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Subject: Re: [PATCH] cgroup: Fix memory leak when parsing multiple source
 parameters
To:     Qinglang Miao <miaoqinglang@huawei.com>, Tejun Heo <tj@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>
CC:     <cgroups@vger.kernel.org>, <linux-kernel@vger.kernel.org>
References: <20201209121322.77665-1-miaoqinglang@huawei.com>
From:   Zefan Li <lizefan@huawei.com>
Message-ID: <a2a23bfb-65a9-96ad-ff54-0248432d166b@huawei.com>
Date:   Tue, 15 Dec 2020 16:39:00 +0800
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On 2020/12/9 20:13, Qinglang Miao wrote:
> A memory leak is found in cgroup1_parse_param() when multiple source
> parameters overwrite fc->source in the fs_context struct without free.
> 
> unreferenced object 0xffff888100d930e0 (size 16):
>   comm "mount", pid 520, jiffies 4303326831 (age 152.783s)
>   hex dump (first 16 bytes):
>     74 65 73 74 6c 65 61 6b 00 00 00 00 00 00 00 00  testleak........
>   backtrace:
>     [<000000003e5023ec>] kmemdup_nul+0x2d/0xa0
>     [<00000000377dbdaa>] vfs_parse_fs_string+0xc0/0x150
>     [<00000000cb2b4882>] generic_parse_monolithic+0x15a/0x1d0
>     [<000000000f750198>] path_mount+0xee1/0x1820
>     [<0000000004756de2>] do_mount+0xea/0x100
>     [<0000000094cafb0a>] __x64_sys_mount+0x14b/0x1f0
> 
> Fix this bug by permitting a single source parameter and rejecting with
> an error all subsequent ones.
> 
> Fixes: 8d2451f4994f ("cgroup1: switch to option-by-option parsing")
> Reported-by: Hulk Robot <hulkci@huawei.com>
> Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
> ---
>  kernel/cgroup/cgroup-v1.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
> index 191c329e4..1fd7d3d18 100644
> --- a/kernel/cgroup/cgroup-v1.c
> +++ b/kernel/cgroup/cgroup-v1.c
> @@ -908,6 +908,9 @@ int cgroup1_parse_param(struct fs_context *fc, struct fs_parameter *param)
>  	opt = fs_parse(fc, cgroup1_fs_parameters, param, &result);
>  	if (opt == -ENOPARAM) {
>  		if (strcmp(param->key, "source") == 0) {
> +			if (fc->source)
> +				return invalf(fc, "Multiple sources not
> +						  supported");

"never break user-visible strings such as printk messages because
that breaks the ability to grep for them.", quoted from CodingStyle.

Please fix this and you can add

	Reviewed-by: Zefan Li <lizefan@huawei.com>