From: Laszlo Ersek Date: Tue, 19 Jan 2016 14:17:20 +0100 Subject: [PATCH] e1000: eliminate infinite loops on out-of-bounds transfer start 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. This is CVE-2016-1981. Cc: "Michael S. Tsirkin" Cc: Petr Matousek Cc: Stefano Stabellini Cc: Prasad Pandit Cc: Michael Roth Cc: Jason Wang Cc: qemu-stable@nongnu.org RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1296044 Signed-off-by: Laszlo Ersek Reviewed-by: Jason Wang Signed-off-by: Jason Wang (cherry picked from commit dd793a74882477ca38d49e191110c17dfee51dcc) --- 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/hw/net/e1000.c +++ b/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);