linux-kernel - Re: [PATCH] net/mlx5e: fix a double-free in arfs_create

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Date: Mon, 8 Jan 2024 17:12:06 +0800 (GMT+08:00)
From: alexious@....edu.cn
To: "Simon Horman" <horms@...nel.org>
Cc: "Saeed Mahameed" <saeedm@...dia.com>, 
	"Leon Romanovsky" <leon@...nel.org>, 
	"David S. Miller" <davem@...emloft.net>, 
	"Eric Dumazet" <edumazet@...gle.com>, 
	"Jakub Kicinski" <kuba@...nel.org>, 
	"Paolo Abeni" <pabeni@...hat.com>, 
	"Maor Gottlieb" <maorg@...lanox.com>, netdev@...r.kernel.org, 
	linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net/mlx5e: fix a double-free in arfs_create_groups



> On Sun, Dec 24, 2023 at 04:13:48PM +0800, Zhipeng Lu wrote:
> > When `in` allocated by kvzalloc fails, arfs_create_groups will free
> > ft->g and return an error. However, arfs_create_table, the only caller of
> > arfs_create_groups, will hold this error and call to
> > mlx5e_destroy_flow_table, in which the ft->g will be freed again.
> > 
> > Fixes: 1cabe6b0965e ("net/mlx5e: Create aRFS flow tables")
> > Signed-off-by: Zhipeng Lu <alexious@....edu.cn>
> 
> Thanks,
> 
> I agree this addresses the issue that you describe.
> And as a minimal fix it looks good.
> 
> Reviewed-by: Simon Horman <horms@...nel.org>
> 
> However, I would like to suggest that some clean-up work could
> take place as a follow-up.
> 
> I think that the error handling in this area of the code
> is rather fragile. This is because initialisation is not necessarily
> unwound on error within the function that initialisation occurs.
> 
> I think it would be better if arfs_create_groups():
> 
> 1. Released allocates resources it allocates, including ft->g and
>    elements of ft->g, on error.
> 2. This was achieved by using a goto unwind ladder.
> 3. The caller treated ft->g as uninitialised if
>    arfs_create_groups fails.
>
 
Agree, I think a unwind ladder for arfs_create_groups is much better.
I'll follow this idea to send a v2 patch later.
Another comment below.

> Likewise, I think that:
> 
> * arfs_create_groups, should initialise ft->num_groups
> 
> And further, logic similar to the above should guide
> how arfs_create_table() initialises ft->t and cleans it
> up on error.
> 

I think that ft->t you mentioned refers to mlx5_create_flow_table.
I'd like to make the life cycle of ft->t similar to ft->g in arfs_create_groups, 
but it needs to add an argument for mlx5_create_flow_table to transfer ft to 
it. However, mlx5_create_flow_table is called in more than 30 different places 
throughout the kernel. So such modification could be another refactoring patch
but may be out of this fix patch's duty.

> I did not look at the code beyond the scope described above.
> But the above are general principles that may well apply in
> other nearby code too.
> 
> ...