Re: [PATCH v7 01/22] x86/decompressor: Don't rely on upper 32 bits of GPRs being preserved

[Borislav Petkov]

On Fri, Jul 28, 2023 at 11:08:55AM +0200, Ard Biesheuvel wrote:
> The 4-to-5 level mode switch trampoline disables long mode and paging in
> order to be able to flick the LA57 bit. According to section 3.4.1.1 of
> the x86 architecture manual [0], we should not rely on 64-bit GPRs

Please use passive voice in your commit message: no "we" or "I", etc,
and describe your changes in imperative mood.

> retaining the upper 32 bits of their contents across such a mode switch.
> 
> Given that RBP, RBX and RSI are live at this point, let's preserve them
> on the stack, along with the return address that might be above 4G as
> well.
> 
> [0] Intel® 64 and IA-32 Architectures Software Developer’s Manual, Volume 1: Basic Architecture
> 
>   "Because the upper 32 bits of 64-bit general-purpose registers are
>    undefined in 32-bit modes, the upper 32 bits of any general-purpose
>    register are not preserved when switching from 64-bit mode to a 32-bit
>    mode (to protected mode or compatibility mode). Software must not
>    depend on these bits to maintain a value after a 64-bit to 32-bit
>    mode switch."
> 
> Fixes: 194a9749c73d650c ("x86/boot/compressed/64: Handle 5-level paging boot if kernel is above 4G")
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> ---
>  arch/x86/boot/compressed/head_64.S | 23 +++++++++++++++-----
>  1 file changed, 17 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index 03c4328a88cbd5d0..f7c11a0018477de8 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -459,11 +459,22 @@ SYM_CODE_START(startup_64)
>  	/* Save the trampoline address in RCX */
>  	movq	%rax, %rcx
>  
> +	/* Set up 32-bit addressable stack */
> +	leaq	TRAMPOLINE_32BIT_STACK_END(%rcx), %rsp
> +
>  	/*
> -	 * Load the address of trampoline_return() into RDI.
> -	 * It will be used by the trampoline to return to the main code.
> +	 * Load the address of trampoline_return() into RDI and push it onto
> +	 * the stack so it will survive 32-bit truncation due to the 32-bit

I think you should explain here what that SDM excerpt above says so that
it is clear what "32-bit truncation" is meant.

> +	 * protected mode switch. It will be used by the trampoline to return
> +	 * to the main code.
>  	 */
>  	leaq	trampoline_return(%rip), %rdi
> +	pushq	%rdi
> +
> +	/* Preserve other live 64-bit registers */
> +	pushq	%rbp
> +	pushq	%rbx
> +	pushq	%rsi
>  
>  	/* Switch to compatibility mode (CS.L = 0 CS.D = 1) via far return */
>  	pushq	$__KERNEL32_CS

What is more interesting is why is this trampoline crap unconditional?

/me goes and does git archeology...

194a9749c73d ("x86/boot/compressed/64: Handle 5-level paging boot if kernel is above 4G")

    We go through the trampoline even if we don't have to: if we're already
    in 5-level paging mode or if we don't need to switch to it. This way the
    trampoline gets tested on every boot.

Well, f*ck no.

All my machines don't have 5level pages. And I bet 5level pages is still
not in the majority of the machines out there.

This all needs to be abstracted away into a separate function which
exits early if no 5 level support.

Kirill, please fix this.

Thx.

-- 
Regards/Gruss,
    Boris.

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