linux-kernel - Re: [PATCH] signal: always clear sa

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Date:	Mon, 11 Mar 2013 20:28:05 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	linux-kernel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	Emese Revfy <re.emese@...il.com>,
	PaX Team <pageexec@...email.hu>
Subject: Re: [PATCH] signal: always clear sa_restorer on execve

On 03/11, Kees Cook wrote:
>
> When the new signal handlers are set up for a fork, the location of
> sa_restorer is not cleared, leaking a parent process's address space
> location to children. This allows for a potential bypass of the parent's
> ASLR by examining the sa_restorer value returned when calling sigaction().

I don't understand.

fork() should not change restorer/etc, and the child has the same address
space anyway. There is no any leak and the patch can't make any difference
in this case because flush_signal_handlers() is not called by fork().

> @@ -485,6 +485,9 @@ flush_signal_handlers(struct task_struct *t, int force_default)
>  		if (force_default || ka->sa.sa_handler != SIG_IGN)
>  			ka->sa.sa_handler = SIG_DFL;
>  		ka->sa.sa_flags = 0;
> +#ifdef __ARCH_HAS_SA_RESTORER
> +		ka->sa.sa_restorer = NULL;
> +#endif

However, exec sets SIG_DFL but keeps ->sa_restorer, so probably this
patch makes sense anyway.

Oleg.

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