Re: [PATCH] fs/seq_file.c: Fix a UAF vulnerability in seq_release()

From: Alexander Duyck
Date: Wed Jul 10 2019 - 11:02:01 EST

On Wed, Jul 10, 2019 at 3:26 AM bsauce <bsauce00@xxxxxxxxx> 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@xxxxxxxxx>

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