Re: [PATCH] selinux: move user accesses in selinuxfs out of locked regions

[Jann Horn]

On Tue, Jun 26, 2018 at 2:15 PM Stephen Smalley <sds@xxxxxxxxxxxxx> wrote:
>
> On 06/25/2018 12:34 PM, Jann Horn wrote:
> > If a user is accessing a file in selinuxfs with a pointer to a userspace
> > buffer that is backed by e.g. a userfaultfd, the userspace access can
> > stall indefinitely, which can block fsi->mutex if it is held.
> >
> > For sel_read_policy(), remove the locking, since this method doesn't seem
> > to access anything that requires locking.
> >
> > For sel_read_bool(), move the user access below the locked region.
> >
> > For sel_write_bool() and sel_commit_bools_write(), move the user access
> > up above the locked region.
> >
> > Cc: stable@xxxxxxxxxxxxxxx
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
>
> Only question I have is wrt the Fixes line, i.e. was this an issue until userfaultfd was introduced, and if not,
> do we need it to be back-ported any further than the commit which introduced it.

You can also use FUSE, if the system is configured appropriately:
Mount a FUSE filesystem, mmap() a file from it, then pass a pointer to
the mmapped region to a syscall. AFAICS FUSE was added to the kernel
in commit d8a5ba45457e4a22aa39c939121efd7bb6c76672, first in
v2.6.16.28.

> Otherwise, you can add my
> Acked-by: Stephen Smalley <sds@xxxxxxxxxxxxx>

This patch should go through Paul Moore's tree, right?

> > ---
> >  security/selinux/selinuxfs.c | 77 ++++++++++++++++--------------------
> >  1 file changed, 33 insertions(+), 44 deletions(-)
> >
> > diff --git a/security/selinux/selinuxfs.c b/security/selinux/selinuxfs.c
> > index f3d374d2ca04..065f8cea84e3 100644
> > --- a/security/selinux/selinuxfs.c
> > +++ b/security/selinux/selinuxfs.c
> > @@ -445,18 +445,13 @@ static ssize_t sel_read_policy(struct file *filp, char __user *buf,
> >       struct policy_load_memory *plm = filp->private_data;
> >       int ret;
> >
> > -     mutex_lock(&fsi->mutex);
> > -
> >       ret = avc_has_perm(&selinux_state,
> >                          current_sid(), SECINITSID_SECURITY,
> >                         SECCLASS_SECURITY, SECURITY__READ_POLICY, NULL);
> >       if (ret)
> > -             goto out;
> > +             return ret;
> >
> > -     ret = simple_read_from_buffer(buf, count, ppos, plm->data, plm->len);
> > -out:
> > -     mutex_unlock(&fsi->mutex);
> > -     return ret;
> > +     return simple_read_from_buffer(buf, count, ppos, plm->data, plm->len);
> >  }
> >
> >  static vm_fault_t sel_mmap_policy_fault(struct vm_fault *vmf)
> > @@ -1188,25 +1183,29 @@ static ssize_t sel_read_bool(struct file *filep, char __user *buf,
> >       ret = -EINVAL;
> >       if (index >= fsi->bool_num || strcmp(name,
> >                                            fsi->bool_pending_names[index]))
> > -             goto out;
> > +             goto out_unlock;
> >
> >       ret = -ENOMEM;
> >       page = (char *)get_zeroed_page(GFP_KERNEL);
> >       if (!page)
> > -             goto out;
> > +             goto out_unlock;
> >
> >       cur_enforcing = security_get_bool_value(fsi->state, index);
> >       if (cur_enforcing < 0) {
> >               ret = cur_enforcing;
> > -             goto out;
> > +             goto out_unlock;
> >       }
> >       length = scnprintf(page, PAGE_SIZE, "%d %d", cur_enforcing,
> >                         fsi->bool_pending_values[index]);
> > -     ret = simple_read_from_buffer(buf, count, ppos, page, length);
> > -out:
> >       mutex_unlock(&fsi->mutex);
> > +     ret = simple_read_from_buffer(buf, count, ppos, page, length);
> > +out_free:
> >       free_page((unsigned long)page);
> >       return ret;
> > +
> > +out_unlock:
> > +     mutex_unlock(&fsi->mutex);
> > +     goto out_free;
> >  }
> >
> >  static ssize_t sel_write_bool(struct file *filep, const char __user *buf,
> > @@ -1219,6 +1218,17 @@ static ssize_t sel_write_bool(struct file *filep, const char __user *buf,
> >       unsigned index = file_inode(filep)->i_ino & SEL_INO_MASK;
> >       const char *name = filep->f_path.dentry->d_name.name;
> >
> > +     if (count >= PAGE_SIZE)
> > +             return -ENOMEM;
> > +
> > +     /* No partial writes. */
> > +     if (*ppos != 0)
> > +             return -EINVAL;
> > +
> > +     page = memdup_user_nul(buf, count);
> > +     if (IS_ERR(page))
> > +             return PTR_ERR(page);
> > +
> >       mutex_lock(&fsi->mutex);
> >
> >       length = avc_has_perm(&selinux_state,
> > @@ -1233,22 +1243,6 @@ static ssize_t sel_write_bool(struct file *filep, const char __user *buf,
> >                                            fsi->bool_pending_names[index]))
> >               goto out;
> >
> > -     length = -ENOMEM;
> > -     if (count >= PAGE_SIZE)
> > -             goto out;
> > -
> > -     /* No partial writes. */
> > -     length = -EINVAL;
> > -     if (*ppos != 0)
> > -             goto out;
> > -
> > -     page = memdup_user_nul(buf, count);
> > -     if (IS_ERR(page)) {
> > -             length = PTR_ERR(page);
> > -             page = NULL;
> > -             goto out;
> > -     }
> > -
> >       length = -EINVAL;
> >       if (sscanf(page, "%d", &new_value) != 1)
> >               goto out;
> > @@ -1280,6 +1274,17 @@ static ssize_t sel_commit_bools_write(struct file *filep,
> >       ssize_t length;
> >       int new_value;
> >
> > +     if (count >= PAGE_SIZE)
> > +             return -ENOMEM;
> > +
> > +     /* No partial writes. */
> > +     if (*ppos != 0)
> > +             return -EINVAL;
> > +
> > +     page = memdup_user_nul(buf, count);
> > +     if (IS_ERR(page))
> > +             return PTR_ERR(page);
> > +
> >       mutex_lock(&fsi->mutex);
> >
> >       length = avc_has_perm(&selinux_state,
> > @@ -1289,22 +1294,6 @@ static ssize_t sel_commit_bools_write(struct file *filep,
> >       if (length)
> >               goto out;
> >
> > -     length = -ENOMEM;
> > -     if (count >= PAGE_SIZE)
> > -             goto out;
> > -
> > -     /* No partial writes. */
> > -     length = -EINVAL;
> > -     if (*ppos != 0)
> > -             goto out;
> > -
> > -     page = memdup_user_nul(buf, count);
> > -     if (IS_ERR(page)) {
> > -             length = PTR_ERR(page);
> > -             page = NULL;
> > -             goto out;
> > -     }
> > -
> >       length = -EINVAL;
> >       if (sscanf(page, "%d", &new_value) != 1)
> >               goto out;
> >
>