[RFC PATCH ghak9 2/3] audit: Add a function to log the path of an fd

[Ondrej Mosnacek]

On Fri, Jul 13, 2018 at 5:17 PM Richard Guy Briggs <rgb at redhat.com> wrote:
> On 2018-07-12 13:36, Ondrej Mosnacek wrote:
> > The function logs an FD_PATH record that is associated with the current
> > syscall. The record associates the given file descriptor with the
> > current path of the file under it (if it is possible to retrieve such
> > path). The reader of the log can then logically connect this information
> > to the syscall arguments from the SYSCALL record (based on the syscall
> > type).
> >
> > Record format:
> > type=FD_PATH msg=audit(...): fd=<file descriptor> path=<file path>
> >
> > Signed-off-by: Ondrej Mosnacek <omosnace at redhat.com>
> > ---
> >  include/linux/audit.h | 10 ++++++++++
> >  kernel/auditsc.c      | 36 ++++++++++++++++++++++++++++++++++++
> >  2 files changed, 46 insertions(+)
> >
> > diff --git a/include/linux/audit.h b/include/linux/audit.h
> > index 9334fbef7bae..95d338bb603a 100644
> > --- a/include/linux/audit.h
> > +++ b/include/linux/audit.h
> > @@ -356,6 +356,7 @@ extern void __audit_log_capset(const struct cred *new, const struct cred *old);
> >  extern void __audit_mmap_fd(int fd, int flags);
> >  extern void __audit_log_kern_module(char *name);
> >  extern void __audit_fanotify(unsigned int response);
> > +extern void __audit_fd_path(int fd);
> >
> >  static inline void audit_ipc_obj(struct kern_ipc_perm *ipcp)
> >  {
> > @@ -458,6 +459,12 @@ static inline void audit_fanotify(unsigned int response)
> >               __audit_fanotify(response);
> >  }
> >
> > +static inline void audit_fd_path(int fd)
> > +{
> > +     if (fd >= 0 && !audit_dummy_context())
>
> Isn't an fd of 0 valid?

It is treated as valid by the above condition (it only rejects
negative values), so I'm not sure if you mean "valid" or "invalid"...
I suppose an fd of 0 is unlikely to be used as dirfd in openat(2) et
al., but in general it is a valid fd and I don't think we should
explicitly exclude it here. The corresponding syscalls' input checks
will already filter out values that are invalid for them.

>
> > +             __audit_fd_path(fd);
> > +}
> > +
> >  extern int audit_n_rules;
> >  extern int audit_signals;
> >  #else /* CONFIG_AUDITSYSCALL */
> > @@ -584,6 +591,9 @@ static inline void audit_log_kern_module(char *name)
> >  static inline void audit_fanotify(unsigned int response)
> >  { }
> >
> > +static inline void audit_fd_path(int fd)
> > +{ }
> > +
> >  static inline void audit_ptrace(struct task_struct *t)
> >  { }
> >  #define audit_n_rules 0
> > diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> > index d762e0b8160e..82dad69213a2 100644
> > --- a/kernel/auditsc.c
> > +++ b/kernel/auditsc.c
> > @@ -74,6 +74,8 @@
> >  #include <linux/string.h>
> >  #include <linux/uaccess.h>
> >  #include <linux/fsnotify_backend.h>
> > +#include <linux/file.h>
> > +#include <linux/dcache.h>
> >  #include <uapi/linux/limits.h>
> >
> >  #include "audit.h"
> > @@ -2422,6 +2424,40 @@ void __audit_fanotify(unsigned int response)
> >               AUDIT_FANOTIFY, "resp=%u", response);
> >  }
> >
> > +void __audit_fd_path(int fd)
> > +{
> > +     struct audit_buffer *ab;
> > +     struct file *file;
> > +     char *buf, *path;
> > +
> > +     if (!audit_enabled)
> > +             return;
> > +
> > +     file = fget_raw(fd);
> > +     if (!file)
> > +             return;
> > +
> > +     buf = kmalloc(PATH_MAX, GFP_KERNEL);
> > +     if (!buf)
>
> I think we need an fput(file) here.

Indeed we do, will fix in next revision.

>
> > +             return;
> > +
> > +     path_get(&file->f_path);
> > +     path = d_absolute_path(&file->f_path, buf, PATH_MAX);
> > +     path_put(&file->f_path);
> > +     fput(file);
> > +     if (!path || IS_ERR(path))
> > +             goto free_buf;
> > +
> > +     ab = audit_log_start(audit_context(), GFP_KERNEL, AUDIT_FD_PATH);
> > +     if (unlikely(!ab))
> > +             goto free_buf;
> > +     audit_log_format(ab, "fd=%i path=", fd);
> > +     audit_log_untrustedstring(ab, path);
> > +     audit_log_end(ab);
> > +free_buf:
> > +     kfree(buf);
> > +}
> > +
> >  static void audit_log_task(struct audit_buffer *ab)
> >  {
> >       kuid_t auid, uid;
>
> - RGB
>
> --
> Richard Guy Briggs <rgb at redhat.com>
> Sr. S/W Engineer, Kernel Security, Base Operating Systems
> Remote, Ottawa, Red Hat Canada
> IRC: rgb, SunRaycer
> Voice: +1.647.777.2635, Internal: (81) 32635

-- 
Ondrej Mosnacek <omosnace at redhat dot com>
Associate Software Engineer, Security Technologies
Red Hat, Inc.