x86/boot/KASLR: Fix kexec crash due to 'virt_addr' calculation bug

Kernel text KASLR is separated into physical address and virtual
address randomization. And for virtual address randomization, we
only randomiza to get an offset between 16M and KERNEL_IMAGE_SIZE.
So the initial value of 'virt_addr' should be LOAD_PHYSICAL_ADDR,
but not the original kernel loading address 'output'.

The bug will cause kernel boot failure if kernel is loaded at a different
position than the address, 16M, which is decided at compiled time.
Kexec/kdump is such practical case.

To fix it, just assign LOAD_PHYSICAL_ADDR to virt_addr as initial
value.
Tested-by: Dave Young <dyoung@redhat.com>
Signed-off-by: Baoquan He <bhe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 8391c73c ("x86/KASLR: Randomize virtual address separately")
Link: http://lkml.kernel.org/r/1498567146-11990-3-git-send-email-bhe@redhat.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/boot/compressed/kaslr.c

@@ -564,9 +564,6 @@ void choose_random_location(unsigned long input,
 {
 	unsigned long random_addr, min_addr;
 
-	/* By default, keep output position unchanged. */
-	*virt_addr = *output;
-
 	if (cmdline_find_option_bool("nokaslr")) {
 		warn("KASLR disabled: 'nokaslr' on cmdline.");
 		return;

arch/x86/boot/compressed/misc.c

@@ -338,7 +338,7 @@ asmlinkage __visible void *extract_kernel(void *rmode, memptr heap,
 				  unsigned long output_len)
 {
 	const unsigned long kernel_total_size = VO__end - VO__text;
-	unsigned long virt_addr = (unsigned long)output;
+	unsigned long virt_addr = LOAD_PHYSICAL_ADDR;
 
 	/* Retain x86 boot parameters pointer passed from startup_32/64. */
 	boot_params = rmode;
@@ -399,7 +399,7 @@ asmlinkage __visible void *extract_kernel(void *rmode, memptr heap,
 #ifndef CONFIG_RELOCATABLE
 	if ((unsigned long)output != LOAD_PHYSICAL_ADDR)
 		error("Destination address does not match LOAD_PHYSICAL_ADDR");
-	if ((unsigned long)output != virt_addr)
+	if (virt_addr != LOAD_PHYSICAL_ADDR)
 		error("Destination virtual address changed when not relocatable");
 #endif
 

arch/x86/boot/compressed/misc.h

@@ -81,8 +81,6 @@ static inline void choose_random_location(unsigned long input,
 					  unsigned long output_size,
 					  unsigned long *virt_addr)
 {
-	/* No change from existing output location. */
-	*virt_addr = *output;
 }
 #endif