Re: [PATCHv2 2/5] x86/boot/compressed/64: Find a place for 32-bit trampoline

[Kirill A. Shutemov]

On Tue, Feb 27, 2018 at 06:42:14PM +0300, Kirill A. Shutemov wrote:
> If a bootloader enables 64-bit mode with 4-level paging, we might need to
> switch over to 5-level paging. The switching requires the disabling of
> paging, which works fine if kernel itself is loaded below 4G.
> 
> But if the bootloader puts the kernel above 4G (not sure if anybody does
> this), we would lose control as soon as paging is disabled, because the
> code becomes unreachable to the CPU.
> 
> To handle the situation, we need a trampoline in lower memory that would
> take care of switching on 5-level paging.
> 
> This patch finds a spot in low memory for a trampoline.
> 
> The heuristic is based on code in reserve_bios_regions().
> 
> We find the end of low memory based on BIOS and EBDA start addresses.
> The trampoline is put just before end of low memory. It's mimic approach
> taken to allocate memory for realtime trampoline.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Tested-by: Borislav Petkov <bp@xxxxxxx>
> ---
>  arch/x86/boot/compressed/misc.c       |  4 ++++
>  arch/x86/boot/compressed/pgtable.h    | 11 +++++++++++
>  arch/x86/boot/compressed/pgtable_64.c | 34 ++++++++++++++++++++++++++++++++++
>  3 files changed, 49 insertions(+)
>  create mode 100644 arch/x86/boot/compressed/pgtable.h
> 
> diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
> index b50c42455e25..e58409667b13 100644
> --- a/arch/x86/boot/compressed/misc.c
> +++ b/arch/x86/boot/compressed/misc.c
> @@ -14,6 +14,7 @@
>  
>  #include "misc.h"
>  #include "error.h"
> +#include "pgtable.h"
>  #include "../string.h"
>  #include "../voffset.h"
>  
> @@ -372,6 +373,9 @@ asmlinkage __visible void *extract_kernel(void *rmode, memptr heap,
>  	debug_putaddr(output_len);
>  	debug_putaddr(kernel_total_size);
>  
> +	/* Report address of 32-bit trampoline */
> +	debug_putaddr(trampoline_32bit);
> +
>  	/*
>  	 * The memory hole needed for the kernel is the larger of either
>  	 * the entire decompressed kernel plus relocation table, or the

0-day found problem with the patch on 32-bit config.

Here's fixup:

diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index e58409667b13..8e4b55dd5df9 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -373,8 +373,10 @@ asmlinkage __visible void *extract_kernel(void *rmode, memptr heap,
 	debug_putaddr(output_len);
 	debug_putaddr(kernel_total_size);
 
+#ifdef CONFIG_X86_64
 	/* Report address of 32-bit trampoline */
 	debug_putaddr(trampoline_32bit);
+#endif
 
 	/*
 	 * The memory hole needed for the kernel is the larger of either
-- 
 Kirill A. Shutemov