[PATCH] arm64: ignore memory outside of the linear range

On Mon, Aug 17, 2015 at 11:35:46AM +0100, Ard Biesheuvel wrote:

On 17 August 2015 at 11:43, Will Deacon [off-list ref] wrote:

quoted

Hi Ard,

On Sat, Aug 15, 2015 at 01:13:44PM +0100, Ard Biesheuvel wrote:

quoted

We need to ensure that we don't try to map system RAM ranges whose
offset relative to the start of the kernel image exceeds the size of
the linear range. This may happen even on systems that don't have
huge amounts of RAM if it is laid out very sparsely.

Signed-off-by: Ard Biesheuvel <redacted>
---

This is the minimal fix for addressing the issue we discussed. I dropped
the other changes for now, let's revisit those when (if) my patches for
decoupling the kernel mapping from the linear mapping are back under
discussion.

I will leave it up to the maintainers whether this constitutes a bugfix or
not, but since this has never worked from the beginning afaict, I don't
think it belongs in stable per se.

 arch/arm64/mm/init.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index ad87ce826cce..c65e57d4c3e7 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c

@@ -158,6 +158,19 @@ early_param("mem", early_mem);

 void __init arm64_memblock_init(void)
 {
+     /*
+      * Remove the memory that we will not be able to cover
+      * with the linear mapping.
+      */
+     const s64 linear_region_size = -(s64)PAGE_OFFSET;
+
+     if (memstart_addr + linear_region_size < memblock_end_of_DRAM()) {
+             pr_warn("Ignoring memory outside of linear range (0x%012llx - 0x%012llx)\n",
+                     memstart_addr + linear_region_size,
+                     (u64)memblock_end_of_DRAM() - 1);
+             memblock_remove(memstart_addr + linear_region_size, ULLONG_MAX);
+     }
+

I think this will interact badly with Mark Salter's patches to relocate
the initrd if it falls outside of the linear mapping (which relies on the
memblocks remaining intact after paging_init):

  https://lkml.org/lkml/2015/8/16/75

Are you sure? By the looks of it, these patches combined would
correctly address the case where no mem= is passed, but the initial
ramdisk is loaded past the end of the linear region.

I.e., memblock_end_of_DRAM() will return the clipped value, which
would be smaller than orig_end, triggering the relocation machinery
which moves it inside the linear region.

Ok. I was trying to consider the case where the initrd is outside of the
linear mapping not because of a restrictive "mem=", but because its out
of range of our page tables. AFAICT, we'll end up attempting to memcpy
the thing back down (but I appreciate that things won't work without
your patch too).

Will