[PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()

[PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()

[Jia-Ju Bai]

In gfs2_alloc_inode(), when kmem_cache_alloc() on line 1724 returns
NULL, ip is assigned to NULL. In this case, "return &ip->i_inode" will
cause a null-pointer dereference.

To fix this null-pointer dereference, NULL is returned when ip is NULL.

This bug is found by a static analysis tool STCheck written by us.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
---
 fs/gfs2/super.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/gfs2/super.c b/fs/gfs2/super.c
index 0acc5834f653..c07c3f4f8451 100644
--- a/fs/gfs2/super.c
+++ b/fs/gfs2/super.c
@@ -1728,8 +1728,9 @@ static struct inode *gfs2_alloc_inode(struct super_block *sb)
 		memset(&ip->i_res, 0, sizeof(ip->i_res));
 		RB_CLEAR_NODE(&ip->i_res.rs_node);
 		ip->i_rahead = 0;
-	}
-	return &ip->i_inode;
+		return &ip->i_inode;
+	} else
+		return NULL;
 }
 
 static void gfs2_free_inode(struct inode *inode)
-- 
2.17.0

[Cluster-devel] [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()

[Jia-Ju Bai]

In gfs2_alloc_inode(), when kmem_cache_alloc() on line 1724 returns
NULL, ip is assigned to NULL. In this case, "return &ip->i_inode" will
cause a null-pointer dereference.

To fix this null-pointer dereference, NULL is returned when ip is NULL.

This bug is found by a static analysis tool STCheck written by us.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
---
 fs/gfs2/super.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/gfs2/super.c b/fs/gfs2/super.c
index 0acc5834f653..c07c3f4f8451 100644
--- a/fs/gfs2/super.c
+++ b/fs/gfs2/super.c
@@ -1728,8 +1728,9 @@ static struct inode *gfs2_alloc_inode(struct super_block *sb)
 		memset(&ip->i_res, 0, sizeof(ip->i_res));
 		RB_CLEAR_NODE(&ip->i_res.rs_node);
 		ip->i_rahead = 0;
-	}
-	return &ip->i_inode;
+		return &ip->i_inode;
+	} else
+		return NULL;
 }
 
 static void gfs2_free_inode(struct inode *inode)
-- 
2.17.0

[Cluster-devel] [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()

[Steven Whitehouse]

Hi,

On 24/07/2019 09:43, Jia-Ju Bai wrote:
> In gfs2_alloc_inode(), when kmem_cache_alloc() on line 1724 returns
> NULL, ip is assigned to NULL. In this case, "return &ip->i_inode" will
> cause a null-pointer dereference.
>
> To fix this null-pointer dereference, NULL is returned when ip is NULL.
>
> This bug is found by a static analysis tool STCheck written by us.

The bug is in the tool I'm afraid. Since i_inode is the first element of 
ip, there is no NULL dereference here. A pointer to ip->i_inode and a 
pointer to ip are one and the same (bar the differing types) which is 
the reason that we return &ip->i_inode rather than just ip,

Steve.


>
> Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
> ---
>   fs/gfs2/super.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/fs/gfs2/super.c b/fs/gfs2/super.c
> index 0acc5834f653..c07c3f4f8451 100644
> --- a/fs/gfs2/super.c
> +++ b/fs/gfs2/super.c
> @@ -1728,8 +1728,9 @@ static struct inode *gfs2_alloc_inode(struct super_block *sb)
>   		memset(&ip->i_res, 0, sizeof(ip->i_res));
>   		RB_CLEAR_NODE(&ip->i_res.rs_node);
>   		ip->i_rahead = 0;
> -	}
> -	return &ip->i_inode;
> +		return &ip->i_inode;
> +	} else
> +		return NULL;
>   }
>   
>   static void gfs2_free_inode(struct inode *inode)

Re: [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()

[Christoph Hellwig]

On Wed, Jul 24, 2019 at 04:43:03PM +0800, Jia-Ju Bai wrote:
> index 0acc5834f653..c07c3f4f8451 100644
> --- a/fs/gfs2/super.c
> +++ b/fs/gfs2/super.c
> @@ -1728,8 +1728,9 @@ static struct inode *gfs2_alloc_inode(struct super_block *sb)
>  		memset(&ip->i_res, 0, sizeof(ip->i_res));
>  		RB_CLEAR_NODE(&ip->i_res.rs_node);
>  		ip->i_rahead = 0;
> -	}
> -	return &ip->i_inode;
> +		return &ip->i_inode;
> +	} else
> +		return NULL;
>  }

No need for an else after a return.  You probably just want to
return early for the NULL case.

[Cluster-devel] [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()

[Christoph Hellwig]

On Wed, Jul 24, 2019 at 04:43:03PM +0800, Jia-Ju Bai wrote:
> index 0acc5834f653..c07c3f4f8451 100644
> --- a/fs/gfs2/super.c
> +++ b/fs/gfs2/super.c
> @@ -1728,8 +1728,9 @@ static struct inode *gfs2_alloc_inode(struct super_block *sb)
>  		memset(&ip->i_res, 0, sizeof(ip->i_res));
>  		RB_CLEAR_NODE(&ip->i_res.rs_node);
>  		ip->i_rahead = 0;
> -	}
> -	return &ip->i_inode;
> +		return &ip->i_inode;
> +	} else
> +		return NULL;
>  }

No need for an else after a return.  You probably just want to
return early for the NULL case.

[Cluster-devel] [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()

[Christoph Hellwig]

On Wed, Jul 24, 2019 at 09:48:38AM +0100, Steven Whitehouse wrote:
> Hi,
> 
> On 24/07/2019 09:43, Jia-Ju Bai wrote:
> > In gfs2_alloc_inode(), when kmem_cache_alloc() on line 1724 returns
> > NULL, ip is assigned to NULL. In this case, "return &ip->i_inode" will
> > cause a null-pointer dereference.
> > 
> > To fix this null-pointer dereference, NULL is returned when ip is NULL.
> > 
> > This bug is found by a static analysis tool STCheck written by us.
> 
> The bug is in the tool I'm afraid. Since i_inode is the first element of ip,
> there is no NULL dereference here. A pointer to ip->i_inode and a pointer to
> ip are one and the same (bar the differing types) which is the reason that
> we return &ip->i_inode rather than just ip,

But that doesn't help if ip is NULL, as dereferencing a field in in
still remains invalid behavior.

[Cluster-devel] [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()

[Edwin Török]

On 24/07/2019 11:02, Christoph Hellwig wrote:
> On Wed, Jul 24, 2019 at 09:48:38AM +0100, Steven Whitehouse wrote:
>> Hi,
>>
>> On 24/07/2019 09:43, Jia-Ju Bai wrote:
>>> In gfs2_alloc_inode(), when kmem_cache_alloc() on line 1724 returns
>>> NULL, ip is assigned to NULL. In this case, "return &ip->i_inode" will
>>> cause a null-pointer dereference.
>>>
>>> To fix this null-pointer dereference, NULL is returned when ip is NULL.
>>>
>>> This bug is found by a static analysis tool STCheck written by us.
>>
>> The bug is in the tool I'm afraid. Since i_inode is the first element of ip,
>> there is no NULL dereference here. A pointer to ip->i_inode and a pointer to
>> ip are one and the same (bar the differing types) which is the reason that
>> we return &ip->i_inode rather than just ip,
> 
> But that doesn't help if ip is NULL, as dereferencing a field in in
> still remains invalid behavior.
> 

According to C99 you may be right that it is undefined behaviour, that just happens to work correctly on current versions
of GCC, see [1].
Although as pointed out in [1] this undefined behaviour was relied upon to implement the offsetof macro
(nowadays that should be a compiler built-in, see include/linux/stddef.h).

I wish that cases like these would be more clearly defined in the C standard, at least as implementation defined behavior such that GCC
could then be explicit about what semantics it wants and document it here: https://gcc.gnu.org/onlinedocs/gcc/#toc-C-Implementation-Defined-Behavior
It is difficult to write future-proof code if the compiler can keep changing its mind about semantics of code that it has previously accepted.

[1] https://software.intel.com/en-us/blogs/2015/04/20/null-pointer-dereferencing-causes-undefined-behavior

Best regards,
--Edwin

[Cluster-devel] [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()

[Steven Whitehouse]

Hi,

On 24/07/2019 11:02, Christoph Hellwig wrote:
> On Wed, Jul 24, 2019 at 09:48:38AM +0100, Steven Whitehouse wrote:
>> Hi,
>>
>> On 24/07/2019 09:43, Jia-Ju Bai wrote:
>>> In gfs2_alloc_inode(), when kmem_cache_alloc() on line 1724 returns
>>> NULL, ip is assigned to NULL. In this case, "return &ip->i_inode" will
>>> cause a null-pointer dereference.
>>>
>>> To fix this null-pointer dereference, NULL is returned when ip is NULL.
>>>
>>> This bug is found by a static analysis tool STCheck written by us.
>> The bug is in the tool I'm afraid. Since i_inode is the first element of ip,
>> there is no NULL dereference here. A pointer to ip->i_inode and a pointer to
>> ip are one and the same (bar the differing types) which is the reason that
>> we return &ip->i_inode rather than just ip,
> But that doesn't help if ip is NULL, as dereferencing a field in in
> still remains invalid behavior.
>
>
We are not dereferencing it though really, we are taking the address of 
the field... we could have written:

return (struct inode *)ip;

and it would have the same effect, so far as I can tell. I don't mind 
changing it, if that is perhaps a clearer way to write the same thing, 
rather than &ip->i_inode;

Steve.

[Cluster-devel] [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()

[Christoph Hellwig]

On Wed, Jul 24, 2019 at 11:22:46AM +0100, Steven Whitehouse wrote:
> and it would have the same effect, so far as I can tell. I don't mind
> changing it, if that is perhaps a clearer way to write the same thing,
> rather than &ip->i_inode;

The cleanest thing is to not rely on any of that magic and write it
like all other file systems:

	ip = kmem_cache_alloc
	if (!ip)
		retuturn NULL;

	...

	return &ip->i_inode;

Absolutely not point in trying to be clever here.

[Cluster-devel] [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()

[Steven Whitehouse]

Hi,

On 24/07/2019 11:27, Christoph Hellwig wrote:
> On Wed, Jul 24, 2019 at 11:22:46AM +0100, Steven Whitehouse wrote:
>> and it would have the same effect, so far as I can tell. I don't mind
>> changing it, if that is perhaps a clearer way to write the same thing,
>> rather than &ip->i_inode;
> The cleanest thing is to not rely on any of that magic and write it
> like all other file systems:
>
> 	ip = kmem_cache_alloc
> 	if (!ip)
> 		retuturn NULL;
>
> 	...
>
> 	return &ip->i_inode;
>
> Absolutely not point in trying to be clever here.

Yes, that works too,

Steve.

[Cluster-devel] [PATCH] fs: gfs2: Fix a null-pointer dereference in gfs2_alloc_inode()

[Andreas Gruenbacher]

On Wed, 24 Jul 2019 at 12:55, Steven Whitehouse <swhiteho@redhat.com> wrote:
> On 24/07/2019 11:27, Christoph Hellwig wrote:
> > On Wed, Jul 24, 2019 at 11:22:46AM +0100, Steven Whitehouse wrote:
> >> and it would have the same effect, so far as I can tell. I don't mind
> >> changing it, if that is perhaps a clearer way to write the same thing,
> >> rather than &ip->i_inode;
> > The cleanest thing is to not rely on any of that magic and write it
> > like all other file systems:
> >
> >       ip = kmem_cache_alloc
> >       if (!ip)
> >               retuturn NULL;
> >
> >       ...
> >
> >       return &ip->i_inode;
> >
> > Absolutely not point in trying to be clever here.
>
> Yes, that works too,

I'll change that ...

Andreas