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[PATCH-for-6.2 v3 1/2] hw/block/fdc: Kludge missing floppy drive to fix
From: |
Philippe Mathieu-Daudé |
Subject: |
[PATCH-for-6.2 v3 1/2] hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196 |
Date: |
Thu, 18 Nov 2021 13:06:34 +0100 |
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.
Cc: qemu-stable@nongnu.org
Fixes: CVE-2021-20196
Reported-by: Gaoning Pan (Ant Security Light-Year Lab) <pgn@zju.edu.cn>
BugLink: https://bugs.launchpad.net/qemu/+bug/1912780
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/338
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
hw/block/fdc.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)
diff --git a/hw/block/fdc.c b/hw/block/fdc.c
index d21b717b7d6..6f94b6a6daa 100644
--- a/hw/block/fdc.c
+++ b/hw/block/fdc.c
@@ -1161,7 +1161,19 @@ 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_new(qemu_get_aio_context(), 0, BLK_PERM_ALL);
+ }
+ return cur_drv;
}
/* Status A register : 0x00 (read-only) */
--
2.31.1
- [PATCH-for-6.2 v3 0/2] hw/block/fdc: Fix CVE-2021-20196, Philippe Mathieu-Daudé, 2021/11/18
- [PATCH-for-6.2 v3 1/2] hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196,
Philippe Mathieu-Daudé <=
- [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Philippe Mathieu-Daudé, 2021/11/18
- Re: [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Hanna Reitz, 2021/11/23
- Re: [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Philippe Mathieu-Daudé, 2021/11/23
- Re: [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Hanna Reitz, 2021/11/23
- Re: [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Philippe Mathieu-Daudé, 2021/11/24
- Re: [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Hanna Reitz, 2021/11/24
- Re: [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Philippe Mathieu-Daudé, 2021/11/24
- Re: [PATCH-for-6.2 v3 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196, Alexander Bulekov, 2021/11/23