[PATCH] arm64/mm: Introduce a variable to hold base address of linear region

[bhsharma@redhat.com (Bhupesh Sharma)]

The start of the linear region map on a KASLR enabled ARM64 machine -
which supports a compatible EFI firmware (with EFI_RNG_PROTOCOL
support), is no longer correctly represented by the PAGE_OFFSET macro,
since it is defined as:

    (UL(1) << (VA_BITS - 1)) + 1)

So taking an example of a platform with VA_BITS=48, this gives a static
value of:
PAGE_OFFSET = 0xffff800000000000

However, for the KASLR case, we use the 'memstart_offset_seed'
to randomize the linear region - since 'memstart_addr' indicates the
start of physical RAM, we randomize the same on basis
of 'memstart_offset_seed' value.

As the PAGE_OFFSET value is used presently by several user space
tools (for e.g. makedumpfile and crash tools) to determine the start
of linear region and hence to read addresses (like PT_NOTE fields) from
'/proc/kcore' for the non-KASLR boot cases, so it would be better to
use 'memblock_start_of_DRAM()' value (converted to virtual) as
the start of linear region for the KASLR cases and default to
the PAGE_OFFSET value for non-KASLR cases to indicate the start of
linear region.

I tested this on my qualcomm (which supports EFI_RNG_PROTOCOL)
and apm mustang (which does not support EFI_RNG_PROTOCOL) arm64 boards
and was able to use a modified user space utility (like kexec-tools and
makedumpfile) to determine the start of linear region correctly for
both the KASLR and non-KASLR boot cases.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: James Morse <james.morse@arm.com>
Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
---
 arch/arm64/include/asm/memory.h | 3 +++
 arch/arm64/kernel/arm64ksyms.c  | 1 +
 arch/arm64/mm/init.c            | 3 +++
 3 files changed, 7 insertions(+)

diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h
index 49d99214f43c..bfd0915ecaf8 100644
--- a/arch/arm64/include/asm/memory.h
+++ b/arch/arm64/include/asm/memory.h
@@ -178,6 +178,9 @@ extern s64			memstart_addr;
 /* PHYS_OFFSET - the physical address of the start of memory. */
 #define PHYS_OFFSET		({ VM_BUG_ON(memstart_addr & 1); memstart_addr; })
 
+/* the virtual base of the linear region. */
+extern s64			linear_reg_start_addr;
+
 /* the virtual base of the kernel image (minus TEXT_OFFSET) */
 extern u64			kimage_vaddr;
 
diff --git a/arch/arm64/kernel/arm64ksyms.c b/arch/arm64/kernel/arm64ksyms.c
index d894a20b70b2..a92238ea45ff 100644
--- a/arch/arm64/kernel/arm64ksyms.c
+++ b/arch/arm64/kernel/arm64ksyms.c
@@ -42,6 +42,7 @@ EXPORT_SYMBOL(__arch_copy_in_user);
 
 	/* physical memory */
 EXPORT_SYMBOL(memstart_addr);
+EXPORT_SYMBOL(linear_reg_start_addr);
 
 	/* string / mem functions */
 EXPORT_SYMBOL(strchr);
diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 325cfb3b858a..29447adb0eef 100644
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -60,6 +60,7 @@
  * that cannot be mistaken for a real physical address.
  */
 s64 memstart_addr __ro_after_init = -1;
+s64 linear_reg_start_addr __ro_after_init = PAGE_OFFSET;
 phys_addr_t arm64_dma_phys_limit __ro_after_init;
 
 #ifdef CONFIG_BLK_DEV_INITRD
@@ -452,6 +453,8 @@ void __init arm64_memblock_init(void)
 		}
 	}
 
+	linear_reg_start_addr = __phys_to_virt(memblock_start_of_DRAM());
+
 	/*
 	 * Register the kernel text, kernel data, initrd, and initial
 	 * pagetables with memblock.
-- 
2.7.4