sysfs: make sure read buffer is zeroed

13c589d5b0ac ("sysfs: use seq_file when reading regular files")
switched sysfs from custom read implementation to seq_file to enable
later transition to kernfs. After the change, the buffer passed to
->show() is acquired through seq_get_buf(); unfortunately, this
introduces a subtle behavior change. Before the commit, the buffer
passed to ->show() was always zero as it was allocated using
get_zeroed_page(). Because seq_file doesn't clear buffers on
allocation and neither does seq_get_buf(), after the commit, depending
on the behavior of ->show(), we may end up exposing uninitialized data
to userland thus possibly altering userland visible behavior and
leaking information.

Fix it by explicitly clearing the buffer.

Signed-off-by: Tejun Heo <[e-mail]>
Reported-by: Ron <[e-mail]>
Fixes: 13c589d5b0ac ("sysfs: use seq_file when reading regular files")
Cc: stable <[e-mail]> # 3.13+
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

fs/sysfs/file.c

···
 	ssize_t count;
 	char *buf;
 
-	/* acquire buffer and ensure that it's >= PAGE_SIZE */
+	/* acquire buffer and ensure that it's >= PAGE_SIZE and clear */
 	count = seq_get_buf(sf, &buf);
 	if (count < PAGE_SIZE) {
 		seq_commit(sf, -1);
 		return 0;
 	}
+	memset(buf, 0, PAGE_SIZE);
 
 	/*
 	 * Invoke show().  Control may reach here via seq file lseek even