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[81.185.172.31]) by smtp.gmail.com with ESMTPSA id e3sm1788460wrt.93.2019.07.10.03.58.44 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 10 Jul 2019 03:58:45 -0700 (PDT) Subject: Re: [PATCH] fs/seq_file.c: Fix a UAF vulnerability in seq_release() To: bsauce , alexander.h.duyck@intel.com Cc: vbabka@suse.cz, mgorman@suse.de, l.stach@pengutronix.de, vdavydov.dev@gmail.com, akpm@linux-foundation.org, alex@ghiti.fr, adobriyan@gmail.com, mike.kravetz@oracle.com, rientjes@google.com, rppt@linux.vnet.ibm.com, mhocko@suse.com, ksspiers@google.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <1562754389-29217-1-git-send-email-bsauce00@gmail.com> From: Eric Dumazet Message-ID: <32e544a6-575e-a47e-fd8a-647145ac1972@gmail.com> Date: Wed, 10 Jul 2019 12:58:43 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.1 MIME-Version: 1.0 In-Reply-To: <1562754389-29217-1-git-send-email-bsauce00@gmail.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/10/19 12:26 PM, bsauce 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 > --- > 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; > } > This makes no sense, since m is freed right away anyway. So whatever is trying to 'reuse' m is in big trouble.