Re: [PATCH RFC 5/9] ARM: Add L1 PTE non-secure mapping

Russell King - ARM Linux
2014-07-21T16:46:48Z

On Mon, Jul 21, 2014 at 03:47:16PM +0100, Daniel Thompson wrote:
> From: Marek Vasut <marex@denx.de>
>
> Add new device type, MT_DEVICE_NS. This type sets the NS bit in L1 PTE [1].
> Accesses to a memory region which is mapped this way generate non-secure
> access to that memory area. One must be careful here, since the NS bit is
> only available in L1 PTE, therefore when creating the mapping, the mapping
> must be at least 1 MiB big and must be aligned to 1 MiB. If that condition
> was false, the kernel would use regular L2 page mapping for this area instead
> and the NS bit setting would be ineffective.

Right, so this says that PTE mappings are not permissible.

> + [MT_DEVICE_NS] = { /* Non-secure accesses from secure mode */
> + .prot_pte = PROT_PTE_DEVICE | L_PTE_MT_DEV_SHARED |
> + L_PTE_SHARED,
> + .prot_l1 = PMD_TYPE_TABLE,

However, by filling in prot_pte and prot_l1, you're telling the code that
it /can/ setup such a mapping. This is screwed.

If you want to deny anything but section mappings (because they don't work)
then you omit prot_pte and prot_l1. With those omitted, if someone tries
to abuse this mapping type, then this check in create_mapping() will
trigger:

if (type->prot_l1 == 0 && ((addr | phys | length) & ~SECTION_MASK)) {
printk(KERN_WARNING "BUG: map for 0x%08llx at 0x%08lx can not "
"be mapped using pages, ignoring.\n",
(long long)__pfn_to_phys(md->pfn), addr);
return;
}

ioremap doesn't have that check; it assumes that it will always be setting
up PTE mappings via ioremap_page_range(). In fact, on many platforms
that's the only option.

So making this interface available via ioremap() seems pointless - but
more importantly it's extremely error-prone. So, MT_DEVICE_NS shouldn't
be using 4 at all, shouldn't be in asm/io.h, but should be with the
private MT_* definitions in map.h.

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