Re: [PATCH 2/2] sysctl: Restore the selinux path based label lookup for sysctls.

Stephen Smalley
2007-02-07T21:17:11Z

On Wed, 2007-02-07 at 13:24 -0500, Stephen Smalley wrote:
> On Tue, 2007-02-06 at 14:21 -0700, Eric W. Biederman wrote:
> > This time instead of generating the generating the paths from proc_dir_entries
> > generate the labels from the names in the sysctl ctl_tables themselves. This
> > removes an unnecessary layer of indirection, allows this to work even when
> > procfs support is not compiled into the kernel, and especially allows it
> > to work now that ctl_tables no longer have a proc_dir_entry field.
>
> Thanks, looks sane.
>
> > I continue passing "proc" into genfs sid although that is complete nonsense
> > to allow existing selinux policies to work without modification.
> >
> > Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
>
> Acked-by: Stephen Smalley <sds@tycho.nsa.gov>

Hmmm...but in testing the patch, I don't seem to (consistently) reach
these checks when accessing via /proc/sys. I see that you are caching
the mode information and using it in some cases rather than calling the
sysctl_perm function.

One related but separate issue is that the /proc/sys inode labeling is
also affected by the sysctl patch series. Those inodes used to be
labeled by selinux_proc_get_sid (from selinux_d_instantiate), but that
no longer works, so they now fall back to the superblock SID (generic
proc label). That changes the inode permission checks on an attempt to
access a /proc/sys node and will likely cause denials under current
policy for confined domains since one wouldn't generally be writing to
the generic proc label. If you always called sysctl_perm from the proc
sysctl code, we could possibly dispense with inode permission checking
on those inodes, e.g. marking them private.

>
> > ---
> > security/selinux/hooks.c | 43 +++++++++++++++++++++++++++++++++++++++++--
> > 1 files changed, 41 insertions(+), 2 deletions(-)
> >
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > index 3a36057..c17a8dd 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -1424,6 +1424,41 @@ static int selinux_capable(struct task_struct *tsk, int cap)
> > return task_has_capability(tsk,cap);
> > }
> >
> > +static int selinux_sysctl_get_sid(ctl_table *table, u16 tclass, u32 *sid)
> > +{
> > + int buflen, rc;
> > + char *buffer, *path, *end;
> > +
> > + rc = -ENOMEM;
> > + buffer = (char*)__get_free_page(GFP_KERNEL);
> > + if (!buffer)
> > + goto out;
> > +
> > + buflen = PAGE_SIZE;
> > + end = buffer+buflen;
> > + *--end = '\0';
> > + buflen--;
> > + path = end-1;
> > + *path = '/';
> > + while (table) {
> > + const char *name = table->procname;
> > + size_t namelen = strlen(name);
> > + buflen -= namelen + 1;
> > + if (buflen < 0)
> > + goto out_free;
> > + end -= namelen;
> > + memcpy(end, name, namelen);
> > + *--end = '/';
> > + path = end;
> > + table = table->parent;
> > + }
> > + rc = security_genfs_sid("proc", path, tclass, sid);
> > +out_free:
> > + free_page((unsigned long)buffer);
> > +out:
> > + return rc;
> > +}
> > +
> > static int selinux_sysctl(ctl_table *table, int op)
> > {
> > int error = 0;
> > @@ -1438,8 +1473,12 @@ static int selinux_sysctl(ctl_table *table, int op)
> >
> > tsec = current->security;
> >
> > - /* Use the well-defined sysctl SID. */
> > - tsid = SECINITSID_SYSCTL;
> > + rc = selinux_sysctl_get_sid(table, (op == 0001) ?
> > + SECCLASS_DIR : SECCLASS_FILE, &tsid);
> > + if (rc) {
> > + /* Default to the well-defined sysctl SID. */
> > + tsid = SECINITSID_SYSCTL;
> > + }
> >
> > /* The op values are "defined" in sysctl.c, thereby creating
> > * a bad coupling between this module and sysctl.c */
--
Stephen Smalley
National Security Agency