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*From*: 	Laszlo Ersek

*Subject*: 	[Qemu-devel] [PATCH] e1000: eliminate infinite loops on

out-of-bounds transfer start

*Date*: 	Tue, 19 Jan 2016 14:17:20 +0100

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The start_xmit() and e1000_receive_iov() functions implement DMA transfers

iterating over a set of descriptors that the guest's e1000 driver

prepares:

- the TDLEN and RDLEN registers store the total size of the descriptor

  area,

- while the TDH and RDH registers store the offset (in whole tx / rx

  descriptors) into the area where the transfer is supposed to start.

Each time a descriptor is processed, the TDH and RDH register is bumped

(as appropriate for the transfer direction).

QEMU already contains logic to deal with bogus transfers submitted by the

guest:

- Normally, the transmit case wants to increase TDH from its initial value

  to TDT. (TDT is allowed to be numerically smaller than the initial TDH

  value; wrapping at or above TDLEN bytes to zero is normal.) The failsafe

  that QEMU currently has here is a check against reaching the original

  TDH value again -- a complete wraparound, which should never happen.

- In the receive case RDH is increased from its initial value until

  "total_size" bytes have been received; preferably in a single step, or

  in "s->rxbuf_size" byte steps, if the latter is smaller. However, null

  RX descriptors are skipped without receiving data, while RDH is

  incremented just the same. QEMU tries to prevent an infinite loop

  (processing only null RX descriptors) by detecting whether RDH assumes

  its original value during the loop. (Again, wrapping from RDLEN to 0 is

  normal.)

What both directions miss is that the guest could program TDLEN and RDLEN

so low, and the initial TDH and RDH so high, that these registers will

immediately be truncated to zero, and then never reassume their initial

values in the loop -- a full wraparound will never occur.

The condition that expresses this is:

  xdh_start >= s->mac_reg[XDLEN] / sizeof(desc)

i.e., TDH or RDH start out after the last whole rx or tx descriptor that

fits into the TDLEN or RDLEN sized area.

This condition could be checked before we enter the loops, but

pci_dma_read() / pci_dma_write() knows how to fill in buffers safely for

bogus DMA addresses, so we just extend the existing failsafes with the

above condition.

Cc: "Michael S. Tsirkin" <address@hidden>

Cc: Petr Matousek <address@hidden>

Cc: Stefano Stabellini <address@hidden>

Cc: Prasad Pandit <address@hidden>

Cc: Michael Roth <address@hidden>

Cc: Jason Wang <address@hidden>

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1296044

Signed-off-by: Laszlo Ersek <address@hidden>

Reviewed-by: Jason Wang <address@hidden>

---

Notes:

    Regarding the public posting: we made an honest effort to vet this

    vulnerability, and the impact seems low -- no host side reads/writes,

    "just" a DoS (infinite loop). We decided the patch could be posted

    publicly, for the usual review process. Jason and Prasad checked the

    patch in the internal discussion already, but comments, improvements

    etc. are clearly welcome. The CVE request is underway. Thanks.

 hw/net/e1000.c | 6 ++++--

 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/hw/net/e1000.c b/hw/net/e1000.c

index bec06e9..34d0823 100644

--- a/tools/qemu-xen/hw/net/e1000.c

+++ b/tools/qemu-xen/hw/net/e1000.c

@@ -908,7 +908,8 @@ start_xmit(E1000State *s)

          * bogus values to TDT/TDLEN.

          * there's nothing too intelligent we could do about this.

          */

-        if (s->mac_reg[TDH] == tdh_start) {

+        if (s->mac_reg[TDH] == tdh_start ||

+            tdh_start >= s->mac_reg[TDLEN] / sizeof(desc)) {

             DBGOUT(TXERR, "TDH wraparound @%x, TDT %x, TDLEN %x\n",

                    tdh_start, s->mac_reg[TDT], s->mac_reg[TDLEN]);

             break;

@@ -1165,7 +1166,8 @@ e1000_receive_iov(NetClientState *nc, const struct iovec *iov, int iovcnt)

         if (++s->mac_reg[RDH] * sizeof(desc) >= s->mac_reg[RDLEN])

             s->mac_reg[RDH] = 0;

         /* see comment in start_xmit; same here */

-        if (s->mac_reg[RDH] == rdh_start) {

+        if (s->mac_reg[RDH] == rdh_start ||

+            rdh_start >= s->mac_reg[RDLEN] / sizeof(desc)) {

             DBGOUT(RXERR, "RDH wraparound @%x, RDT %x, RDLEN %x\n",

                    rdh_start, s->mac_reg[RDT], s->mac_reg[RDLEN]);

             set_ics(s, 0, E1000_ICS_RXO);

-- 

1.8.3.1