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

[bsauce]

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 <bsauc...@gmail.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