[PATCH] sysfs: clamp show() return value in sysfs_kf_read()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

sysfs_kf_seq_show() defends against buggy show() callbacks that return
larger than PAGE_SIZE by clamping the value and printing a warning.
sysfs_kf_read(), the prealloc variant, has no such defense.

The only current in-tree user of __ATTR_PREALLOC is drivers/md/md.c,
whose show() callbacks are well-behaved, so this is hardening against
future drivers doing foolish things and out-of-tree code doing even more
foolish things.

Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Danilo Krummrich <dakr@kernel.org>
Cc: NeilBrown <neil@brown.name>
Cc: Tejun Heo <tj@kernel.org>
Fixes: 2b75869bba67 ("sysfs/kernfs: allow attributes to request write buffer be pre-allocated.")
Assisted-by: gregkh_clanker_t1000
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/sysfs/file.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
index 5709cede1d75..25b44fe171a3 100644
--- a/fs/sysfs/file.c
+++ b/fs/sysfs/file.c
@@ -120,6 +120,10 @@ static ssize_t sysfs_kf_read(struct kernfs_open_file *of, char *buf,
 	len = ops->show(kobj, of->kn->priv, buf);
 	if (len < 0)
 		return len;
+	if (len >= (ssize_t)PAGE_SIZE) {
+		printk("fill_read_buffer: %pS returned bad count\n", ops->show);
+		len = PAGE_SIZE - 1;
+	}
 	if (pos) {
 		if (len <= pos)
 			return 0;
-- 
2.54.0