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

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Date:   Wed, 10 Jul 2019 18:26:29 +0800
From:   bsauce <bsauce00@...il.com>
To:     alexander.h.duyck@...el.com
Cc:     vbabka@...e.cz, mgorman@...e.de, l.stach@...gutronix.de,
        vdavydov.dev@...il.com, akpm@...ux-foundation.org, alex@...ti.fr,
        adobriyan@...il.com, mike.kravetz@...cle.com, rientjes@...gle.com,
        rppt@...ux.vnet.ibm.com, mhocko@...e.com, ksspiers@...gle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        bsauce <bsauce00@...il.com>
Subject: [PATCH] fs/seq_file.c: Fix a UAF vulnerability in seq_release()

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>
---
 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;
 }
-- 
2.7.4