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From 94608c59045791dfd35102bc59b792e96f2cfa30 Mon Sep 17 00:00:00 2001
From: Vivek Kumbhar <vkumbhar@mvista.com>
Date: Tue, 29 Nov 2022 15:57:13 +0530
Subject: [PATCH] CVE-2021-20196
Upstream-Status: Backport [https://gitlab.com/qemu-project/qemu/-/commit/1ab95af033a419e7a64e2d58e67dd96b20af5233]
CVE: CVE-2021-20196
Signed-off-by: Vivek Kumbhar <vkumbhar@mvista.com>
hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196
Guest might select another drive on the bus by setting the
DRIVE_SEL bit of the DIGITAL OUTPUT REGISTER (DOR).
The current controller model doesn't expect a BlockBackend
to be NULL. A simple way to fix CVE-2021-20196 is to create
an empty BlockBackend when it is missing. All further
accesses will be safely handled, and the controller state
machines keep behaving correctly.
---
hw/block/fdc.c | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)
diff --git a/hw/block/fdc.c b/hw/block/fdc.c
index ac5d31e8..e128e975 100644
--- a/hw/block/fdc.c
+++ b/hw/block/fdc.c
@@ -58,6 +58,11 @@
} \
} while (0)
+/* Anonymous BlockBackend for empty drive */
+static BlockBackend *blk_create_empty_drive(void)
+{
+ return blk_new(qemu_get_aio_context(), 0, BLK_PERM_ALL);
+}
/********************************************************/
/* qdev floppy bus */
@@ -1356,7 +1361,19 @@ static FDrive *get_drv(FDCtrl *fdctrl, int unit)
static FDrive *get_cur_drv(FDCtrl *fdctrl)
{
- return get_drv(fdctrl, fdctrl->cur_drv);
+ FDrive *cur_drv = get_drv(fdctrl, fdctrl->cur_drv);
+
+ if (!cur_drv->blk) {
+ /*
+ * Kludge: empty drive line selected. Create an anonymous
+ * BlockBackend to avoid NULL deref with various BlockBackend
+ * API calls within this model (CVE-2021-20196).
+ * Due to the controller QOM model limitations, we don't
+ * attach the created to the controller device.
+ */
+ cur_drv->blk = blk_create_empty_drive();
+ }
+ return cur_drv;
}
/* Status A register : 0x00 (read-only) */
--
2.25.1
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