Re: [PATCH] Allow restricting permissions in /proc/sys

[Topi Miettinen]

On 13.11.2019 6.50, Andy Lutomirski wrote:
> On Tue, Nov 12, 2019 at 3:22 PM Christian Brauner
> <christian.brauner@xxxxxxxxxx> wrote:
> > [Cc+ linux-api@xxxxxxxxxxxxxxx]
> >
> > since that's potentially relevant to quite a few people.
> >
> > On Sun, Nov 03, 2019 at 04:55:48PM +0200, Topi Miettinen wrote:
> > > Several items in /proc/sys need not be accessible to unprivileged
> > > tasks. Let the system administrator change the permissions, but only
> > > to more restrictive modes than what the sysctl tables allow.
> > >
> > > Signed-off-by: Topi Miettinen <toiwoton@xxxxxxxxx>
> > > ---
> > >   fs/proc/proc_sysctl.c | 41 +++++++++++++++++++++++++++++++----------
> > >   1 file changed, 31 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> > > index d80989b6c344..88c4ca7d2782 100644
> > > --- a/fs/proc/proc_sysctl.c
> > > +++ b/fs/proc/proc_sysctl.c
> > > @@ -818,6 +818,10 @@ static int proc_sys_permission(struct inode *inode, int
> > > mask)
> > >          if ((mask & MAY_EXEC) && S_ISREG(inode->i_mode))
> > >                  return -EACCES;
> > >
> > > +       error = generic_permission(inode, mask);
> > > +       if (error)
> > > +               return error;
> > > +
> > >          head = grab_header(inode);
> > >          if (IS_ERR(head))
> > >                  return PTR_ERR(head);
> > > @@ -837,9 +841,35 @@ static int proc_sys_setattr(struct dentry *dentry,
> > > struct iattr *attr)
> > >          struct inode *inode = d_inode(dentry);
> > >          int error;
> > >
> > > -       if (attr->ia_valid & (ATTR_MODE | ATTR_UID | ATTR_GID))
> > > +       if (attr->ia_valid & (ATTR_UID | ATTR_GID))
> > >                  return -EPERM;
>
> Supporting at least ATTR_GID would make this much more useful.

Yes, also XATTR/ACL support would be useful. But so far I've tried to 
allow only tightening of permissions.

> > > +       if (attr->ia_valid & ATTR_MODE) {
> > > +               struct ctl_table_header *head = grab_header(inode);
> > > +               struct ctl_table *table = PROC_I(inode)->sysctl_entry;
> > > +               umode_t max_mode = 0777; /* Only these bits may change */
> > > +
> > > +               if (IS_ERR(head))
> > > +                       return PTR_ERR(head);
> > > +
> > > +               if (!table) /* global root - r-xr-xr-x */
> > > +                       max_mode &= ~0222;
> > > +               else /*
> > > +                     * Don't allow permissions to become less
> > > +                     * restrictive than the sysctl table entry
> > > +                     */
> > > +                       max_mode &= table->mode;
>
> Style nit: please put braces around multi-line if and else branches,
> even if they're only multi-line because of comments.

OK, thanks.

> > > +
> > > +               sysctl_head_finish(head);
> > > +
> > > +               /* Execute bits only allowed for directories */
> > > +               if (!S_ISDIR(inode->i_mode))
> > > +                       max_mode &= ~0111;
>
> Why is this needed?

In general, /proc/sys does not allow executable permissions for the 
files, so I've continued this policy.

-Topi