Commit 43b1d3e6 authored by Chris Koch's avatar Chris Koch Committed by Dave Hansen
Browse files

kexec: Allocate kernel above bzImage's pref_address 



A relocatable kernel will relocate itself to pref_address if it is
loaded below pref_address. This means a booted kernel may be relocating
itself to an area with reserved memory on modern systems, potentially
clobbering arbitrary data that may be important to the system.

This is often the case, as the default value of PHYSICAL_START is
0x1000000 and kernels are typically loaded at 0x100000 or above by
bootloaders like iPXE or kexec. GRUB behaves like the approach
implemented here.

Also fixes the documentation around pref_address and PHYSICAL_START to
be accurate.

[ dhansen: changelog tweak ]

Co-developed-by: default avatarCloud Hsu <cloudhsu@google.com>
Signed-off-by: default avatarCloud Hsu <cloudhsu@google.com>
Signed-off-by: default avatarChris Koch <chrisko@google.com>
Signed-off-by: default avatarDave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: default avatarH. Peter Anvin (Intel) <hpa@zytor.com>
Link: https://lore.kernel.org/all/20231215190521.3796022-1-chrisko%40google.com
parent ac456ca0

Documentation/arch/x86/boot.rst

+2 −1
Original line number Diff line number Diff line
@@ -878,7 +878,8 @@ Protocol: 2.10+ 
  address if possible.



  A non-relocatable kernel will unconditionally move itself and to run

  at this address.

  at this address. A relocatable kernel will move itself to this address if it

  loaded below this address.



============	=======

Field name:	init_size

arch/x86/Kconfig

+5 −5
Original line number Diff line number Diff line
@@ -2114,11 +2114,11 @@ config PHYSICAL_START 
	help

	  This gives the physical address where the kernel is loaded.



	  If kernel is a not relocatable (CONFIG_RELOCATABLE=n) then

	  bzImage will decompress itself to above physical address and

	  run from there. Otherwise, bzImage will run from the address where

	  it has been loaded by the boot loader and will ignore above physical

	  address.

	  If the kernel is not relocatable (CONFIG_RELOCATABLE=n) then bzImage

	  will decompress itself to above physical address and run from there.

	  Otherwise, bzImage will run from the address where it has been loaded

	  by the boot loader. The only exception is if it is loaded below the

	  above physical address, in which case it will relocate itself there.



	  In normal kdump cases one does not have to set/change this option

	  as now bzImage can be compiled as a completely relocatable image

arch/x86/kernel/kexec-bzimage64.c

+4 −1
Original line number Diff line number Diff line
@@ -503,7 +503,10 @@ static void *bzImage64_load(struct kimage *image, char *kernel, 
	kbuf.bufsz =  kernel_len - kern16_size;

	kbuf.memsz = PAGE_ALIGN(header->init_size);

	kbuf.buf_align = header->kernel_alignment;

	if (header->pref_address < MIN_KERNEL_LOAD_ADDR)

		kbuf.buf_min = MIN_KERNEL_LOAD_ADDR;

	else

		kbuf.buf_min = header->pref_address;

	kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;

	ret = kexec_add_buffer(&kbuf);

	if (ret)