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

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

[Paul Moore]

On Wed, Jun 4, 2025 at 3:59 AM Mike Rapoport <rppt@kernel.org> wrote:
>
> (added Paul Moore for selinux bits)

Thanks Mike.

I'm adding the LSM and SELinux lists too since there are others that
will be interested as well.

> On Mon, Jun 02, 2025 at 12:17:54PM -0700, Ackerley Tng wrote:
> > The new function, alloc_anon_secure_inode(), returns an inode after
> > running checks in security_inode_init_security_anon().
> >
> > Also refactor secretmem's file creation process to use the new
> > function.
> >
> > Suggested-by: David Hildenbrand <david@redhat.com>
> > Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> > ---
> >  fs/anon_inodes.c   | 22 ++++++++++++++++------
> >  include/linux/fs.h |  1 +
> >  mm/secretmem.c     |  9 +--------
> >  3 files changed, 18 insertions(+), 14 deletions(-)
> >
> > diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
> > index 583ac81669c2..4c3110378647 100644
> > --- a/fs/anon_inodes.c
> > +++ b/fs/anon_inodes.c
> > @@ -55,17 +55,20 @@ static struct file_system_type anon_inode_fs_type = {
> >       .kill_sb        = kill_anon_super,
> >  };
> >
> > -static struct inode *anon_inode_make_secure_inode(
> > -     const char *name,
> > -     const struct inode *context_inode)
> > +static struct inode *anon_inode_make_secure_inode(struct super_block *s,
> > +             const char *name, const struct inode *context_inode,
> > +             bool fs_internal)
> >  {
> >       struct inode *inode;
> >       int error;
> >
> > -     inode = alloc_anon_inode(anon_inode_mnt->mnt_sb);
> > +     inode = alloc_anon_inode(s);
> >       if (IS_ERR(inode))
> >               return inode;
> > -     inode->i_flags &= ~S_PRIVATE;
> > +
> > +     if (!fs_internal)
> > +             inode->i_flags &= ~S_PRIVATE;
> > +
> >       error = security_inode_init_security_anon(inode, &QSTR(name),
> >                                                 context_inode);
> >       if (error) {
> > @@ -75,6 +78,12 @@ static struct inode *anon_inode_make_secure_inode(
> >       return inode;
> >  }
> >
> > +struct inode *alloc_anon_secure_inode(struct super_block *s, const char *name)
> > +{
> > +     return anon_inode_make_secure_inode(s, name, NULL, true);
> > +}
> > +EXPORT_SYMBOL_GPL(alloc_anon_secure_inode);
> > +
> >  static struct file *__anon_inode_getfile(const char *name,
> >                                        const struct file_operations *fops,
> >                                        void *priv, int flags,
> > @@ -88,7 +97,8 @@ static struct file *__anon_inode_getfile(const char *name,
> >               return ERR_PTR(-ENOENT);
> >
> >       if (make_inode) {
> > -             inode = anon_inode_make_secure_inode(name, context_inode);
> > +             inode = anon_inode_make_secure_inode(anon_inode_mnt->mnt_sb,
> > +                                                  name, context_inode, false);
> >               if (IS_ERR(inode)) {
> >                       file = ERR_CAST(inode);
> >                       goto err;
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index 016b0fe1536e..0fded2e3c661 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -3550,6 +3550,7 @@ extern int simple_write_begin(struct file *file, struct address_space *mapping,
> >  extern const struct address_space_operations ram_aops;
> >  extern int always_delete_dentry(const struct dentry *);
> >  extern struct inode *alloc_anon_inode(struct super_block *);
> > +extern struct inode *alloc_anon_secure_inode(struct super_block *, const char *);
> >  extern int simple_nosetlease(struct file *, int, struct file_lease **, void **);
> >  extern const struct dentry_operations simple_dentry_operations;
> >
> > diff --git a/mm/secretmem.c b/mm/secretmem.c
> > index 1b0a214ee558..c0e459e58cb6 100644
> > --- a/mm/secretmem.c
> > +++ b/mm/secretmem.c
> > @@ -195,18 +195,11 @@ static struct file *secretmem_file_create(unsigned long flags)
> >       struct file *file;
> >       struct inode *inode;
> >       const char *anon_name = "[secretmem]";
> > -     int err;
> >
> > -     inode = alloc_anon_inode(secretmem_mnt->mnt_sb);
> > +     inode = alloc_anon_secure_inode(secretmem_mnt->mnt_sb, anon_name);
> >       if (IS_ERR(inode))
> >               return ERR_CAST(inode);
>
> I don't think we should not hide secretmem and guest_memfd inodes from
> selinux, so clearing S_PRIVATE for them is not needed and you can just drop
> fs_internal parameter in anon_inode_make_secure_inode()

It's especially odd since I don't see any comments or descriptions
about why this is being done.  The secretmem change is concerning as
this is user accessible and marking the inode with S_PRIVATE will
bypass a number of LSM/SELinux access controls, possibly resulting in
a security regression (one would need to dig a bit deeper to see what
is possible with secretmem and which LSM/SELinux code paths would be
affected).

I'm less familiar with guest_memfd, but generally speaking if
userspace can act on the inode/fd then we likely don't want the
S_PRIVATE flag stripped from the anon_inode.

Ackerley can you provide an explanation about why the change in
S_PRIVATE was necessary?

> > -     err = security_inode_init_security_anon(inode, &QSTR(anon_name), NULL);
> > -     if (err) {
> > -             file = ERR_PTR(err);
> > -             goto err_free_inode;
> > -     }
> > -
> >       file = alloc_file_pseudo(inode, secretmem_mnt, "secretmem",
> >                                O_RDWR, &secretmem_fops);
> >       if (IS_ERR(file))
> > --
> > 2.49.0.1204.g71687c7c1d-goog

-- 
paul-moore.com

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

[Mike Rapoport]

On Wed, Jun 04, 2025 at 05:13:35PM -0400, Paul Moore wrote:
> On Wed, Jun 4, 2025 at 3:59 AM Mike Rapoport <rppt@kernel.org> wrote:
> >
> > (added Paul Moore for selinux bits)
> 
> Thanks Mike.
> 
> I'm adding the LSM and SELinux lists too since there are others that
> will be interested as well.
> 
> > On Mon, Jun 02, 2025 at 12:17:54PM -0700, Ackerley Tng wrote:
> > > The new function, alloc_anon_secure_inode(), returns an inode after
> > > running checks in security_inode_init_security_anon().
> > >
> > > Also refactor secretmem's file creation process to use the new
> > > function.
> > >
> > > Suggested-by: David Hildenbrand <david@redhat.com>
> > > Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> > > ---
> > >  fs/anon_inodes.c   | 22 ++++++++++++++++------
> > >  include/linux/fs.h |  1 +
> > >  mm/secretmem.c     |  9 +--------
> > >  3 files changed, 18 insertions(+), 14 deletions(-)
> > >
> > > diff --git a/fs/anon_inodes.c b/fs/anon_inodes.c
> > > index 583ac81669c2..4c3110378647 100644
> > > --- a/fs/anon_inodes.c
> > > +++ b/fs/anon_inodes.c
> > > @@ -55,17 +55,20 @@ static struct file_system_type anon_inode_fs_type = {
> > >       .kill_sb        = kill_anon_super,
> > >  };
> > >
> > > -static struct inode *anon_inode_make_secure_inode(
> > > -     const char *name,
> > > -     const struct inode *context_inode)
> > > +static struct inode *anon_inode_make_secure_inode(struct super_block *s,
> > > +             const char *name, const struct inode *context_inode,
> > > +             bool fs_internal)
> > >  {
> > >       struct inode *inode;
> > >       int error;
> > >
> > > -     inode = alloc_anon_inode(anon_inode_mnt->mnt_sb);
> > > +     inode = alloc_anon_inode(s);
> > >       if (IS_ERR(inode))
> > >               return inode;
> > > -     inode->i_flags &= ~S_PRIVATE;
> > > +
> > > +     if (!fs_internal)
> > > +             inode->i_flags &= ~S_PRIVATE;
> > > +
> > >       error = security_inode_init_security_anon(inode, &QSTR(name),
> > >                                                 context_inode);
> > >       if (error) {
> > > @@ -75,6 +78,12 @@ static struct inode *anon_inode_make_secure_inode(
> > >       return inode;
> > >  }
> > >
> > > +struct inode *alloc_anon_secure_inode(struct super_block *s, const char *name)
> > > +{
> > > +     return anon_inode_make_secure_inode(s, name, NULL, true);
> > > +}
> > > +EXPORT_SYMBOL_GPL(alloc_anon_secure_inode);
> > > +
> > >  static struct file *__anon_inode_getfile(const char *name,
> > >                                        const struct file_operations *fops,
> > >                                        void *priv, int flags,
> > > @@ -88,7 +97,8 @@ static struct file *__anon_inode_getfile(const char *name,
> > >               return ERR_PTR(-ENOENT);
> > >
> > >       if (make_inode) {
> > > -             inode = anon_inode_make_secure_inode(name, context_inode);
> > > +             inode = anon_inode_make_secure_inode(anon_inode_mnt->mnt_sb,
> > > +                                                  name, context_inode, false);
> > >               if (IS_ERR(inode)) {
> > >                       file = ERR_CAST(inode);
> > >                       goto err;
> > > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > > index 016b0fe1536e..0fded2e3c661 100644
> > > --- a/include/linux/fs.h
> > > +++ b/include/linux/fs.h
> > > @@ -3550,6 +3550,7 @@ extern int simple_write_begin(struct file *file, struct address_space *mapping,
> > >  extern const struct address_space_operations ram_aops;
> > >  extern int always_delete_dentry(const struct dentry *);
> > >  extern struct inode *alloc_anon_inode(struct super_block *);
> > > +extern struct inode *alloc_anon_secure_inode(struct super_block *, const char *);
> > >  extern int simple_nosetlease(struct file *, int, struct file_lease **, void **);
> > >  extern const struct dentry_operations simple_dentry_operations;
> > >
> > > diff --git a/mm/secretmem.c b/mm/secretmem.c
> > > index 1b0a214ee558..c0e459e58cb6 100644
> > > --- a/mm/secretmem.c
> > > +++ b/mm/secretmem.c
> > > @@ -195,18 +195,11 @@ static struct file *secretmem_file_create(unsigned long flags)
> > >       struct file *file;
> > >       struct inode *inode;
> > >       const char *anon_name = "[secretmem]";
> > > -     int err;
> > >
> > > -     inode = alloc_anon_inode(secretmem_mnt->mnt_sb);
> > > +     inode = alloc_anon_secure_inode(secretmem_mnt->mnt_sb, anon_name);
> > >       if (IS_ERR(inode))
> > >               return ERR_CAST(inode);
> >
> > I don't think we should not hide secretmem and guest_memfd inodes from
> > selinux, so clearing S_PRIVATE for them is not needed and you can just drop
> > fs_internal parameter in anon_inode_make_secure_inode()
> 
> It's especially odd since I don't see any comments or descriptions
> about why this is being done.  The secretmem change is concerning as
> this is user accessible and marking the inode with S_PRIVATE will
> bypass a number of LSM/SELinux access controls, possibly resulting in
> a security regression (one would need to dig a bit deeper to see what
> is possible with secretmem and which LSM/SELinux code paths would be
> affected).

secretmem always had S_PRIVATE set because alloc_anon_inode() clears it
anyway and this patch does not change it.
I'm just thinking that it makes sense to actually allow LSM/SELinux
controls that S_PRIVATE bypasses for both secretmem and guest_memfd.
 
-- 
Sincerely yours,
Mike.

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

[Paul Moore]

On Thu, Jun 5, 2025 at 1:50 AM Mike Rapoport <rppt@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.

-- 
paul-moore.com

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

[Ira Weiny]

Paul Moore wrote:
> On Thu, Jun 5, 2025 at 1:50 AM Mike Rapoport <rppt@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?

Ira

[snip]

Re: [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@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]
> 

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

[Mike Rapoport]

On Mon, Jun 16, 2025 at 06:30:09PM +0530, Shivank Garg wrote:
> 
> 
> 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@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.

anon_inode_make_secure_inode() was introduced when more than 10 versions of
secretmem already were posted so it didn't jump at me to replace
alloc_anon_inode() with anon_inode_make_secure_inode().
 
> > 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.

Agree.

> > 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?

Yes, makes sense to me.
 
> Thanks,
> Shivank

-- 
Sincerely yours,
Mike.