linux-kernel - Re: [PATCH] x86/sme: Fix memory encryption if enabled by default and not overridden

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Date: Fri, 26 Jan 2024 16:26:50 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org
Cc: x86@...nel.org, Ard Biesheuvel <ardb@...nel.org>,
 Borislav Petkov <bp@...en8.de>,
 Nikita Zhandarovich <n.zhandarovich@...tech.ru>,
 Kevin Loughlin <kevinloughlin@...gle.com>
Subject: Re: [PATCH] x86/sme: Fix memory encryption if enabled by default and
 not overridden

On 1/26/24 10:39, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@...nel.org>
> 
> Commit cbebd68f59f0 ("x86/mm: Fix use of uninitialized buffer in
> sme_enable()") 'fixed' an issue in sme_enable() detected by static
> analysis, and broke the common case in the process.
> 
> cmdline_find_option() will return < 0 on an error, or when the command
> line argument does not appear at all. In this particular case, the
> latter is not an error condition, and so the early exit is wrong.
> 
> Instead, without mem_encrypt= on the command line, the compile time
> default should be honoured, which could be to enable memory encryption,
> and this is currently broken.
> 
> Fix it by setting sme_me_mask to a preliminary value based on the
> compile time default, and only omitting the command line argument test
> when cmdline_find_option() returns an error.
> 
> Cc: Borislav Petkov (AMD) <bp@...en8.de>
> Cc: Tom Lendacky <thomas.lendacky@....com>
> Cc: Nikita Zhandarovich <n.zhandarovich@...tech.ru>
> Cc: Kevin Loughlin <kevinloughlin@...gle.com>
> Fixes: cbebd68f59f0 ("x86/mm: Fix use of uninitialized buffer in sme_enable()")
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>

Nice catch.

Reviewed-by: Tom Lendacky <thomas.lendacky@....com>

> ---
>   arch/x86/mm/mem_encrypt_identity.c | 5 ++---
>   1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/mm/mem_encrypt_identity.c b/arch/x86/mm/mem_encrypt_identity.c
> index d73aeb16417f..30df4f1725f4 100644
> --- a/arch/x86/mm/mem_encrypt_identity.c
> +++ b/arch/x86/mm/mem_encrypt_identity.c
> @@ -600,15 +600,14 @@ void __init sme_enable(struct boot_params *bp)
>   	cmdline_ptr = (const char *)((u64)bp->hdr.cmd_line_ptr |
>   				     ((u64)bp->ext_cmd_line_ptr << 32));
>   
> +	sme_me_mask = active_by_default ? me_mask : 0;
>   	if (cmdline_find_option(cmdline_ptr, cmdline_arg, buffer, sizeof(buffer)) < 0)
> -		return;
> +		goto out;
>   
>   	if (!strncmp(buffer, cmdline_on, sizeof(buffer)))
>   		sme_me_mask = me_mask;
>   	else if (!strncmp(buffer, cmdline_off, sizeof(buffer)))
>   		sme_me_mask = 0;
> -	else
> -		sme_me_mask = active_by_default ? me_mask : 0;
>   out:
>   	if (sme_me_mask) {
>   		physical_mask &= ~sme_me_mask;