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From:   KP Singh <kpsingh@chromium.org>
Date:   Wed, 16 Sep 2020 14:47:48 +0200
Message-ID: <CACYkzJ7dZuOL9NhzaMmpvGqE3TWYHV=AicNmjgs1aD81TctR0Q@mail.gmail.com>
Subject: Re: [PATCH] ima: Fix NULL pointer dereference in ima_file_hash
To:     Florent Revest <revest@chromium.org>
Cc:     open list <linux-kernel@vger.kernel.org>,
        Linux Security Module list 
        <linux-security-module@vger.kernel.org>,
        Mimi Zohar <zohar@linux.ibm.com>,
        James Morris <jmorris@namei.org>,
        Kees Cook <keescook@chromium.org>,
        Lakshmi Ramasubramanian <nramas@linux.microsoft.com>,
        Jann Horn <jannh@google.com>
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Somehow this patch does not seem to have been picked up by any of the
mailing list archives and I am not sure if this was delivered to the
lists. I will send a v2 and add Florent's Reviewed-by tag.

On Wed, Sep 16, 2020 at 2:41 PM Florent Revest <revest@chromium.org> wrote:
>
> Reviewed-by: Florent Revest <revest@chromium.org>
>
> On Wed, 2020-09-16 at 12:05 +0000, 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>
> > ---
> >  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;
> > +     }
> > +
> >       if (buf) {
> >               size_t copied_size;
> >
>