[PATCH 1/6 v14] ARM: Handle a device tree in lowmem

[Florian Fainelli]

On 10/1/2020 8:22 AM, Linus Walleij wrote:
> If the kernel grows big, it can happen that the kernel does not
> fit in the first memory block. This is normally out-of-spec
> but it turns out that the boot usually survives this.
> 
> What it does not survive is that the prepare_page_table() code
> wipes all PMDs (page table pointers for the virtual memory)
> over lowmem, including the PMD where the attached DTB is
> stored. (The variable is named atags but this only really
> happens for DTBs.)
> 
> Let's deal with this.
> 
> First, this makes the code jump over two PMDs where the DTB
> is stored if it ends up in lowmem: it could happen that the
> DTB crosses a PMD_SIZE boundary so to be on the safe side we
> need to skip over the one we know it starts in and also the
> next.
> 
> Next we make ARM use unflatten_and_copy_device_tree() instead
> of just unflattening it where it is. Currently we just assume
> that the device tree will be in some kind of "safe" memory and
> as proven by KASan it will sometimes end up in the kernel lowmem
> which will create crashes as the kernel will clear all lowmem
> PMDs while initializing the memory manager.
> 
> Finally, we add a new call to the MMU code to go in and clear
> out the PMDs in lowmem if the device tree was there.
> 
> Example boot log on an affected system:
> 
> ATAGs/DTB found in lowmem, skip clearing PMD @0xc3000000
> ATAGs/DTB found in lowmem, skip clearing PMD @0xc3200000
> (...)
> DTB @430af0e0 (physical) copied to @cc394d80 (virtual)
> Clear ATAGs/DTB PMD @0xc3000000
> Clear ATAGs/DTB PMD @0xc3200000
> 
> This fixes the problem where the Qualcomm APQ8060 would not
> boot while using KASan.
> 
> Signed-off-by: Linus Walleij <linus.walleij at linaro.org>

This is a much cleaner version than what we had been carrying in our STB 
Linux distribution which was:

https://github.com/Broadcom/stblinux-4.9/blob/master/linux/arch/arm/kernel/head.S#L251

https://github.com/Broadcom/stblinux-4.9/blob/master/linux/drivers/of/fdt.c#L688
-- 
Florian