linux-kernel - RE: [PATCH 3/6] x86/sev-es: Use __put_user()/__get

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Date:   Wed, 12 May 2021 08:04:33 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Joerg Roedel' <joro@...tes.org>,
        "x86@...nel.org" <x86@...nel.org>,
        Hyunwook Baek <baekhw@...gle.com>
CC:     Joerg Roedel <jroedel@...e.de>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "hpa@...or.com" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Juergen Gross" <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Martin Radev <martin.b.radev@...il.com>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>
Subject: RE: [PATCH 3/6] x86/sev-es: Use __put_user()/__get_user

From: Joerg
> Sent: 12 May 2021 08:55
> 
> From: Joerg Roedel <jroedel@...e.de>
> 
> The put_user() and get_user() functions do checks on the address which is
> passed to them. They check whether the address is actually a user-space
> address and whether its fine to access it. They also call might_fault()
> to indicate that they could fault and possibly sleep.
> 
> All of these checks are neither wanted nor required in the #VC exception
> handler, which can be invoked from almost any context and also for MMIO
> instructions from kernel space on kernel memory. All the #VC handler
> wants to know is whether a fault happened when the access was tried.
> 
> This is provided by __put_user()/__get_user(), which just do the access
> no matter what.

That can't be right at all.
__put/get_user() are only valid on user addresses and will try to
fault in a missing page - so can sleep.

At best this is abused the calls.

	David

> Fixes: f980f9c31a92 ("x86/sev-es: Compile early handler code into kernel image")
> Cc: stable@...r.kernel.org # v5.10+
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
>  arch/x86/kernel/sev.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index 6530a844eb61..110b39345b40 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -342,22 +342,22 @@ static enum es_result vc_write_mem(struct es_em_ctxt *ctxt,
>  	switch (size) {
>  	case 1:
>  		memcpy(&d1, buf, 1);
> -		if (put_user(d1, target))
> +		if (__put_user(d1, target))
>  			goto fault;
>  		break;
>  	case 2:
>  		memcpy(&d2, buf, 2);
> -		if (put_user(d2, target))
> +		if (__put_user(d2, target))
>  			goto fault;
>  		break;
>  	case 4:
>  		memcpy(&d4, buf, 4);
> -		if (put_user(d4, target))
> +		if (__put_user(d4, target))
>  			goto fault;
>  		break;
>  	case 8:
>  		memcpy(&d8, buf, 8);
> -		if (put_user(d8, target))
> +		if (__put_user(d8, target))
>  			goto fault;
>  		break;
>  	default:
> @@ -396,22 +396,22 @@ static enum es_result vc_read_mem(struct es_em_ctxt *ctxt,
> 
>  	switch (size) {
>  	case 1:
> -		if (get_user(d1, s))
> +		if (__get_user(d1, s))
>  			goto fault;
>  		memcpy(buf, &d1, 1);
>  		break;
>  	case 2:
> -		if (get_user(d2, s))
> +		if (__get_user(d2, s))
>  			goto fault;
>  		memcpy(buf, &d2, 2);
>  		break;
>  	case 4:
> -		if (get_user(d4, s))
> +		if (__get_user(d4, s))
>  			goto fault;
>  		memcpy(buf, &d4, 4);
>  		break;
>  	case 8:
> -		if (get_user(d8, s))
> +		if (__get_user(d8, s))
>  			goto fault;
>  		memcpy(buf, &d8, 8);
>  		break;
> --
> 2.31.1

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