seq_file: disallow extremely large seq buffer allocations

commit 8cae8cd8 upstream.

There is no reasonable need for a buffer larger than this, and it avoids
int overflow pitfalls.

Fixes: 058504ed ("fs/seq_file: fallback to vmalloc allocation")
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Reported-by: Qualys Security Advisory <qsa@qualys.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/seq_file.c

@@ -28,6 +28,9 @@ static void *seq_buf_alloc(unsigned long size)
 	void *buf;
 	gfp_t gfp = GFP_KERNEL;
 
+	if (unlikely(size > MAX_RW_COUNT))
+		return NULL;
+
 	/*
 	 * For high order allocations, use __GFP_NORETRY to avoid oom-killing -
 	 * it's better to fall back to vmalloc() than to kill things.  For small