linux-kernel - Re: [PATCH v1] x86: Pin cr4 FSGSBASE

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Date:   Tue, 26 May 2020 08:56:18 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Andi Kleen <andi@...stfloor.org>
Cc:     x86@...nel.org, keescook@...omium.org,
        linux-kernel@...r.kernel.org, sashal@...nel.org,
        Andi Kleen <ak@...ux.intel.com>, stable@...r.kernel.org
Subject: Re: [PATCH v1] x86: Pin cr4 FSGSBASE

On Mon, May 25, 2020 at 10:28:48PM -0700, Andi Kleen wrote:
> From: Andi Kleen <ak@...ux.intel.com>
> 
> Since there seem to be kernel modules floating around that set
> FSGSBASE incorrectly, prevent this in the CR4 pinning. Currently
> CR4 pinning just checks that bits are set, this also checks
> that the FSGSBASE bit is not set, and if it is clears it again.

So we are trying to "protect" ourselves from broken out-of-tree kernel
modules now?  Why stop with this type of check, why not just forbid them
entirely if we don't trust them?  :)

> Note this patch will need to be undone when the full FSGSBASE
> patches are merged. But it's a reasonable solution for v5.2+
> stable at least. Sadly the older kernels don't have the necessary
> infrastructure for this (although a simpler version of this
> could be added there too)
> 
> Cc: stable@...r.kernel.org # v5.2+
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> ---
>  arch/x86/kernel/cpu/common.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
> index bed0cb83fe24..1f5b7871ae9a 100644
> --- a/arch/x86/kernel/cpu/common.c
> +++ b/arch/x86/kernel/cpu/common.c
> @@ -385,6 +385,11 @@ void native_write_cr4(unsigned long val)
>  		/* Warn after we've set the missing bits. */
>  		WARN_ONCE(bits_missing, "CR4 bits went missing: %lx!?\n",
>  			  bits_missing);
> +		if (val & X86_CR4_FSGSBASE) {
> +			WARN_ONCE(1, "CR4 unexpectedly set FSGSBASE!?\n");

Like this will actually be noticed by anyone who calls this?  What is a
user supposed to do about this?

What about those systems that panic-on-warn?

> +			val &= ~X86_CR4_FSGSBASE;

So you just prevented them from setting this, thereby fixing up their
broken code that will never be fixed because you did this?  Why do this?

thanks,

greg k-h