[PATCHv3] Fix range checks in kernfs_get_target_path

[Bernd Edlinger]

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: Bernd Edlinger <bernd.edlinger@xxxxxxxxxx>
---
  fs/kernfs/symlink.c | 5 ++++-
  1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/kernfs/symlink.c b/fs/kernfs/symlink.c
index 08ccabd..774bada 100644
--- a/fs/kernfs/symlink.c
+++ b/fs/kernfs/symlink.c
@@ -63,6 +63,9 @@ static int kernfs_get_target_path(struct kernfs_node
  		if (base == kn)
  			break;
  
+		if ((s - path) + 3 >= PATH_MAX)
+			return -ENAMETOOLONG;
+
  		strcpy(s, "../");
  		s += 3;
  		base = base->parent;
@@ -79,7 +82,7 @@ static int kernfs_get_target_path(struct kernfs_node
  	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 */
-- 
1.9.1