linux-kernel - Re: [PATCH v4 1/3] lsm: introduce new hooks for setting/getting inode fsxattr

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Message-ID: <20250324.aThi9ioghiex@digikod.net>
Date: Mon, 24 Mar 2025 20:27:02 +0100
From: Mickaël Salaün <mic@...ikod.net>
To: Paul Moore <paul@...l-moore.com>
Cc: Andrey Albershteyn <aalbersh@...hat.com>, 
	Richard Henderson <richard.henderson@...aro.org>, Matt Turner <mattst88@...il.com>, 
	Russell King <linux@...linux.org.uk>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will@...nel.org>, Geert Uytterhoeven <geert@...ux-m68k.org>, 
	Michal Simek <monstr@...str.eu>, Thomas Bogendoerfer <tsbogend@...ha.franken.de>, 
	"James E.J. Bottomley" <James.Bottomley@...senpartnership.com>, Helge Deller <deller@....de>, 
	Madhavan Srinivasan <maddy@...ux.ibm.com>, Michael Ellerman <mpe@...erman.id.au>, 
	Nicholas Piggin <npiggin@...il.com>, Christophe Leroy <christophe.leroy@...roup.eu>, 
	Naveen N Rao <naveen@...nel.org>, Heiko Carstens <hca@...ux.ibm.com>, 
	Vasily Gorbik <gor@...ux.ibm.com>, Alexander Gordeev <agordeev@...ux.ibm.com>, 
	Christian Borntraeger <borntraeger@...ux.ibm.com>, Sven Schnelle <svens@...ux.ibm.com>, 
	Yoshinori Sato <ysato@...rs.sourceforge.jp>, Rich Felker <dalias@...c.org>, 
	John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>, "David S. Miller" <davem@...emloft.net>, 
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	Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>, 
	Günther Noack <gnoack@...gle.com>, Arnd Bergmann <arnd@...db.de>, 
	Pali Rohár <pali@...nel.org>, James Morris <jmorris@...ei.org>, 
	"Serge E. Hallyn" <serge@...lyn.com>, linux-alpha@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-m68k@...ts.linux-m68k.org, linux-mips@...r.kernel.org, 
	linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org, linux-s390@...r.kernel.org, 
	linux-sh@...r.kernel.org, sparclinux@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-security-module@...r.kernel.org, linux-api@...r.kernel.org, linux-arch@...r.kernel.org, 
	selinux@...r.kernel.org, Andrey Albershteyn <aalbersh@...nel.org>
Subject: Re: [PATCH v4 1/3] lsm: introduce new hooks for setting/getting
 inode  fsxattr

On Fri, Mar 21, 2025 at 05:32:25PM -0400, Paul Moore wrote:
> On Mar 21, 2025 Andrey Albershteyn <aalbersh@...hat.com> wrote:
> > 
> > Introduce new hooks for setting and getting filesystem extended
> > attributes on inode (FS_IOC_FSGETXATTR).
> > 
> > Cc: selinux@...r.kernel.org
> > Cc: Paul Moore <paul@...l-moore.com>
> > 
> > Signed-off-by: Andrey Albershteyn <aalbersh@...nel.org>
> > ---
> >  fs/ioctl.c                    |  7 ++++++-
> >  include/linux/lsm_hook_defs.h |  4 ++++
> >  include/linux/security.h      | 16 ++++++++++++++++
> >  security/security.c           | 32 ++++++++++++++++++++++++++++++++
> >  4 files changed, 58 insertions(+), 1 deletion(-)
> 
> Thanks Andrey, one small change below, but otherwise this looks pretty
> good.  If you feel like trying to work up the SELinux implementation but
> need some assitance please let me know, I'll be happy to help :)
> 
> > diff --git a/fs/ioctl.c b/fs/ioctl.c
> > index 638a36be31c14afc66a7fd6eb237d9545e8ad997..4434c97bc5dff5a3e8635e28745cd99404ff353e 100644
> > --- a/fs/ioctl.c
> > +++ b/fs/ioctl.c
> > @@ -525,10 +525,15 @@ EXPORT_SYMBOL(fileattr_fill_flags);
> >  int vfs_fileattr_get(struct dentry *dentry, struct fileattr *fa)
> >  {
> >  	struct inode *inode = d_inode(dentry);
> > +	int error;
> >  
> >  	if (!inode->i_op->fileattr_get)
> >  		return -ENOIOCTLCMD;
> >  
> > +	error = security_inode_getfsxattr(inode, fa);
> > +	if (error)
> > +		return error;
> > +
> >  	return inode->i_op->fileattr_get(dentry, fa);
> >  }
> >  EXPORT_SYMBOL(vfs_fileattr_get);
> > @@ -692,7 +697,7 @@ int vfs_fileattr_set(struct mnt_idmap *idmap, struct dentry *dentry,
> >  			fa->flags |= old_ma.flags & ~FS_COMMON_FL;
> >  		}
> >  		err = fileattr_set_prepare(inode, &old_ma, fa);
> > -		if (!err)
> > +		if (!err && !security_inode_setfsxattr(inode, fa))
> >  			err = inode->i_op->fileattr_set(idmap, dentry, fa);
> >  	}
> >  	inode_unlock(inode);
> 
> I don't believe we want to hide or otherwise drop the LSM return code as
> that could lead to odd behavior, e.g. returning 0/success despite not
> having executed the fileattr_set operation.

Yes, this should look something like this:

 		err = fileattr_set_prepare(inode, &old_ma, fa);
 		if (err)
 			goto out;
 		err = security_inode_setfsxattr(dentry, fa);
 		if (err)
 			goto out;
 		err = inode->i_op->fileattr_set(idmap, dentry, fa);
 		if (err)
 			goto out;

> 
> --
> paul-moore.com
>