linux-kernel - Re: [PATCH] Fix direct mapping correctly in ioremap

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Message-ID: <20080214160322.GA27530@elte.hu>
Date:	Thu, 14 Feb 2008 17:03:23 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	torvalds@...l.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] Fix direct mapping correctly in ioremap

* Andi Kleen <andi@...stfloor.org> wrote:

> -	if (ioremap_change_attr(vaddr, size, mode) < 0) {
> +	/* Fix up the direct mapping for the new cache attributes */
> +	err = ioremap_change_attr((unsigned long)__va(phys_addr),
> +				size + offset, mode);

Ugh. This would break the 32-bit kernel - if any phys_addr larger than 
1GB is passed in (which is the common case on 32-bit) then we'll start 
changing the attributes of random (most likely user-space) pages!

That is because on 32-bit __va() does this:

  #define __va(x)                  ((void *)((unsigned long)(x)+PAGE_OFFSET))

where on 32-bit 3GB:1GB split PAGE_OFFSET is defined to 0xc0000000.

So if a driver passes in the physical address of a PCI device with a BAR 
at 0xe1000000 somewhere in the PCI aperture, we'll get 
0xe1000000+0xc0000000 == 0x91000000 - right in the middle of user-space.

Changing attributes there is very wrong. (it could even crash the kernel 
in certain circumstances.)

Have you tried to boot this patch on 32-bit? There are a couple of new 
safety nets in the cpa code that would/should trigger very visibly - 
such as the warning here:

                if (!pte_val(old_pte)) {
                        printk(KERN_WARNING "CPA: called for zero pte. "
                               "vaddr = %lx cpa->vaddr = %lx\n", address,
                                cpa->vaddr);
                        WARN_ON(1);
                        return -EINVAL;
                }

(these are already there in -git)

Please have a look at how we solved the "secondary alias" 64-bit problem 
in x86.git#mm and please resend against x86.git#mm if you still think 
something is missing. Thanks,

	Ingo
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