linux-kernel - [PATCH] Fix range checks in kernfs_get_target

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Message-ID: <AM5PR0701MB26577F11198A56858D55C104E4470@AM5PR0701MB2657.eurprd07.prod.outlook.com>
Date:   Fri, 6 Jul 2018 21:08:49 +0000
From:   Bernd Edlinger <bernd.edlinger@...mail.de>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: [PATCH] Fix range checks in kernfs_get_target_path

The strncpy causes a warning [-Wstringop-truncation] here,
which indicates that it never appends a NUL byte to the path.
The NUL byte is only there because the buffer is allocated
with kzalloc(PAGE_SIZE, GFP_KERNEL), but since the range-check
is also off-by-one, and PAGE_SIZE==PATH_MAX the returned string
will not be zero-terminated if it is exactly PATH_MAX characters.
Furthermore also the initial loop may theoretically exceed PATH_MAX
and cause a fault.

Signed-off-by: Bernd Edlinger <bernd.edlinger@...mail.de>
---
  fs/kernfs/symlink.c | 10 +++++++---
  1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/fs/kernfs/symlink.c b/fs/kernfs/symlink.c
index 08ccabd..c8b7d44a 100644
--- a/fs/kernfs/symlink.c
+++ b/fs/kernfs/symlink.c
@@ -63,7 +63,10 @@ static int kernfs_get_target_path(struct kernfs_node 
*parent,
  		if (base == kn)
  			break;

-		strcpy(s, "../");
+		if ((s - path) + 3 >= PATH_MAX)
+			return -ENAMETOOLONG;
+
+		memcpy(s, "../", 3);
  		s += 3;
  		base = base->parent;
  	}
@@ -79,16 +82,17 @@ 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 */
  	kn = target;
+	s[len] = '\0';
  	while (kn->parent && kn != base) {
  		int slen = strlen(kn->name);

  		len -= slen;
-		strncpy(s + len, kn->name, slen);
+		memcpy(s + len, kn->name, slen);
  		if (len)
  			s[--len] = '/';

-- 
1.9.1