kernfs: Fix range checks in kernfs_get_target_path

mainline inclusion
from mainline-4.20-rc1
commit a75e78f2
category: bugfix
bugzilla: 10347
CVE: NA
---------------------------

The terminating NUL byte is only there because the buffer is
allocated with kzalloc(PAGE_SIZE, GFP_KERNEL), but since the
range-check is off-by-one, and PAGE_SIZE==PATH_MAX, the
returned string may not be zero-terminated if it is exactly
PATH_MAX characters long.  Furthermore also the initial loop
may theoretically exceed PATH_MAX and cause a fault.
Signed-off-by: NBernd Edlinger <bernd.edlinger@hotmail.de>
Acked-by: NTejun Heo <tj@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYufen Yu <yuyufen@huawei.com>
Reviewed-by: NHou Tao <houtao1@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

fs/kernfs/symlink.c

@@ -72,6 +72,9 @@ static int kernfs_get_target_path(struct kernfs_node *parent,
 		if (base == kn)
 			break;
 
+		if ((s - path) + 3 >= PATH_MAX)
+			return -ENAMETOOLONG;
+
 		strcpy(s, "../");
 		s += 3;
 		base = base->parent;
@@ -88,7 +91,7 @@ static int kernfs_get_target_path(struct kernfs_node *parent,
 	if (len < 2)
 		return -EINVAL;
 	len--;
-	if ((s - path) + len > PATH_MAX)
+	if ((s - path) + len >= PATH_MAX)
 		return -ENAMETOOLONG;
 
 	/* reverse fillup of target string from target to base */