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

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Message-ID: <20240103172254.GB31813@kernel.org>
Date: Wed, 3 Jan 2024 17:22:54 +0000
From: Simon Horman <horms@...nel.org>
To: Zhipeng Lu <alexious@....edu.cn>
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.

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 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.

...