[PATCH v9 12/25] security: Introduce file_post_open hook

[Roberto Sassu]

On Fri, 2024-02-09 at 11:12 +0100, Christian Brauner wrote:
> On Mon, Jan 15, 2024 at 07:17:56PM +0100, Roberto Sassu wrote:
> > From: Roberto Sassu <roberto.sassu at huawei.com>
> > 
> > In preparation to move IMA and EVM to the LSM infrastructure, introduce the
> > file_post_open hook. Also, export security_file_post_open() for NFS.
> > 
> > Based on policy, IMA calculates the digest of the file content and
> > extends the TPM with the digest, verifies the file's integrity based on
> > the digest, and/or includes the file digest in the audit log.
> > 
> > LSMs could similarly take action depending on the file content and the
> > access mask requested with open().
> > 
> > The new hook returns a value and can cause the open to be aborted.
> > 
> > Signed-off-by: Roberto Sassu <roberto.sassu at huawei.com>
> > Reviewed-by: Stefan Berger <stefanb at linux.ibm.com>
> > Acked-by: Casey Schaufler <casey at schaufler-ca.com>
> > Reviewed-by: Mimi Zohar <zohar at linux.ibm.com>
> > ---
> >  fs/namei.c                    |  2 ++
> >  fs/nfsd/vfs.c                 |  6 ++++++
> >  include/linux/lsm_hook_defs.h |  1 +
> >  include/linux/security.h      |  6 ++++++
> >  security/security.c           | 17 +++++++++++++++++
> >  5 files changed, 32 insertions(+)
> > 
> > diff --git a/fs/namei.c b/fs/namei.c
> > index 71c13b2990b4..fb93d3e13df6 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -3620,6 +3620,8 @@ static int do_open(struct nameidata *nd,
> >  	error = may_open(idmap, &nd->path, acc_mode, open_flag);
> >  	if (!error && !(file->f_mode & FMODE_OPENED))
> >  		error = vfs_open(&nd->path, file);
> > +	if (!error)
> > +		error = security_file_post_open(file, op->acc_mode);
> 
> What does it do for O_CREAT? IOW, we managed to create that thing and we
> managed to open that thing. Can security_file_post_open() and
> ima_file_check() fail afterwards even for newly created files?

$ strace touch test-file
...
openat(AT_FDCWD, "test-file", O_WRONLY|O_CREAT|O_NOCTTY|O_NONBLOCK, 0666) = -1 EPERM (Operation not permitted)

The open fails, but the file is there. I didn't see warnings/errors in
the kernel log.

Roberto