Re: [RFC][PATCH] arm64: Add support for CONFIG_DEBUG_VIRTUAL

From: Laura Abbott
Date: Wed Nov 02 2016 - 16:28:09 EST

On 10/29/2016 02:39 AM, Ard Biesheuvel wrote:
On 28 October 2016 at 23:07, Laura Abbott <labbott@xxxxxxxxxx> wrote:
diff --git a/arch/arm64/mm/physaddr.c b/arch/arm64/mm/physaddr.c
new file mode 100644
index 0000000..6c271e2
--- /dev/null
+++ b/arch/arm64/mm/physaddr.c
@@ -0,0 +1,17 @@
+#include <linux/mm.h>
+#include <asm/memory.h>
+unsigned long __virt_to_phys(unsigned long x)
+ phys_addr_t __x = (phys_addr_t)x;
+ if (__x & BIT(VA_BITS - 1)) {
+ /* The bit check ensures this is the right range */
+ return (__x & ~PAGE_OFFSET) + PHYS_OFFSET;
+ } else {
+ VIRTUAL_BUG_ON(x < kimage_vaddr || x > (unsigned long)_end);

IIUC, in (3) you were asking if the last check should be '>' or '>='?

To match high_memory, I suspect the latter, as _end doesn't fall within
the mapped virtual address space.

I was actually concerned about if _end would be correct with KASLR.
Ard confirmed that it gets fixed up to be correct. I'll change the
check to check for >=.

While testing this, I found two places with __pa(_end) to get bounds,
one in arm64 code and one in memblock code. x86 gets away with this
because memblock is actually __pa_symbol and x86 does image placement
different and can check against the maximum image size. I think
including _end in __pa_symbol but excluding it from the generic
__virt_to_phys makes sense. It's a bit nicer than doing _end - 1 +
1 everywhere.

Could we redefine __pa_symbol() under CONFIG_DEBUG_VIRTUAL to
something that checks (x >= kimage_vaddr + TEXT_OFFSET || x <=
(unsigned long)_end), i.e., reject linear virtual addresses? (Assuming
my understanding of the meaning of __pa_symbol() is correct)

Yes, that's going to be my approach. I originally prototyped this
with just x >= kimage_vaddr but kimage_vaddr + TEXT_OFFSET is
a tighter bound.