[PATCH] Drivers: hv: vmbus: Fix variable assignments in hv_ringbuffer_read()

[PATCH] Drivers: hv: vmbus: Fix variable assignments in hv_ringbuffer_read()

[Andres Beltran]

Assignments to buffer_actual_len and requestid happen before packetlen
is checked to be within buflen. If this condition is true,
hv_ringbuffer_read() returns with these variables already set to some
value even though no data is actually read. This might create
inconsistencies in any routine calling hv_ringbuffer_read(). Assign values
to such pointers after the packetlen check.

Signed-off-by: Andres Beltran <lkmlabelt@gmail.com>
---
 drivers/hv/ring_buffer.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
index 356e22159e83..e277ce7372a4 100644
--- a/drivers/hv/ring_buffer.c
+++ b/drivers/hv/ring_buffer.c
@@ -350,12 +350,13 @@ int hv_ringbuffer_read(struct vmbus_channel *channel,
 
 	offset = raw ? 0 : (desc->offset8 << 3);
 	packetlen = (desc->len8 << 3) - offset;
-	*buffer_actual_len = packetlen;
-	*requestid = desc->trans_id;
 
 	if (unlikely(packetlen > buflen))
 		return -ENOBUFS;
 
+	*buffer_actual_len = packetlen;
+	*requestid = desc->trans_id;
+
 	/* since ring is double mapped, only one copy is necessary */
 	memcpy(buffer, (const char *)desc + offset, packetlen);
 
-- 
2.25.1

Re: [PATCH] Drivers: hv: vmbus: Fix variable assignments in hv_ringbuffer_read()

[Stephen Hemminger]

On Fri, 24 Jul 2020 09:46:06 -0700
"Andres Beltran" <lkmlabelt@gmail.com> wrote:

> Assignments to buffer_actual_len and requestid happen before packetlen
> is checked to be within buflen. If this condition is true,
> hv_ringbuffer_read() returns with these variables already set to some
> value even though no data is actually read. This might create
> inconsistencies in any routine calling hv_ringbuffer_read(). Assign values
> to such pointers after the packetlen check.
> 
> Signed-off-by: Andres Beltran <lkmlabelt@gmail.com>
> ---
>  drivers/hv/ring_buffer.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
> index 356e22159e83..e277ce7372a4 100644
> --- a/drivers/hv/ring_buffer.c
> +++ b/drivers/hv/ring_buffer.c
> @@ -350,12 +350,13 @@ int hv_ringbuffer_read(struct vmbus_channel
> *channel,
>  
>  	offset = raw ? 0 : (desc->offset8 << 3);
>  	packetlen = (desc->len8 << 3) - offset;
> -	*buffer_actual_len = packetlen;
> -	*requestid = desc->trans_id;
>  
>  	if (unlikely(packetlen > buflen))
>  		return -ENOBUFS;
>  
> +	*buffer_actual_len = packetlen;
> +	*requestid = desc->trans_id;
> +
>  	/* since ring is double mapped, only one copy is necessary */
>  	memcpy(buffer, (const char *)desc + offset, packetlen);
>  

What is the rationale for this change, it may break other code.

A common API model in Windows world where this originated
is to have a call where caller first
makes request and then if the requested buffer is not big enough the
caller look at the actual length and allocate a bigger buffer.

Did you audit all the users of this API to make sure they aren't doing that.

RE: [PATCH] Drivers: hv: vmbus: Fix variable assignments in hv_ringbuffer_read()

[Haiyang Zhang]

> -----Original Message-----
> From: Stephen Hemminger <stephen@networkplumber.org>
> Sent: Friday, July 24, 2020 1:11 PM
> To: Andres Beltran <lkmlabelt@gmail.com>
> Cc: KY Srinivasan <kys@microsoft.com>; Haiyang Zhang
> <haiyangz@microsoft.com>; Stephen Hemminger <sthemmin@microsoft.com>;
> wei.liu@kernel.org; linux-hyperv@vger.kernel.org; linux-
> kernel@vger.kernel.org; Michael Kelley <mikelley@microsoft.com>;
> parri.andrea@gmail.com; Saruhan Karademir <skarade@microsoft.com>
> Subject: Re: [PATCH] Drivers: hv: vmbus: Fix variable assignments in
> hv_ringbuffer_read()
> 
> On Fri, 24 Jul 2020 09:46:06 -0700
> "Andres Beltran" <lkmlabelt@gmail.com> wrote:
> 
> > Assignments to buffer_actual_len and requestid happen before packetlen
> > is checked to be within buflen. If this condition is true,
> > hv_ringbuffer_read() returns with these variables already set to some
> > value even though no data is actually read. This might create
> > inconsistencies in any routine calling hv_ringbuffer_read(). Assign values
> > to such pointers after the packetlen check.
> >
> > Signed-off-by: Andres Beltran <lkmlabelt@gmail.com>
> > ---
> >  drivers/hv/ring_buffer.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/hv/ring_buffer.c b/drivers/hv/ring_buffer.c
> > index 356e22159e83..e277ce7372a4 100644
> > --- a/drivers/hv/ring_buffer.c
> > +++ b/drivers/hv/ring_buffer.c
> > @@ -350,12 +350,13 @@ int hv_ringbuffer_read(struct vmbus_channel
> > *channel,
> >
> >  	offset = raw ? 0 : (desc->offset8 << 3);
> >  	packetlen = (desc->len8 << 3) - offset;
> > -	*buffer_actual_len = packetlen;
> > -	*requestid = desc->trans_id;
> >
> >  	if (unlikely(packetlen > buflen))
> >  		return -ENOBUFS;
> >
> > +	*buffer_actual_len = packetlen;
> > +	*requestid = desc->trans_id;
> > +
> >  	/* since ring is double mapped, only one copy is necessary */
> >  	memcpy(buffer, (const char *)desc + offset, packetlen);
> >
> 
> What is the rationale for this change, it may break other code.
> 
> A common API model in Windows world where this originated
> is to have a call where caller first
> makes request and then if the requested buffer is not big enough the
> caller look at the actual length and allocate a bigger buffer.
> 
> Did you audit all the users of this API to make sure they aren't doing that.

I took a quick look. I saw a case which treats the ringbuffer_read as 
successful if the buffer_actual_len > 0. So if hv_ringbuffer_read() should 
not be fixed this way, then all callers like balloon_onchannelcallback() 
need a fix.

1476 static void balloon_onchannelcallback(void *context)
1477 {
1478         struct hv_device *dev = context;
1479         u32 recvlen;
1480         u64 requestid;
1481         struct dm_message *dm_msg;
1482         struct dm_header *dm_hdr;
1483         struct hv_dynmem_device *dm = hv_get_drvdata(dev);
1484         struct dm_balloon *bal_msg;
1485         struct dm_hot_add *ha_msg;
1486         union dm_mem_page_range *ha_pg_range;
1487         union dm_mem_page_range *ha_region;
1488
1489         memset(recv_buffer, 0, sizeof(recv_buffer));
1490         vmbus_recvpacket(dev->channel, recv_buffer,
1491                          HV_HYP_PAGE_SIZE, &recvlen, &requestid);
1492
1493         if (recvlen > 0) {
1494                 dm_msg = (struct dm_message *)recv_buffer;
1495                 dm_hdr = &dm_msg->hdr;

Thanks,
- Haiyang

Re: [PATCH] Drivers: hv: vmbus: Fix variable assignments in hv_ringbuffer_read()

[Andres Beltran]

On Fri, Jul 24, 2020 at 1:10 PM Stephen Hemminger
<stephen@networkplumber.org> wrote:
> What is the rationale for this change, it may break other code.
>
> A common API model in Windows world where this originated
> is to have a call where caller first
> makes request and then if the requested buffer is not big enough the
> caller look at the actual length and allocate a bigger buffer.
>
> Did you audit all the users of this API to make sure they aren't doing that.
>

The rationale for the change was to solve instances like the one
@Haiyang Zhang pointed out, especially in hv_utils, which needs
additional hardening. Unfortunately, there is an instance in
hv_pci_onchannelcallback() that does what you just described. Thus,
the fix will have to be made to all the callers of vmbus_recvpacket()
and vmbus_recvpacket_raw() to make sure they check the return value,
which most callers are not doing now. Thanks for pointing out this
behavior. I was not aware that the length can be checked by callers to
allocate a bigger buffer.

RE: [PATCH] Drivers: hv: vmbus: Fix variable assignments in hv_ringbuffer_read()

[Haiyang Zhang]

> -----Original Message-----
> From: Andres Beltran <lkmlabelt@gmail.com>
> Sent: Friday, July 24, 2020 7:04 PM
> To: Stephen Hemminger <stephen@networkplumber.org>
> Cc: KY Srinivasan <kys@microsoft.com>; Haiyang Zhang
> <haiyangz@microsoft.com>; Stephen Hemminger <sthemmin@microsoft.com>;
> Wei Liu <wei.liu@kernel.org>; linux-hyperv@vger.kernel.org; linux-
> kernel@vger.kernel.org; Michael Kelley <mikelley@microsoft.com>; Andrea
> Parri <parri.andrea@gmail.com>; Saruhan Karademir
> <skarade@microsoft.com>
> Subject: Re: [PATCH] Drivers: hv: vmbus: Fix variable assignments in
> hv_ringbuffer_read()
> 
> On Fri, Jul 24, 2020 at 1:10 PM Stephen Hemminger
> <stephen@networkplumber.org> wrote:
> > What is the rationale for this change, it may break other code.
> >
> > A common API model in Windows world where this originated
> > is to have a call where caller first
> > makes request and then if the requested buffer is not big enough the
> > caller look at the actual length and allocate a bigger buffer.
> >
> > Did you audit all the users of this API to make sure they aren't doing that.
> >
> 
> The rationale for the change was to solve instances like the one
> @Haiyang Zhang pointed out, especially in hv_utils, which needs
> additional hardening. Unfortunately, there is an instance in
> hv_pci_onchannelcallback() that does what you just described. Thus,
> the fix will have to be made to all the callers of vmbus_recvpacket()
> and vmbus_recvpacket_raw() to make sure they check the return value,
> which most callers are not doing now. Thanks for pointing out this
> behavior. I was not aware that the length can be checked by callers to
> allocate a bigger buffer.

To prevent future coding error, please add code comments for 
hv_ringbuffer_read() to indicate that the buffer_actual_len may be 
nonzero when the function fails, and should not be used to 
determine if the function succeeds or not.

Thanks,
- Haiyang