linux-kernel - Re: [PATCH v4 4/9] epoll: Add implementation for epoll_ctl

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Date:	Tue, 10 Mar 2015 09:59:35 -0400
From:	Dan Rosenberg <dan.j.rosenberg@...il.com>
To:	Fam Zheng <famz@...hat.com>, linux-kernel@...r.kernel.org
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Andy Lutomirski <luto@...capital.net>,
	David Herrmann <dh.herrmann@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	David Drysdale <drysdale@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Vivek Goyal <vgoyal@...hat.com>,
	Mike Frysinger <vapier@...too.org>,
	Theodore Ts'o <tytso@....edu>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Rashika Kheria <rashika.kheria@...il.com>,
	Hugh Dickins <hughd@...gle.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
	Josh Triplett <josh@...htriplett.org>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Omar Sandoval <osandov@...ndov.com>,
	Jonathan Corbet <corbet@....net>, shane.seymour@...com
Subject: Re: [PATCH v4 4/9] epoll: Add implementation for epoll_ctl_batch

On 03/09/2015 09:49 PM, Fam Zheng wrote:
> +	if (!cmds || ncmds <= 0 || ncmds > EP_MAX_BATCH)
> +		return -EINVAL;
> +	cmd_size = sizeof(struct epoll_ctl_cmd) * ncmds;
> +	/* TODO: optimize for small arguments like select/poll with a stack
> +	 * allocated buffer */
> +
> +	kcmds = kmalloc(cmd_size, GFP_KERNEL);
> +	if (!kcmds)
> +		return -ENOMEM;
You probably want to define EP_MAX_BATCH as some sane value much less
than INT_MAX/(sizeof(struct epoll_ctl_cmd)). While this avoids the
integer overflow from before, any user can cause the kernel to kmalloc
up to INT_MAX bytes. Probably not a huge deal because it's freed at the
end of the syscall, but generally not a great idea.

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