Re: [PATCH v3] fs: introduce getfsxattrat and setfsxattrat syscalls

Paul Moore
2025-02-25T02:38:08Z

On Mon, Feb 24, 2025 at 11:00 AM Andrey Albershteyn <aalbersh@redhat.com> wrote:
> On 2025-02-21 16:08:33, Mickaël Salaün wrote:
> > It looks security checks are missing. With IOCTL commands, file
> > permissions are checked at open time, but with these syscalls the path
> > is only resolved but no specific access seems to be checked (except
> > inode_owner_or_capable via vfs_fileattr_set).

...

> > On Tue, Feb 11, 2025 at 06:22:47PM +0100, Andrey Albershteyn wrote:

...

> > > +SYSCALL_DEFINE4(setfsxattrat, int, dfd, const char __user *, filename,
> > > + struct fsxattr __user *, fsx, unsigned int, at_flags)
> > > +{
> > > + CLASS(fd, dir)(dfd);
> > > + struct fileattr fa;
> > > + struct path filepath;
> > > + int error;
> > > + unsigned int lookup_flags = 0;
> > > +
> > > + if ((at_flags & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH)) != 0)
> > > + return -EINVAL;
> > > +
> > > + if (at_flags & AT_SYMLINK_FOLLOW)
> > > + lookup_flags |= LOOKUP_FOLLOW;
> > > +
> > > + if (at_flags & AT_EMPTY_PATH)
> > > + lookup_flags |= LOOKUP_EMPTY;
> > > +
> > > + if (fd_empty(dir))
> > > + return -EBADF;
> > > +
> > > + if (copy_fsxattr_from_user(&fa, fsx))
> > > + return -EFAULT;
> > > +
> > > + error = user_path_at(dfd, filename, lookup_flags, &filepath);
> > > + if (error)
> > > + return error;
> > > +
> > > + error = mnt_want_write(filepath.mnt);
> > > + if (!error) {
> >
> > security_inode_setattr() should probably be called too.
>
> Aren't those checks for something different - inode attributes
> ATTR_*?
> (sorry, the naming can't be more confusing)
>
> Looking into security_inode_setattr() it seems to expect struct
> iattr, which works with inode attributes (mode, time, uid/gid...).
> These new syscalls work with filesystem inode extended flags/attributes
> FS_XFLAG_* in fsxattr->fsx_xflags. Let me know if I missing
> something here

A valid point. While these are two different operations, with
different structs/types, I suspect that most LSMs will consider them
to be roughly equivalent from an access control perspective, which is
why I felt the existing security_inode_{set,get}attr() hooks seemed
appropriate. However, there likely is value in keeping the ATTR and
FSX operations separate; those LSMs that wish to treat them the same
can easily do so in their respective LSM callbacks.

With all this in mind, I think it probably makes sense to create two
new LSM hooks, security_inode_{get,set}fsxattr(). The get hook should
probably be placed inside vfs_fileattr_get() just before the call to
the inode's fileattr_get() method, and the set hook should probably be
placed inside vfs_fileattr_set(), inside the inode lock and after a
successful call to fileattr_set_prepare().

Does that sound better to everyone?

--
paul-moore.com