From 94608c59045791dfd35102bc59b792e96f2cfa30 Mon Sep 17 00:00:00 2001 From: Vivek Kumbhar 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 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