Commit f7f742beb6 for qemu.org

commit f7f742beb685f66e48fbdbf373b054acfe210844
Author: Alex Bennée <alex.bennee@linaro.org>
Date:   Mon Jul 13 12:34:17 2026 +0100

    hw/dma: don't allow weird transfer lengths for bcm2835

    The datasheet doesn't explicitly say that TXFR_LEN has to be word
    aligned but the fact there is a DMA_D_WIDTH flag to select between 32
    bit and 128 bit strongly implies that is how it works. The downstream
    rpi kernel also goes to efforts to not write sub-4 byte lengths so
    lets:

      - fail when mis-programmed and report GUEST_ERROR
      - catch setting D_WIDTH for 128 bit and report UNIMP

    Yodel did some digging into the specs (see discussion link):

    {A} AMBA AXI Protocol Version: 2.0 Specification
        https://documentation-service.arm.com/static/64256e84314e245d086bc88f

    {B} BCM2835 ARM Peripherals
        https://datasheets.raspberrypi.com/bcm2835/bcm2835-peripherals.pdf

    [1] {A} (p. 10-2)
    [2] {B} (p. 51)
    [3] {A} (p. 14-5)
    [4] {A} (p. 4-3)
    [5] {A} (p. 9-4)
    [6] {B} (p. 53)

    However was unable to come up with an unambiguous conclusion without
    testing on the real hardware. So in the absence of certainty and for
    the sake of addressing the DoS I suggest we merge as is for now.

    Link: https://lore.kernel.org/all/20251111105429.3993300-1-alex.bennee@linaro.org/
    Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3201
    Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
    Message-id: 20260710131500.2323848-1-alex.bennee@linaro.org
    Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
    Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

diff --git a/hw/dma/bcm2835_dma.c b/hw/dma/bcm2835_dma.c
index d07459de2d..b842aef4b7 100644
--- a/hw/dma/bcm2835_dma.c
+++ b/hw/dma/bcm2835_dma.c
@@ -86,6 +86,23 @@ static void bcm2835_dma_update(BCM2835DMAState *s, unsigned c)
         }
         xlen_td = xlen;

+        if (ch->ti & BCM2708_DMA_D_WIDTH) {
+            qemu_log_mask(LOG_UNIMP, "%s: 128bit transfers not yet supported", __func__);
+            ch->cs |= BCM2708_DMA_ERR;
+            break;
+        }
+
+        /*
+         * Datasheet implies 32bit or 128bit transfers only
+         *
+         * TODO: test on real HW and report back.
+         */
+        if (xlen & 0x3) {
+            qemu_log_mask(LOG_GUEST_ERROR, "%s: bad transfer size\n", __func__);
+            ch->cs |= BCM2708_DMA_ERR;
+            break;
+        }
+
         while (ylen != 0) {
             /* Normal transfer mode */
             while (xlen != 0) {