x86, kaslr: Return location from decompress_kernel

This allows decompress_kernel to return a new location for the kernel to
be relocated to. Additionally, enforces CONFIG_PHYSICAL_START as the
minimum relocation position when building with CONFIG_RELOCATABLE.

With CONFIG_RANDOMIZE_BASE set, the choose_kernel_location routine
will select a new location to decompress the kernel, though here it is
presently a no-op. The kernel command line option "nokaslr" is introduced
to bypass these routines.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: http://lkml.kernel.org/r/1381450698-28710-3-git-send-email-keescook@chromium.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/arch/x86/boot/compressed/head_32.S b/arch/x86/boot/compressed/head_32.S
index 5d6f6891b188..9116aac232c7 100644
--- a/arch/x86/boot/compressed/head_32.S
+++ b/arch/x86/boot/compressed/head_32.S
@@ -117,9 +117,11 @@ preferred_addr:
         addl    %eax, %ebx
         notl    %eax
         andl    %eax, %ebx
-#else
-        movl    $LOAD_PHYSICAL_ADDR, %ebx
+        cmpl    $LOAD_PHYSICAL_ADDR, %ebx
+        jge     1f
 #endif
+        movl    $LOAD_PHYSICAL_ADDR, %ebx
+1:
 
         /* Target address to relocate to for decompression */
         addl    $z_extract_offset, %ebx
@@ -191,14 +193,14 @@ relocated:
         leal    boot_heap(%ebx), %eax
         pushl   %eax            /* heap area */
         pushl   %esi            /* real mode pointer */
-        call    decompress_kernel
+        call    decompress_kernel /* returns kernel location in %eax */
         addl    $24, %esp
 
 /*
  * Jump to the decompressed kernel.
  */
         xorl    %ebx, %ebx
-        jmp     *%ebp
+        jmp     *%eax
 
 /*
  * Stack and heap for uncompression