[Qemu-devel] [PATCH v2 0/2] rtl8139: Fix buffer overflow in standard mode

[Qemu-devel] [PATCH v2 0/2] rtl8139: Fix buffer overflow in standard mode

[Vladislav Yasevich]

When rtl8139 card is running in standard mode, it is very easy
to overlflow and the receive buffer and get into a siutation
where all packets are dropped.  Simply reproduction case is
to ping the guest from the host with 6500 byte packets.  

There are actually 2 problems here.
 1) When the rtl8129 buffer is overflow, the card emulation
    returns the size of the packet back to queue transmission.
    This signals successful reception even though the packet
    has been dropped.  The proper solution is to return 0, so
    that the packet is re-queued and will be resubmitted later.

 2) When packets are sized such that the fragments end up completely
    filling the receive buffer without overflow, the device thinks
    that the buffer is actually empty (instead of full).  This causes
    next packet to over-write the existing packets.  With the above
    ping reproducer, ever ICMP packet fills the buffer and thus keeps
    overwriting the previous packet and never waking up the guest.
    The solution here is track the number of unread bytes separately
    so we would know if we have anything in buffer to read or not.

V2: instead of tracking buffer_full condition, changed the code, as
    suggested by Stefan Hajnoczi, to track the number of unread bytes
    instead.  We initialize it to 0 at the start, adjust it on every
    receive from the network and read from the guest and can set
    the number of unread of bytes to full buffer size when the buffer
    full.

Vladislav Yasevich (2):
  rtl8139: Do not consume the packet during overflow in standard mode.
  rtl8139: correctly track full receive buffer in standard mode

 hw/net/rtl8139.c | 44 +++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 39 insertions(+), 5 deletions(-)

-- 
1.9.3

[Qemu-devel] [PATCH 1/2] rtl8139: Do not consume the packet during overflow in standard mode.

[Vladislav Yasevich]

When operation in standard mode, we currently return the size
of packet during buffer overflow.  This consumes the overflow
packet.  Return 0 instead so we can re-process the overflow packet
when we have room.

This fixes issues with lost/dropped fragments of large messages.

Signed-off-by: Vladislav Yasevich <vyasevic@redhat.com>
---
 hw/net/rtl8139.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/net/rtl8139.c b/hw/net/rtl8139.c
index edbb61c..359e001 100644
--- a/hw/net/rtl8139.c
+++ b/hw/net/rtl8139.c
@@ -1157,7 +1157,7 @@ static ssize_t rtl8139_do_receive(NetClientState *nc, const uint8_t *buf, size_t
             s->IntrStatus |= RxOverflow;
             ++s->RxMissed;
             rtl8139_update_irq(s);
-            return size_;
+            return 0;
         }
 
         packet_header |= RxStatusOK;
-- 
1.9.3

[Qemu-devel] [PATCH 2/2] rtl8139: correctly track full receive buffer in standard mode

[Vladislav Yasevich]

In standard operation mode, when the receive ring buffer
is full, the buffer actually appears empty to the driver since
the RxBufAddr (the location we wirte new data to) and RxBufPtr
(the location guest would stat reading from) are the same.
As a result, the call to rtl8139_RxBufferEmpty ends up
returning true indicating that the receive buffer is empty.
This would result in the next packet overwriting the recevie buffer
again and stalling receive operations.

This patch tracks the number of unread bytes in the rxbuffer
using an unused C+ register.  On every read and write, the
number is adjsted and the special case of a full buffer is also
trapped.

The C+ register trick is used to simplify migration and not require
a new machine type.  This register is not used in regular mode
and C+ mode doesn't have the same issue.

Signed-off-by: Vladislav Yasevich <vyasevic@redhat.com>
---
 hw/net/rtl8139.c | 40 ++++++++++++++++++++++++++++++++++++----
 1 file changed, 36 insertions(+), 4 deletions(-)

diff --git a/hw/net/rtl8139.c b/hw/net/rtl8139.c
index 359e001..381dedb 100644
--- a/hw/net/rtl8139.c
+++ b/hw/net/rtl8139.c
@@ -794,6 +794,27 @@ static bool rtl8139_cp_rx_valid(RTL8139State *s)
     return !(s->RxRingAddrLO == 0 && s->RxRingAddrHI == 0);
 }
 
+static uint32_t rtl8139_unread_bytes(RTL8139State *s)
+{
+    return MOD2(s->RxBufferSize + s->RxBufPtr - s->RxBufAddr, s->RxBufferSize);
+}
+
+static void rtl8139_set_unread_bytes(RTL8139State *s, uint32_t unread)
+{
+    /* In standard mode, C+ RxDesc isn't used.  Reuse it
+     * to store the number of unread bytes.
+     */
+    s->currCPlusRxDesc = unread;
+}
+
+static uint32_t rtl8139_get_unread_bytes(RTL8139State *s)
+{
+    /* In standard mode, C+ RxDesc isn't used.  Reuse it
+     * to store the number of unread bytes in linear buffer.
+     */
+    return s->currCPlusRxDesc;
+}
+
 static int rtl8139_can_receive(NetClientState *nc)
 {
     RTL8139State *s = qemu_get_nic_opaque(nc);
@@ -810,8 +831,7 @@ static int rtl8139_can_receive(NetClientState *nc)
            This is a hack to work around slirp deficiencies anyway.  */
         return 1;
     } else {
-        avail = MOD2(s->RxBufferSize + s->RxBufPtr - s->RxBufAddr,
-                     s->RxBufferSize);
+        avail = rtl8139_get_unread_bytes(s);
         return (avail == 0 || avail >= 1514 || (s->IntrMask & RxOverflow));
     }
 }
@@ -1144,7 +1164,7 @@ static ssize_t rtl8139_do_receive(NetClientState *nc, const uint8_t *buf, size_t
         DPRINTF("in ring Rx mode ================\n");
 
         /* begin ring receiver mode */
-        int avail = MOD2(s->RxBufferSize + s->RxBufPtr - s->RxBufAddr, s->RxBufferSize);
+        int avail = rtl8139_get_unread_bytes(s);
 
         /* if receiver buffer is empty then avail == 0 */
 
@@ -1178,6 +1198,14 @@ static ssize_t rtl8139_do_receive(NetClientState *nc, const uint8_t *buf, size_t
         /* correct buffer write pointer */
         s->RxBufAddr = MOD2((s->RxBufAddr + 3) & ~0x3, s->RxBufferSize);
 
+        /* Catch buffer full condition.  Without this, we end up
+        * assuming that buffer is empty.
+        */
+        rtl8139_set_unread_bytes(s,
+                                 (s->RxBufAddr == s->RxBufPtr) ?
+                                 s->RxBufferSize :
+                                 rtl8139_unread_bytes(s));
+
         /* now we can signal we have received something */
 
         DPRINTF("received: rx buffer length %d head 0x%04x read 0x%04x\n",
@@ -1204,6 +1232,7 @@ static void rtl8139_reset_rxring(RTL8139State *s, uint32_t bufferSize)
     s->RxBufferSize = bufferSize;
     s->RxBufPtr  = 0;
     s->RxBufAddr = 0;
+    rtl8139_set_unread_bytes(s, 0);
 }
 
 static void rtl8139_reset(DeviceState *d)
@@ -1412,7 +1441,7 @@ static void rtl8139_ChipCmd_write(RTL8139State *s, uint32_t val)
 
 static int rtl8139_RxBufferEmpty(RTL8139State *s)
 {
-    int unread = MOD2(s->RxBufferSize + s->RxBufAddr - s->RxBufPtr, s->RxBufferSize);
+    int unread = rtl8139_get_unread_bytes(s);
 
     if (unread != 0)
     {
@@ -2601,6 +2630,9 @@ static void rtl8139_RxBufPtr_write(RTL8139State *s, uint32_t val)
     /* this value is off by 16 */
     s->RxBufPtr = MOD2(val + 0x10, s->RxBufferSize);
 
+    /* We just read data, update unread bytes */
+    rtl8139_set_unread_bytes(s, rtl8139_unread_bytes(s));
+
     /* more buffer space may be available so try to receive */
     qemu_flush_queued_packets(qemu_get_queue(s->nic));
 
-- 
1.9.3

Re: [Qemu-devel] [PATCH v2 0/2] rtl8139: Fix buffer overflow in standard mode

[Vlad Yasevich]

On 08/26/2015 03:51 PM, Vladislav Yasevich wrote:
> When rtl8139 card is running in standard mode, it is very easy
> to overlflow and the receive buffer and get into a siutation
> where all packets are dropped.  Simply reproduction case is
> to ping the guest from the host with 6500 byte packets.  
> 
> There are actually 2 problems here.
>  1) When the rtl8129 buffer is overflow, the card emulation
>     returns the size of the packet back to queue transmission.
>     This signals successful reception even though the packet
>     has been dropped.  The proper solution is to return 0, so
>     that the packet is re-queued and will be resubmitted later.
> 
>  2) When packets are sized such that the fragments end up completely
>     filling the receive buffer without overflow, the device thinks
>     that the buffer is actually empty (instead of full).  This causes
>     next packet to over-write the existing packets.  With the above
>     ping reproducer, ever ICMP packet fills the buffer and thus keeps
>     overwriting the previous packet and never waking up the guest.
>     The solution here is track the number of unread bytes separately
>     so we would know if we have anything in buffer to read or not.
> 
> V2: instead of tracking buffer_full condition, changed the code, as
>     suggested by Stefan Hajnoczi, to track the number of unread bytes
>     instead.  We initialize it to 0 at the start, adjust it on every
>     receive from the network and read from the guest and can set
>     the number of unread of bytes to full buffer size when the buffer
>     full.
> 
> Vladislav Yasevich (2):
>   rtl8139: Do not consume the packet during overflow in standard mode.
>   rtl8139: correctly track full receive buffer in standard mode
> 

Self nack.  The second patch is wrong.  Will resubmit when fixed.

-vlad

>  hw/net/rtl8139.c | 44 +++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 39 insertions(+), 5 deletions(-)
>