[PATCH] arm64: mm: phys_mem_access_prot() reports non-kernel memory as device memory

[Will Deacon]

On Mon, Sep 05, 2016 at 09:43:03AM +0100, James Morse wrote:
> Running 'acpidump' on an arm64 system causes the program to SIGSEGV, and
> yield the following trace:
> [   71.623572] acpidump[2159]: unhandled alignment fault (11) at 0xffff8a4a909c, esr 0x92000021
> [   71.632030] pgd = ffff8003ecf7d000
> [   71.635420] [ffff8a4a909c] *pgd=00000083ec370003, *pud=00000083e7225003, *pmd=00000083ec3f1003, *pte=01600083ff1c3fc3
> [   71.646042]
> [   71.647524] CPU: 0 PID: 2159 Comm: acpidump Tainted: G        W I     4.8.0-rc4 #5081
> [   71.655352] Hardware name: AMD Overdrive/Supercharger/Default string, BIOS ROD1002C 04/08/2016
> [   71.663957] task: ffff8003eac41900 task.stack: ffff8003e7210000
> [   71.669871] PC is at 0xffff8a3c77d4
> [   71.673353] LR is at 0x404934
> [   71.676314] pc : [<0000ffff8a3c77d4>] lr : [<0000000000404934>] pstate: 00000000
> [   71.683702] sp : 0000ffffd0e39af0
> 
> Acpidump digs through /sys/ to find the physical address of the ACPI tables,
> then reads them from /dev/mem. The code that provides /dev/mem uses
> phys_mem_access_prot() to determine the protection type it should use.
> 
> Arm64's phys_mem_access_prot() reports any memory region that isn't part
> of the linear map as device memory. This breaks Acpidump as the acpi tables
> are neither device memory nor part of the linear map.
> 
> Change this check to use memblock_is_memory() instead. On an EFI system
> any region the efi memory map described as any of WB/WC/WT appears in
> memblock.memory. On a non-EFI system, setup_machine_fdt() will cause
> memory nodes in the DT to be added to memblock.memory.
> 
> /dev/mem is the only user of phys_mem_access_prot()
> 
> Reported-by: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
> Signed-off-by: James Morse <james.morse at arm.com>
> ---
> Stable?
> Acpi was introduced for v4.1 but hidden behind 'expert'.
> 'expert' was removed for v4.7.
> 
>  arch/arm64/mm/mmu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 4989948d1feb..96a2f327fd2c 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -63,7 +63,7 @@ static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused;
>  pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
>  			      unsigned long size, pgprot_t vma_prot)
>  {
> -	if (!pfn_valid(pfn))
> +	if (!memblock_is_memory(pfn << PAGE_SHIFT))

Hmm, this looks pretty weird to me. pfn_valid currently calls
memblock_is_map_memory, so now we have this slightly paradoxical situation
of having some memory marked NOMAP but wanting to map it to userspace.

So our abstractions don't seem to align with reality. Why are the ACPI
tables getting marked as NOMAP to being with?

Will