[PATCH 1/2] fs: Provide function that allocates a secure anonymous inode

[Shivank Garg]

On 6/6/2025 8:39 PM, Ira Weiny wrote:
> Paul Moore wrote:
>> On Thu, Jun 5, 2025 at 1:50 AM Mike Rapoport <rppt at kernel.org> wrote:
>>>
>>> secretmem always had S_PRIVATE set because alloc_anon_inode() clears it
>>> anyway and this patch does not change it.
>>
>> Yes, my apologies, I didn't look closely enough at the code.
>>
>>> I'm just thinking that it makes sense to actually allow LSM/SELinux
>>> controls that S_PRIVATE bypasses for both secretmem and guest_memfd.
>>
>> It's been a while since we added the anon_inode hooks so I'd have to
>> go dig through the old thread to understand the logic behind marking
>> secretmem S_PRIVATE, especially when the
>> anon_inode_make_secure_inode() function cleared it.  It's entirely
>> possible it may have just been an oversight.
> 
> I'm jumping in where I don't know what I'm talking about...
> 
> But my reading of the S_PRIVATE flag is that the memory can't be mapped by
> user space.  So for guest_memfd() we need !S_PRIVATE because it is
> intended to be mapped by user space.  So we want the secure checks.
> 
> I think secretmem is the same.
> 
> Do I have that right?


Hi Mike, Paul,

If I understand correctly,
we need to clear the S_PRIVATE flag for all secure inodes. The S_PRIVATE flag was previously
set for  secretmem (via alloc_anon_inode()), which caused security checks to be 
bypassed - this was unintentional since the original anon_inode_make_secure_inode() 
was already clearing it.

Both secretmem and guest_memfd create file descriptors
(memfd_create/kvm_create_guest_memfd)
so they should be subject to LSM/SELinux security policies rather than bypassing them with S_PRIVATE?

static struct inode *anon_inode_make_secure_inode(struct super_block *s,
		const char *name, const struct inode *context_inode)
{
...
	/* Clear S_PRIVATE for all inodes*/
	inode->i_flags &= ~S_PRIVATE;
...
}

Please let me know if this conclusion makes sense?

Thanks,
Shivank

> 
> Ira
> 
> [snip]
>