linux-kernel - Re: [PATCH] arm: Use kernel mm when updating section permissions

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Date:	Thu, 5 Nov 2015 16:27:19 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Laura Abbott <labbott@...hat.com>
Cc:	Laura Abbott <labbott@...oraproject.org>,
	Kees Cook <keescook@...omium.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] arm: Use kernel mm when updating section permissions

On Thu, Nov 05, 2015 at 08:20:42AM -0800, Laura Abbott wrote:
> On 11/05/2015 01:46 AM, Russell King - ARM Linux wrote:
> >On Wed, Nov 04, 2015 at 05:00:39PM -0800, Laura Abbott wrote:
> >>Currently, read only permissions are not being applied even
> >>when CONFIG_DEBUG_RODATA is set. This is because section_update
> >>uses current->mm for adjusting the page tables. current->mm
> >>need not be equivalent to the kernel version. Use pgd_offset_k
> >>to get the proper page directory for updating.
> >
> >What are you trying to achieve here?  You can't use these functions
> >at run time (after the first thread has been spawned) to change
> >permissions, because there will be multiple copies of the kernel
> >section mappings, and those copies will not get updated.
> >
> >In any case, this change will probably break kexec and ftrace, as
> >the running thread will no longer see the updated page tables.
> >
> 
> I think I was hitting that exact problem with multiple copies
> not getting updated. The section_update code was being called
> and I was seeing the tables get updated but nothing was being
> applied when I tried to write to text or check the debugfs
> page table. The current flow is:
> 
> rest_init -> kernel_thread(kernel_init) and from that thread
> mark_rodata_ro. So mark_rodata_ro is always going to happen
> in a thread.
> 
> Do we need to update for both init_mm and the first running
> thread?

The "first running thread" is merely coincidental for things like kexec.

Hmm.  Actually, I think the existing code _should_ be fine.  At the
point where mark_rodata_ro() is, we should still be using init_mm, so
updating the current threads page tables should actually be updating
the swapper_pg_dir.

The other cases (kexec and ftrace) I think are also fine as they
stand - we want to be changing the currently active page tables
there.

So, I really think we do not want to be using pgd_offset_k() here at
all - but we need to find out what's changed to cause (presumably)
mark_rodata_ro() not to be hitting the swapper page tables.  Maybe
some debug in mark_rodata_ro() to find out what current->mm is?

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