[PATCH 0/8] xen: harden frontends against malicious backends

[PATCH 0/8] xen: harden frontends against malicious backends

[Juergen Gross]

Xen backends of para-virtualized devices can live in dom0 kernel, dom0
user land, or in a driver domain. This means that a backend might
reside in a less trusted environment than the Xen core components, so
a backend should not be able to do harm to a Xen guest (it can still
mess up I/O data, but it shouldn't be able to e.g. crash a guest by
other means or cause a privilege escalation in the guest).

Unfortunately many frontends in the Linux kernel are fully trusting
their respective backends. This series is starting to fix the most
important frontends: console, disk and network.

It was discussed to handle this as a security problem, but the topic
was discussed in public before, so it isn't a real secret.

Juergen Gross (8):
  xen: sync include/xen/interface/io/ring.h with Xen's newest version
  xen/blkfront: read response from backend only once
  xen/blkfront: don't take local copy of a request from the ring page
  xen/blkfront: don't trust the backend response data blindly
  xen/netfront: read response from backend only once
  xen/netfront: don't read data from request on the ring page
  xen/netfront: don't trust the backend response data blindly
  xen/hvc: replace BUG_ON() with negative return value

 drivers/block/xen-blkfront.c    | 118 +++++++++-----
 drivers/net/xen-netfront.c      | 184 ++++++++++++++-------
 drivers/tty/hvc/hvc_xen.c       |  15 +-
 include/xen/interface/io/ring.h | 278 ++++++++++++++++++--------------
 4 files changed, 369 insertions(+), 226 deletions(-)

-- 
2.26.2

[PATCH 8/8] xen/hvc: replace BUG_ON() with negative return value

[Juergen Gross]

Xen frontends shouldn't BUG() in case of illegal data received from
their backends. So replace the BUG_ON()s when reading illegal data from
the ring page with negative return values.

Signed-off-by: Juergen Gross <jgross@suse.com>
---
 drivers/tty/hvc/hvc_xen.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
index 92c9a476defc..30d7ffb1e04c 100644
--- a/drivers/tty/hvc/hvc_xen.c
+++ b/drivers/tty/hvc/hvc_xen.c
@@ -86,6 +86,11 @@ static int __write_console(struct xencons_info *xencons,
 	cons = intf->out_cons;
 	prod = intf->out_prod;
 	mb();			/* update queue values before going on */
+
+	if (WARN_ONCE((prod - cons) > sizeof(intf->out),
+		      "Illegal ring page indices"))
+		return -EINVAL;
+
 	BUG_ON((prod - cons) > sizeof(intf->out));
 
 	while ((sent < len) && ((prod - cons) < sizeof(intf->out)))
@@ -114,7 +119,10 @@ static int domU_write_console(uint32_t vtermno, const char *data, int len)
 	 */
 	while (len) {
 		int sent = __write_console(cons, data, len);
-		
+
+		if (sent < 0)
+			return sent;
+
 		data += sent;
 		len -= sent;
 
@@ -138,7 +146,10 @@ static int domU_read_console(uint32_t vtermno, char *buf, int len)
 	cons = intf->in_cons;
 	prod = intf->in_prod;
 	mb();			/* get pointers before reading ring */
-	BUG_ON((prod - cons) > sizeof(intf->in));
+
+	if (WARN_ONCE((prod - cons) > sizeof(intf->in),
+		      "Illegal ring page indices"))
+		return -EINVAL;
 
 	while (cons != prod && recv < len)
 		buf[recv++] = intf->in[MASK_XENCONS_IDX(cons++, intf->in)];
-- 
2.26.2

Re: [PATCH 8/8] xen/hvc: replace BUG_ON() with negative return value

[Christophe Leroy]

Le 13/05/2021 à 12:03, Juergen Gross a écrit :
> Xen frontends shouldn't BUG() in case of illegal data received from
> their backends. So replace the BUG_ON()s when reading illegal data from
> the ring page with negative return values.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>   drivers/tty/hvc/hvc_xen.c | 15 +++++++++++++--
>   1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
> index 92c9a476defc..30d7ffb1e04c 100644
> --- a/drivers/tty/hvc/hvc_xen.c
> +++ b/drivers/tty/hvc/hvc_xen.c
> @@ -86,6 +86,11 @@ static int __write_console(struct xencons_info *xencons,
>   	cons = intf->out_cons;
>   	prod = intf->out_prod;
>   	mb();			/* update queue values before going on */
> +
> +	if (WARN_ONCE((prod - cons) > sizeof(intf->out),
> +		      "Illegal ring page indices"))
> +		return -EINVAL;
> +
>   	BUG_ON((prod - cons) > sizeof(intf->out));

Why keep the BUG_ON() ?


>   
>   	while ((sent < len) && ((prod - cons) < sizeof(intf->out)))
> @@ -114,7 +119,10 @@ static int domU_write_console(uint32_t vtermno, const char *data, int len)
>   	 */
>   	while (len) {
>   		int sent = __write_console(cons, data, len);
> -		
> +
> +		if (sent < 0)
> +			return sent;
> +
>   		data += sent;
>   		len -= sent;
>   
> @@ -138,7 +146,10 @@ static int domU_read_console(uint32_t vtermno, char *buf, int len)
>   	cons = intf->in_cons;
>   	prod = intf->in_prod;
>   	mb();			/* get pointers before reading ring */
> -	BUG_ON((prod - cons) > sizeof(intf->in));
> +
> +	if (WARN_ONCE((prod - cons) > sizeof(intf->in),
> +		      "Illegal ring page indices"))
> +		return -EINVAL;
>   
>   	while (cons != prod && recv < len)
>   		buf[recv++] = intf->in[MASK_XENCONS_IDX(cons++, intf->in)];
> 

Re: [PATCH 8/8] xen/hvc: replace BUG_ON() with negative return value

[Juergen Gross]

[-- Attachment #1.1.1: Type: text/plain, Size: 1232 bytes --]

On 13.05.21 12:16, Christophe Leroy wrote:
> 
> 
> Le 13/05/2021 à 12:03, Juergen Gross a écrit :
>> Xen frontends shouldn't BUG() in case of illegal data received from
>> their backends. So replace the BUG_ON()s when reading illegal data from
>> the ring page with negative return values.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>>   drivers/tty/hvc/hvc_xen.c | 15 +++++++++++++--
>>   1 file changed, 13 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
>> index 92c9a476defc..30d7ffb1e04c 100644
>> --- a/drivers/tty/hvc/hvc_xen.c
>> +++ b/drivers/tty/hvc/hvc_xen.c
>> @@ -86,6 +86,11 @@ static int __write_console(struct xencons_info 
>> *xencons,
>>       cons = intf->out_cons;
>>       prod = intf->out_prod;
>>       mb();            /* update queue values before going on */
>> +
>> +    if (WARN_ONCE((prod - cons) > sizeof(intf->out),
>> +              "Illegal ring page indices"))
>> +        return -EINVAL;
>> +
>>       BUG_ON((prod - cons) > sizeof(intf->out));
> 
> Why keep the BUG_ON() ?

Oh, failed to delete it. Thanks for noticing.


Juergen

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Re: [PATCH 8/8] xen/hvc: replace BUG_ON() with negative return value

[Greg Kroah-Hartman]

On Thu, May 13, 2021 at 12:03:02PM +0200, Juergen Gross wrote:
> Xen frontends shouldn't BUG() in case of illegal data received from
> their backends. So replace the BUG_ON()s when reading illegal data from
> the ring page with negative return values.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>  drivers/tty/hvc/hvc_xen.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
> index 92c9a476defc..30d7ffb1e04c 100644
> --- a/drivers/tty/hvc/hvc_xen.c
> +++ b/drivers/tty/hvc/hvc_xen.c
> @@ -86,6 +86,11 @@ static int __write_console(struct xencons_info *xencons,
>  	cons = intf->out_cons;
>  	prod = intf->out_prod;
>  	mb();			/* update queue values before going on */
> +
> +	if (WARN_ONCE((prod - cons) > sizeof(intf->out),
> +		      "Illegal ring page indices"))
> +		return -EINVAL;

How nice, you just rebooted on panic-on-warn systems :(

> +
>  	BUG_ON((prod - cons) > sizeof(intf->out));

Why keep this line?

Please just fix this up properly, if userspace can trigger this, then
both the WARN_ON() and BUG_ON() are not correct and need to be correctly
handled.


>  
>  	while ((sent < len) && ((prod - cons) < sizeof(intf->out)))
> @@ -114,7 +119,10 @@ static int domU_write_console(uint32_t vtermno, const char *data, int len)
>  	 */
>  	while (len) {
>  		int sent = __write_console(cons, data, len);
> -		
> +
> +		if (sent < 0)
> +			return sent;
> +
>  		data += sent;
>  		len -= sent;
>  
> @@ -138,7 +146,10 @@ static int domU_read_console(uint32_t vtermno, char *buf, int len)
>  	cons = intf->in_cons;
>  	prod = intf->in_prod;
>  	mb();			/* get pointers before reading ring */
> -	BUG_ON((prod - cons) > sizeof(intf->in));
> +
> +	if (WARN_ONCE((prod - cons) > sizeof(intf->in),
> +		      "Illegal ring page indices"))
> +		return -EINVAL;

Same here, you still just paniced a machine :(

thanks,

greg k-h

Re: [PATCH 8/8] xen/hvc: replace BUG_ON() with negative return value

[Juergen Gross]

[-- Attachment #1.1.1: Type: text/plain, Size: 1447 bytes --]

On 13.05.21 12:25, Greg Kroah-Hartman wrote:
> On Thu, May 13, 2021 at 12:03:02PM +0200, Juergen Gross wrote:
>> Xen frontends shouldn't BUG() in case of illegal data received from
>> their backends. So replace the BUG_ON()s when reading illegal data from
>> the ring page with negative return values.
>>
>> Signed-off-by: Juergen Gross <jgross@suse.com>
>> ---
>>   drivers/tty/hvc/hvc_xen.c | 15 +++++++++++++--
>>   1 file changed, 13 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
>> index 92c9a476defc..30d7ffb1e04c 100644
>> --- a/drivers/tty/hvc/hvc_xen.c
>> +++ b/drivers/tty/hvc/hvc_xen.c
>> @@ -86,6 +86,11 @@ static int __write_console(struct xencons_info *xencons,
>>   	cons = intf->out_cons;
>>   	prod = intf->out_prod;
>>   	mb();			/* update queue values before going on */
>> +
>> +	if (WARN_ONCE((prod - cons) > sizeof(intf->out),
>> +		      "Illegal ring page indices"))
>> +		return -EINVAL;
> 
> How nice, you just rebooted on panic-on-warn systems :(
> 
>> +
>>   	BUG_ON((prod - cons) > sizeof(intf->out));
> 
> Why keep this line?

Failed to delete it, sorry.

> 
> Please just fix this up properly, if userspace can trigger this, then
> both the WARN_ON() and BUG_ON() are not correct and need to be correctly
> handled.

It can be triggered by the console backend, but I agree a WARN isn't the
way to go here.


Juergen

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Re: [PATCH 0/8] xen: harden frontends against malicious backends

[Marek Marczykowski-Górecki]

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On Thu, May 13, 2021 at 12:02:54PM +0200, Juergen Gross wrote:
> Xen backends of para-virtualized devices can live in dom0 kernel, dom0
> user land, or in a driver domain. This means that a backend might
> reside in a less trusted environment than the Xen core components, so
> a backend should not be able to do harm to a Xen guest (it can still
> mess up I/O data, but it shouldn't be able to e.g. crash a guest by
> other means or cause a privilege escalation in the guest).
> 
> Unfortunately many frontends in the Linux kernel are fully trusting
> their respective backends. This series is starting to fix the most
> important frontends: console, disk and network.
> 
> It was discussed to handle this as a security problem, but the topic
> was discussed in public before, so it isn't a real secret.

Is it based on patches we ship in Qubes[1] and also I've sent here some
years ago[2]? I see a lot of similarities. If not, you may want to
compare them.

[1] https://github.com/QubesOS/qubes-linux-kernel/
[2] https://lists.xenproject.org/archives/html/xen-devel/2018-04/msg02336.html


> Juergen Gross (8):
>   xen: sync include/xen/interface/io/ring.h with Xen's newest version
>   xen/blkfront: read response from backend only once
>   xen/blkfront: don't take local copy of a request from the ring page
>   xen/blkfront: don't trust the backend response data blindly
>   xen/netfront: read response from backend only once
>   xen/netfront: don't read data from request on the ring page
>   xen/netfront: don't trust the backend response data blindly
>   xen/hvc: replace BUG_ON() with negative return value
> 
>  drivers/block/xen-blkfront.c    | 118 +++++++++-----
>  drivers/net/xen-netfront.c      | 184 ++++++++++++++-------
>  drivers/tty/hvc/hvc_xen.c       |  15 +-
>  include/xen/interface/io/ring.h | 278 ++++++++++++++++++--------------
>  4 files changed, 369 insertions(+), 226 deletions(-)
> 
> -- 
> 2.26.2
> 
> 

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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