[PATCH driver-core-linus] sysfs: make sure read buffer is zeroed

[Tejun Heo <tj@kernel.org>]

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 <tj@kernel.org>
Reported-by: Ron <ron@debian.org> 
Fixes: 13c589d5b0ac ("sysfs: use seq_file when reading regular files")
---
 fs/sysfs/file.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index 28cc1acd..e9ef59b 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -47,12 +47,13 @@ static int sysfs_kf_seq_show(struct seq_file *sf, void *v)
 	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