Re: [PATCH] kexec: allocate kernel above bzImage's pref_address

[Dave Hansen]

On 12/15/23 11:05, Chris Koch wrote:
> 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 this patch does.
> 
> Also fixes the documentation around pref_address and PHYSICAL_START to
> be accurate.

Are you reporting a bug and is this a bug fix?  It's not super clear
from the changelog.


> diff --git a/Documentation/arch/x86/boot.rst b/Documentation/arch/x86/boot.rst
> index 22cc7a040dae..49bea8986620 100644
> --- a/Documentation/arch/x86/boot.rst
> +++ b/Documentation/arch/x86/boot.rst
> @@ -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.

I think we should avoid saying the same things over and over again in
different spots.

Here, it doesn't really help to enumerate the different interpretations
of 'pref_address'.  All that matters is that the bootloader can avoid
the overhead of a later copy if it can place the kernel at
'pref_address'.  The exact reasons that various kernels might decide to
relocate are unimportant here.

>  ============	=======
>  Field name:	init_size
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 3762f41bb092..1370f43328d7 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -2109,11 +2109,11 @@ 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.

I kinda dislike how this is written.  It's written almost like code
where you're spelling out the conditions.  I prefer something much
higher-level.

	This gives a minimum physical address at which the kernel can be
	loaded.

	CONFIG_RELOCATABLE=n kernels will be decompressed to and must
	run at PHYSICAL_START exactly.

	CONFIG_RELOCATABLE=y kernels can run at any address above
	PHYSICAL_START.  If a kernel is loaded below PHYSICAL_START, it
	will relocate itself to PHYSICAL_START.

>  	  In normal kdump cases one does not have to set/change this option
>  	  as now bzImage can be compiled as a completely relocatable image
> diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
> index a61c12c01270..5dcd232d58bf 100644
> --- a/arch/x86/kernel/kexec-bzimage64.c
> +++ b/arch/x86/kernel/kexec-bzimage64.c
> @@ -498,7 +498,10 @@ 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;
> -	kbuf.buf_min = MIN_KERNEL_LOAD_ADDR;
> +	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)

Comment, please.

It isn't clear from this hunk why or how this fixes the bug.  How does
this manage to avoid clobbering reserved areas?