[PATCHv3] Fix range checks in kernfs_get_target_path

[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@hotmail.de>
---
  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

Re: [PATCHv3] Fix range checks in kernfs_get_target_path

[Bernd Edlinger]

Ping...
Sorry, I had actually completely forgotten about this one.

On 07/07/18 19:52, Bernd Edlinger wrote:
> 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@hotmail.de>
> ---
>   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 */

Re: [PATCHv3] Fix range checks in kernfs_get_target_path

[Tejun Heo]

On Sun, Aug 26, 2018 at 10:55:24AM +0000, Bernd Edlinger wrote:
> Ping...
> Sorry, I had actually completely forgotten about this one.
> 
> On 07/07/18 19:52, Bernd Edlinger wrote:
> > 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@hotmail.de>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun