[PATCH 2/2] hw/sd/sdcard: Do not allow invalid SD card sizes

["Philippe Mathieu-Daudé" <f4bug@amsat.org>]

QEMU allows to create SD card with unrealistic sizes. This could work,
but some guests (at least Linux) consider sizes that are not a power
of 2 as a firmware bug and fix the card size to the next power of 2.

Before CVE-2020-13253 fix, this would allow OOB read/write accesses
past the image size end.

CVE-2020-13253 has been fixed as:

    Read command is rejected if BLOCK_LEN_ERROR or ADDRESS_ERROR
    occurred and no data transfer is performed.

    Write command is rejected if BLOCK_LEN_ERROR or ADDRESS_ERROR
    occurred and no data transfer is performed.

    WP_VIOLATION errors are not modified: the error bit is set, we
    stay in receive-data state, wait for a stop command. All further
    data transfer is ignored. See the check on sd->card_status at the
    beginning of sd_read_data() and sd_write_data().

While this is the correct behavior, in case QEMU create smaller SD
cards, guests still try to access past the image size end, and QEMU
considers this is an invalid address, thus "all further data transfer
is ignored". This is wrong and make the guest looping until
eventually timeouts.

Fix by not allowing invalid SD card sizes.  Suggesting the expected
size as a hint:

  $ qemu-system-arm -M orangepi-pc -drive file=rootfs.ext2,if=sd,format=raw
  qemu-system-arm: Invalid SD card size: 60 MiB (expecting at least 64 MiB)

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
 hw/sd/sd.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/hw/sd/sd.c b/hw/sd/sd.c
index cb81487e5c..c45106b78e 100644
--- a/hw/sd/sd.c
+++ b/hw/sd/sd.c
@@ -32,6 +32,7 @@
 
 #include "qemu/osdep.h"
 #include "qemu/units.h"
+#include "qemu/cutils.h"
 #include "hw/irq.h"
 #include "hw/registerfields.h"
 #include "sysemu/block-backend.h"
@@ -2130,11 +2131,26 @@ static void sd_realize(DeviceState *dev, Error **errp)
     }
 
     if (sd->blk) {
+        int64_t blk_size;
+
         if (blk_is_read_only(sd->blk)) {
             error_setg(errp, "Cannot use read-only drive as SD card");
             return;
         }
 
+        blk_size = blk_getlength(sd->blk);
+        if (blk_size > 0 && !is_power_of_2(blk_size)) {
+            int64_t blk_size_aligned = pow2ceil(blk_size);
+            char *blk_size_str = size_to_str(blk_size);
+            char *blk_size_aligned_str = size_to_str(blk_size_aligned);
+
+            error_setg(errp, "Invalid SD card size: %s (expecting at least %s)",
+                       blk_size_str, blk_size_aligned_str);
+            g_free(blk_size_str);
+            g_free(blk_size_aligned_str);
+            return;
+        }
+
         ret = blk_set_perm(sd->blk, BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE,
                            BLK_PERM_ALL, errp);
         if (ret < 0) {
-- 
2.21.3