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

From: David Laight <David.Laight@ACULAB.COM>
To: 'Joerg Roedel' <joro@8bytes.org>,
	"x86@kernel.org" <x86@kernel.org>,
	Hyunwook Baek <baekhw@google.com>
Cc: Joerg Roedel <jroedel@suse.de>,
	"stable@vger.kernel.org" <stable@vger.kernel.org>,
	"hpa@zytor.com" <hpa@zytor.com>,
	Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Jiri Slaby <jslaby@suse.cz>,
	Dan Williams <dan.j.williams@intel.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	"Juergen Gross" <jgross@suse.com>,
	Kees Cook <keescook@chromium.org>,
	David Rientjes <rientjes@google.com>,
	Cfir Cohen <cfir@google.com>, Erdem Aktas <erdemaktas@google.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mike Stunes <mstunes@vmware.com>,
	Sean Christopherson <seanjc@google.com>,
	Martin Radev <martin.b.radev@gmail.com>,
	Arvind Sankar <nivedita@alum.mit.edu>,
	"linux-coco@lists.linux.dev" <linux-coco@lists.linux.dev>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"virtualization@lists.linux-foundation.org"
	<virtualization@lists.linux-foundation.org>
Subject: RE: [PATCH 3/6] x86/sev-es: Use __put_user()/__get_user
Date: Wed, 12 May 2021 08:04:33 +0000	[thread overview]
Message-ID: <0496626f018d4d27a8034a4822170222@AcuMS.aculab.com> (raw)
In-Reply-To: <20210512075445.18935-4-joro@8bytes.org>

From: Joerg
> Sent: 12 May 2021 08:55
> 
> From: Joerg Roedel <jroedel@suse.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@vger.kernel.org # v5.10+
> Signed-off-by: Joerg Roedel <jroedel@suse.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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