Re: [PATCH v5 11/38] selinux: reject writable opens of policy file, drop mmap shared/write check

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.

Acked-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Reviewed-by: Jann Horn <jannh@google.com>
Acked-by: Paul Moore <paul@paul-moore.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261009-b4-mmap-prepare-vma-flag-sanify-v5-0-923b70125b1e@kernel.org?part=11