The documentation of the "Set palette" mailbox property at
https://github.com/raspberrypi/firmware/wiki/Mailbox-property-interface#set-palette
says it has the form:

    Length: 24..1032
    Value:
        u32: offset: first palette index to set (0-255)
        u32: length: number of palette entries to set (1-256)
        u32...: RGBA palette values (offset to offset+length-1)

We get this wrong in a couple of ways:
 * we aren't checking the offset and length are in range, so the guest
   can make us spin for a long time by providing a large length
 * the bounds check on our loop is wrong: we should iterate through
   'length' palette entries, not 'length - offset' entries

Fix the loop to implement the bounds checks and get the loop
condition right. In the process, make the variables local to
this switch case, rather than function-global, so it's clearer
what type they are when reading the code.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20240723131029.1159908-2-peter.maydell@linaro.org
---
 hw/misc/bcm2835_property.c | 27 ++++++++++++++++-----------
 1 file changed, 16 insertions(+), 11 deletions(-)

diff --git a/hw/misc/bcm2835_property.c b/hw/misc/bcm2835_property.c
index 63de3db6215..e28fdca9846 100644
--- a/hw/misc/bcm2835_property.c
+++ b/hw/misc/bcm2835_property.c
@@ -31,7 +31,6 @@ static void bcm2835_property_mbox_push(BCM2835PropertyState *s, uint32_t value)
     size_t resplen;
     uint32_t tmp;
     int n;
-    uint32_t offset, length, color;
     uint32_t start_num, number, otp_row;
 
     /*
@@ -274,19 +273,25 @@ static void bcm2835_property_mbox_push(BCM2835PropertyState *s, uint32_t value)
             resplen = 16;
             break;
         case RPI_FWREQ_FRAMEBUFFER_SET_PALETTE:
-            offset = ldl_le_phys(&s->dma_as, value + 12);
-            length = ldl_le_phys(&s->dma_as, value + 16);
-            n = 0;
-            while (n < length - offset) {
-                color = ldl_le_phys(&s->dma_as, value + 20 + (n << 2));
-                stl_le_phys(&s->dma_as,
-                            s->fbdev->vcram_base + ((offset + n) << 2), color);
-                n++;
+        {
+            uint32_t offset = ldl_le_phys(&s->dma_as, value + 12);
+            uint32_t length = ldl_le_phys(&s->dma_as, value + 16);
+            int resp;
+
+            if (offset > 255 || length < 1 || length > 256) {
+                resp = 1; /* invalid request */
+            } else {
+                for (uint32_t e = 0; e < length; e++) {
+                    uint32_t color = ldl_le_phys(&s->dma_as, value + 20 + (e << 2));
+                    stl_le_phys(&s->dma_as,
+                                s->fbdev->vcram_base + ((offset + e) << 2), color);
+                }
+                resp = 0;
             }
-            stl_le_phys(&s->dma_as, value + 12, 0);
+            stl_le_phys(&s->dma_as, value + 12, resp);
             resplen = 4;
             break;
-
+        }
         case RPI_FWREQ_FRAMEBUFFER_GET_NUM_DISPLAYS:
             stl_le_phys(&s->dma_as, value + 12, 1);
             resplen = 4;
-- 
2.34.1