linux-kernel - Re: [PATCH] fs/seq_file.c: Fix a UAF vulnerability in seq

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <CAKgT0UcWy9kpwhkk9zPbdgj896GzqLV0P7dGMQAUAPr9rURApw@mail.gmail.com>
Date:   Wed, 10 Jul 2019 08:01:47 -0700
From:   Alexander Duyck <alexander.duyck@...il.com>
To:     bsauce <bsauce00@...il.com>
Cc:     "Duyck, Alexander H" <alexander.h.duyck@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>, Mel Gorman <mgorman@...e.de>,
        h <l.stach@...gutronix.de>, vdavydov.dev@...il.com,
        Andrew Morton <akpm@...ux-foundation.org>, alex@...ti.fr,
        adobriyan@...il.com, mike.kravetz@...cle.com,
        David Rientjes <rientjes@...gle.com>, rppt@...ux.vnet.ibm.com,
        Michal Hocko <mhocko@...e.com>, ksspiers@...gle.com,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fs/seq_file.c: Fix a UAF vulnerability in seq_release()

On Wed, Jul 10, 2019 at 3:26 AM bsauce <bsauce00@...il.com> wrote:
>
> In seq_release(), 'm->buf' points to a chunk. It is freed but not cleared to null right away. It can be reused by seq_read() or srm_env_proc_write().
> For example, /arch/alpha/kernel/srm_env.c provide several interfaces to userspace, like 'single_release', 'seq_read' and 'srm_env_proc_write'.
> Thus in userspace, one can exploit this UAF vulnerability to escape privilege.
> Even if 'm->buf' is cleared by kmem_cache_free(), one can still create several threads to exploit this vulnerability.
> And 'm->buf' should be cleared right after being freed.
>
> Signed-off-by: bsauce <bsauce00@...il.com>

So I am pretty sure this "Signed-off-by" line is incorrect. Take a
look in Documentation/process/submitting-patches.rst for more
information. It specifically it calls out that you need to use your
real name, no pseudonyms.

> ---
>  fs/seq_file.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/fs/seq_file.c b/fs/seq_file.c
> index abe27ec..de5e266 100644
> --- a/fs/seq_file.c
> +++ b/fs/seq_file.c
> @@ -358,6 +358,7 @@ int seq_release(struct inode *inode, struct file *file)
>  {
>         struct seq_file *m = file->private_data;
>         kvfree(m->buf);
> +       m->buf = NULL;
>         kmem_cache_free(seq_file_cache, m);
>         return 0;
>  }

As has already been pointed out we are calling kmem_cache_free on m in
the very next line. As such setting m->buf to NULL would have no
effect as m will be freed and could be reused/overwritten at that
point.