Re: [PATCH v2] ima: Fix NULL pointer dereference in ima_file_hash

[KP Singh <kpsingh@chromium.org>]

On Wed, Sep 16, 2020 at 6:00 PM Mimi Zohar <zohar@linux.ibm.com> wrote:
>
> On Wed, 2020-09-16 at 14:49 +0200, KP Singh wrote:
> > From: KP Singh <kpsingh@google.com>
> >
> > ima_file_hash can be called when there is no iint->ima_hash available
> > even though the inode exists in the integrity cache.
> >
> > An example where this can happen (suggested by Jann Horn):
> >
> > Process A does:
> >
> >       while(1) {
> >               unlink("/tmp/imafoo");
> >               fd = open("/tmp/imafoo", O_RDWR|O_CREAT|O_TRUNC, 0700);
> >               if (fd == -1) {
> >                       perror("open");
> >                       continue;
> >               }
> >               write(fd, "A", 1);
> >               close(fd);
> >       }
> >
> > and Process B does:
> >
> >       while (1) {
> >               int fd = open("/tmp/imafoo", O_RDONLY);
> >               if (fd == -1)
> >                       continue;
> >               char *mapping = mmap(NULL, 0x1000, PROT_READ|PROT_EXEC,
> >                                    MAP_PRIVATE, fd, 0);
> >               if (mapping != MAP_FAILED)
> >                       munmap(mapping, 0x1000);
> >               close(fd);
> >       }
> >
> > Due to the race to get the iint->mutex between ima_file_hash and
> > process_measurement iint->ima_hash could still be NULL.
> >
> > Fixes: 6beea7afcc72 ("ima: add the ability to query the cached hash of a given file")
> > Signed-off-by: KP Singh <kpsingh@google.com>
> > Reviewed-by: Florent Revest <revest@chromium.org>
> > ---
> >  security/integrity/ima/ima_main.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
> > index 8a91711ca79b..4c86cd4eece0 100644
> > --- a/security/integrity/ima/ima_main.c
> > +++ b/security/integrity/ima/ima_main.c
> > @@ -531,6 +531,16 @@ int ima_file_hash(struct file *file, char *buf, size_t buf_size)
> >               return -EOPNOTSUPP;
> >
> >       mutex_lock(&iint->mutex);
> > +
> > +     /*
> > +      * ima_file_hash can be called when ima_collect_measurement has still
> > +      * not been called, we might not always have a hash.
> > +      */
> > +     if (!iint->ima_hash) {
> > +             mutex_unlock(&iint->mutex);
> > +             return -EOPNOTSUPP;
> > +     }
> > +
>
> Not having a file hash is rather common (e.g. mknodat, prior to the
> file being closed).  Before appraising the integrity of a file, it
> checks whether it is a new file (eg. IMA_NEW_FILE), but, unfortunately,
> the flag is only set for those files in the appraise policy.

Makes sense.

>
> The patch looks fine, but you might want to reflect not having a file
> hash is common in the patch description.
>

Thanks! Will send another revision with an updated description.

- KP

> Mimi
>
> >       if (buf) {
> >               size_t copied_size;
> >
>
>