On Tue, Sep 8, 2026 at 4:07 PM Lorenzo Stoakes (ARM) <[email protected]> wrote: > > The policy file has no write method and is exposed read-only (S_IRUGO in > selinux_files[]), yet sel_open_policy() performs no open mode check, so a > CAP_DAC_OVERRIDE caller can open it O_RDWR. Reject FMODE_WRITE at open, as > kernfs does. > > The file can then never be mapped with FMODE_WRITE, so do_mmap() always > clears VM_MAYWRITE and VM_SHARED for MAP_SHARED mappings and the VM_SHARED > check in sel_mmap_policy() cannot be reached. Remove it. > > This also stops sel_mmap_policy() clearing VM_MAYWRITE on a mapping that is > neither a PFN map nor a mixed map, ahead of the core enforcing that only > such mappings may do so. > > Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]>
We should likely do something similar for the other sel_open_*() and sel_mmap_*() functions too. In particular, truncation of the status file can bring down systemd since it is mapped by libselinux. Acked-by: Stephen Smalley <[email protected]> > --- > security/selinux/selinuxfs.c | 11 +++-------- > 1 file changed, 3 insertions(+), 8 deletions(-) > > diff --git a/security/selinux/selinuxfs.c b/security/selinux/selinuxfs.c > index c7d91476971c..545a6f89f9e7 100644 > --- a/security/selinux/selinuxfs.c > +++ b/security/selinux/selinuxfs.c > @@ -340,6 +340,9 @@ static int sel_open_policy(struct inode *inode, struct > file *filp) > struct policy_load_memory *plm = NULL; > int rc; > > + if (filp->f_mode & FMODE_WRITE) > + return -EACCES; > + > rc = avc_has_perm(current_sid(), SECINITSID_SECURITY, > SECCLASS_SECURITY, SECURITY__READ_POLICY, NULL); > if (rc) > @@ -424,14 +427,6 @@ static const struct vm_operations_struct > sel_mmap_policy_ops = { > > static int sel_mmap_policy(struct file *filp, struct vm_area_struct *vma) > { > - if (vma->vm_flags & VM_SHARED) { > - /* do not allow mprotect to make mapping writable */ > - vm_flags_clear(vma, VM_MAYWRITE); > - > - if (vma->vm_flags & VM_WRITE) > - return -EACCES; > - } > - > vm_flags_set(vma, VM_DONTEXPAND | VM_DONTDUMP); > vma->vm_ops = &sel_mmap_policy_ops; > > > -- > 2.55.0 >
