xattrs, xattrs are going to be
> > > > > > written to the disk with the initxattr() callback of filesystems.
> > > > > > 
> > > > > > There is a small mistake above (XATTR_SMACK_TRANSMUTE instead
> > > > > > of XATTR_NAME_SMACKTRANSMUTE, as we are providing just the suffix).
> > > > > but I'm pretty sure the __vfs_setxattr() call is necessary to get
> > > > > the attribute written out. With your change the in-memory inode will
> > > > > get the attribute, but if you reboot it won't be on the directory.
> > > > > 
> > > > > > 95 Passed, 0 Failed, 100% Success rate
> > > > > > 
> > > > > > There was a test failing in dir-transmute.sh, before I fixed the xattr
> > > > > > name.
> > > > > > 
> > > > > > Thanks
> > > > > > 
> > > > > > Roberto
> > > > > > 
> > > > > > > know it's transmuting, but it won't get to disk without the __vfs_setxattr()
> > > > > > > here in smack_d_instantiate(). Now, it's been a long time since that code
> > > > > > > was written, so I could be wrong, but I'm pretty sure about that.
> > > > > > > 
> > > > > > > I think that you should be fine with the changes in smack_init_inode_security(),
> > > > > > > and leaving smack_d_instantiate() untouched. 
> > > > > > > 
> > > > > > > >  			if (rc >= 0)
> > > > > > > >  				transflag = SMK_INODE_TRANSMUTE;
> > > > > > > >  		}

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