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Subject: Re: [PATCH v2] smackfs: restrict bytes count in smackfs write
 functions
To:     Sabyrzhan Tasbolatov <snovitoll@gmail.com>
Cc:     andreyknvl@google.com, casey@schaufler-ca.com, jmorris@namei.org,
        linux-kernel@vger.kernel.org,
        linux-security-module@vger.kernel.org, mhocko@suse.com,
        serge@hallyn.com,
        syzbot+a71a442385a0b2815497@syzkaller.appspotmail.com
References: <3c35f7cc-1c8d-2fa0-6bc9-bde4e96017ce@i-love.sakura.ne.jp>
 <20210128115801.1096425-1-snovitoll@gmail.com>
From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <5271074f-930a-46e9-8ece-2cc65d45dc19@i-love.sakura.ne.jp>
Date:   Thu, 28 Jan 2021 21:59:33 +0900
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On 2021/01/28 20:58, Sabyrzhan Tasbolatov wrote:
> @@ -2005,6 +2009,9 @@ static ssize_t smk_write_onlycap(struct file *file, const char __user *buf,
>  	if (!smack_privileged(CAP_MAC_ADMIN))
>  		return -EPERM;
>  
> +	if (count > PAGE_SIZE)
> +		return -EINVAL;
> +
>  	data = memdup_user_nul(buf, count);
>  	if (IS_ERR(data))
>  		return PTR_ERR(data);
> @@ -2740,10 +2754,13 @@ static ssize_t smk_write_relabel_self(struct file *file, const char __user *buf,
>  		return -EPERM;
>  
>  	/*
> +	 * No partial write.
>  	 * Enough data must be present.
>  	 */
>  	if (*ppos != 0)
>  		return -EINVAL;
> +	if (count == 0 || count > PAGE_SIZE)
> +		return -EINVAL;
>  
>  	data = memdup_user_nul(buf, count);
>  	if (IS_ERR(data))
> 

Doesn't this change break legitimate requests like

  char buffer[20000];

  memset(buffer, ' ', sizeof(buffer));
  memcpy(buffer + sizeof(buffer) - 10, "foo", 3);
  write(fd, buffer, sizeof(buffer));

?