Re: [PATCH 11/39] selinux: reject writable opens of policy file, drop mmap shared/write check

On Tue, Sep 08, 2026 at 10:22:20PM +0200, Jann Horn wrote:

On Tue, Sep 8, 2026 at 10:07 PM Lorenzo Stoakes (ARM) [off-list ref] wrote:

quoted

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.

You should also be able to remove the sel_mmap_policy_ops.page_mkwrite
handler, which is only for shared-writable faults, right?

Yeah I think you're right, though I wonder if it's a bit out of scope for this
change as it's focused on removing a VMA_MAYWRITE_BIT clearance for a
non-kernel-owned VMA.

So maybe one for a follow up?

quoted

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) <ljs@kernel.org>

Reviewed-by: Jann Horn <jannh@google.com>

Thanks!

--
Cheers, Lorenzo