linux-kernel - Re: [PATCH] Re: [SPARC32] NULL pointer derefference

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Message-Id: <20070731.020219.78708972.davem@davemloft.net>
Date:	Tue, 31 Jul 2007 02:02:19 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	mark@...hpc.demon.co.uk
Cc:	aaw@...gle.com, akpm@...ux-foundation.org,
	linux-arch@...r.kernel.org, sparclinux@...r.kernel.org,
	wli@...omorphy.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Re: [SPARC32] NULL pointer derefference

From: Mark Fortescue <mark@...hpc.demon.co.uk>
Date: Tue, 31 Jul 2007 08:55:20 +0100 (BST)

> I have formulated a patch that prevents the update_mmu_cache from doing 
> enything if there is no context available. This apears to have no 
> immediate, undesirable side effects.
> 
> This worked better than the alternative of setting up a context to work with.
> 
> Can you for see any issues in doing this?
> 
> If not, can you check+apply the attached (un-mangled) patch.

Thanks for tracking this down Mark.

The issue is that, when exec()'ing to userspace from a kernel thread,
we need activate_context() to be invoked before we try to touch
userspace at all.  This new argument handling is invoking
get_user_pages() before that happens.

activate_context() happens via flush_old_exec(), but that occurs via
load_elf_binary() et al. which is long after the argument fetching
code runs in fs/exec.c that is using get_user_pages().

(Mark, hint: activate_context() is defined to switch_mm() on
 sparc32, which is sun4c_switch_mm() which you thought was only
 invoked from context switches :-))

Touching userspace before activate_context() is questionable at best,
in my opinion.  But I can't come up with a good way to fix this right
now other than Mark's sparc patch, so I will apply it.

Thanks again Mark!

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