[RFC PATCH] rtl8139: fix stack overflow if RxBuf overlaps MMIO

[Qiuhao Li]

Fix Bug 1910826 [1] / OSS-Fuzz Issue 29224 [2].

In rtl8139.c, the function rtl8139_RxBuf_write, which sets the RxBuf
(Receive Buffer Start Address), doesn't check if this buffer overlaps our
MMIO region. So if the guest machine set the transmit mode to loopback, put
the RxBuf at the address of TSD (Transmit Status of Descriptor, MMIO), and
trigger a frame transfer by directly writing to the TSD, an infinite
recursion will occur:

rtl8139_ioport_write (to TSD) -> rtl8139_io_writel -> rtl8139_transmit ->
rtl8139_transmit_one -> rtl8139_transfer_frame -> rtl8139_do_receive ->
rtl8139_write_buffer -> pci_dma_write (to TSD) -> ... ->
rtl8139_ioport_write (to TSD)

This patch adds a check to ensure the maximum possible RxBuf [3] won't
overlap the MMIO region.

P.S. There is a more concise reproducer with comments [4], which may help :)

[1] https://bugs.launchpad.net/bugs/1910826
[2] https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=29224
[3] https://www.cs.usfca.edu/~cruse/cs326f04/RTL8139D_DataSheet.pdf
    5.7 Transmit Configuration Register
[4] https://bugs.launchpad.net/qemu/+bug/1910826/comments/1

Signed-off-by: Qiuhao Li <Qiuhao.Li@outlook.com>
Reported-by: Alexander Bulekov <alxndr@bu.edu>
---
 hw/net/rtl8139.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/hw/net/rtl8139.c b/hw/net/rtl8139.c
index ba5ace1ab7..aa7a38220d 100644
--- a/hw/net/rtl8139.c
+++ b/hw/net/rtl8139.c
@@ -2567,6 +2567,18 @@ static void rtl8139_RxBuf_write(RTL8139State *s, 
uint32_t val)
 {
     DPRINTF("RxBuf write val=0x%08x\n", val);
 
+    PCIDevice *d = PCI_DEVICE(s);
+    uint64_t mmio_addr = d->io_regions[1].addr;
+    uint64_t mmio_size = d->io_regions[1].size;
+
+    #define MAX_Rx_BUFFER_LENGTH (64 * 1024 + 16) /* RxConfig 12-11 = 0b11 */
+
+    if (val < mmio_addr + mmio_size && val + MAX_Rx_BUFFER_LENGTH > mmio_addr) 
{
+        DPRINTF("The receive buffer may overlap with the MMIO region.\n");
+        DPRINTF("mmio_addr: 0x%lx, mmio_size: 0x%lx\n", mmio_addr, mmio_size);
+        return;
+    }
+
     s->RxBuf = val;
 
     /* may need to reset rxring here */
-- 
2.25.1