Re: [PATCH v2] selinux: preserve user SID across nested backing files

[Karl Mehltretter]

On Thu, Aug 27, 2026 at 03:39:10PM +0100, Paul Moore wrote:
> If you could share the distro and some more details on the testing
> methodology that would be helpful.

The original test used a small BusyBox initramfs.

I repeated it on arm64 QEMU with kernel v7.2-rc1-9-gfd6e2388a3ea, using
both BusyBox and Fedora Cloud Base 44 userspace. Both gave the same result
with SELinux enforcing:

  unpatched: mprotect(PROT_READ) failed with EACCES and the expected
             base_t -> mounter_t:fd { use } denial
  patched:   mprotect(PROT_READ) succeeded

The test mounted two nested overlays as mounter_t, changed back to base_t,
mapped the outer file, then called mprotect(). Both guests used the same
purpose-built policy, not the Fedora stock policy.

> Additional comments below, and while this patch is useful in fixing some
> of the problems (thank you!), we obviously need to fix the others as
> well.  Is this something you think you will be able to work on?

I also tested current mainline commit cf72cbb39da84, which contains
f2381b546e7e ("fs: fix user path of nested backing files"). That fixes the
top path and inode, but exposed the remaining intermediate-mounter issue:

direct mmap(PROT_EXEC) was denied against middle_file_t, while
mmap(PROT_NONE) followed by mprotect(PROT_EXEC) succeeded.

I implemented a second patch which preserves and rechecks each intermediate
path, mounter SID, and file-description SID. With both patches, direct mmap
and deferred mprotect are denied against middle_file_t, and the original SID
propagation test still passes.

Together with f2381b546e7e, this covers the remaining issues. I will send v3
as a two-patch series.

Thanks,
Karl