linux-kernel - Re: [PATCH] xfs: do not call xfs_buf_hash

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Date:   Fri, 20 Jan 2017 13:26:12 -0600
From:   Eric Sandeen <sandeen@...deen.net>
To:     Colin King <colin.king@...onical.com>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] xfs: do not call xfs_buf_hash_destroy on a NULL pag

On 1/20/17 8:26 AM, Colin King wrote:
> From: Colin Ian King <colin.king@...onical.com>
> 
> If pag cannot be allocated, the current error exit path will trip
> a null pointer deference error when calling xfs_buf_hash_destroy
> with a null pag.  Fix this by adding a new error exit lable and
> jumping to this, avoiding the hash destroy and unnecessary kmem_free
> on pag.
> 
> Fixes CoverityScan CID#1397628 ("Dereference after null check")
> 
> Signed-off-by: Colin Ian King <colin.king@...onical.com>

Hm, I think this leaves the code with issues.

> ---
>  fs/xfs/xfs_mount.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_mount.c b/fs/xfs/xfs_mount.c
> index 9b9540d..4e66cd19 100644
> --- a/fs/xfs/xfs_mount.c
> +++ b/fs/xfs/xfs_mount.c
> @@ -207,7 +207,7 @@ xfs_initialize_perag(
>  
>  		pag = kmem_zalloc(sizeof(*pag), KM_MAYFAIL);
>  		if (!pag)
> -			goto out_unwind;
> +			goto out_unwind_pags;

So let's say we got to index == 3 at the top of the loop, and
this fails.

We succeeded in initializing 0, 1, and 2, but 3 failed.

So we go to out_unwind_pags with index == 3...

>  		pag->pag_agno = index;
>  		pag->pag_mount = mp;
>  		spin_lock_init(&pag->pag_ici_lock);
> @@ -242,6 +242,7 @@ xfs_initialize_perag(
>  out_unwind:
>  	xfs_buf_hash_destroy(pag);
>  	kmem_free(pag);
> +out_unwind_pags:

... where index == 3, and:

>  	for (; index > first_initialised; index--) {
>  		pag = radix_tree_delete(&mp->m_perag_tree, index);

this should fail, because it never got inserted, and...

>  		xfs_buf_hash_destroy(pag);

this still tries to destroy a NULL pag, no?

There also seems to be an existing issue w/the code where ag 0 is
never torn down in the error case, because first_initialized doesn't
stay set to 0:

                if (!first_initialised)
                        first_initialised = index;

And we don't even tear down ag 1, because:

>  	for (; index > first_initialised; index--) {
>  		pag = radix_tree_delete(&mp->m_perag_tree, index);

when the loop reaches the first initialized AG, it stops.

So we seem to always leak at least 2 if we managed to get far enough
to initialize them.

-Eric

>