[PATCH] fs: export anon_inode_make_secure_inode() and fix secretmem LSM bypass

[Christian Brauner]

On Thu, Jun 19, 2025 at 02:01:22PM +0300, Mike Rapoport wrote:
> On Thu, Jun 19, 2025 at 12:38:25PM +0200, Christian Brauner wrote:
> > On Thu, Jun 19, 2025 at 11:13:49AM +0200, Vlastimil Babka wrote:
> > > On 6/19/25 09:31, Shivank Garg wrote:
> > > > Export anon_inode_make_secure_inode() to allow KVM guest_memfd to create
> > > > anonymous inodes with proper security context. This replaces the current
> > > > pattern of calling alloc_anon_inode() followed by
> > > > inode_init_security_anon() for creating security context manually.
> > > > 
> > > > This change also fixes a security regression in secretmem where the
> > > > S_PRIVATE flag was not cleared after alloc_anon_inode(), causing
> > > > LSM/SELinux checks to be bypassed for secretmem file descriptors.
> > > > 
> > > > As guest_memfd currently resides in the KVM module, we need to export this
> > > 
> > > Could we use the new EXPORT_SYMBOL_GPL_FOR_MODULES() thingy to make this
> > > explicit for KVM?
> > 
> > Oh? Enlighten me about that, if you have a second, please. 
> 
> From Documentation/core-api/symbol-namespaces.rst:
> 
> The macro takes a comma separated list of module names, allowing only those
> modules to access this symbol. Simple tail-globs are supported.
> 
> For example::
> 
>   EXPORT_SYMBOL_GPL_FOR_MODULES(preempt_notifier_inc, "kvm,kvm-*")
> 
> will limit usage of this symbol to modules whoes name matches the given
> patterns.

Is that still mostly advisory and can still be easily circumenvented?