From b154791e7b6d4ca5cdcd54443484d97360bd7ad2 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Philippe=20Mathieu-Daud=C3=A9?= Date: Wed, 24 Nov 2021 17:15:34 +0100 Subject: [PATCH 1/3] hw/block/fdc: Extract blk_create_empty_drive() MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit We are going to re-use this code in the next commit, so extract it as a new blk_create_empty_drive() function. Inspired-by: Hanna Reitz Signed-off-by: Philippe Mathieu-Daudé Message-id: 20211124161536.631563-2-philmd@redhat.com Signed-off-by: John Snow --- hw/block/fdc.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/hw/block/fdc.c b/hw/block/fdc.c index fa933cd326..1dbf3f6028 100644 --- a/hw/block/fdc.c +++ b/hw/block/fdc.c @@ -61,6 +61,12 @@ } 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 */ @@ -486,8 +492,7 @@ static void floppy_drive_realize(DeviceState *qdev, Error **errp) } if (!dev->conf.blk) { - /* Anonymous BlockBackend for an empty drive */ - dev->conf.blk = blk_new(qemu_get_aio_context(), 0, BLK_PERM_ALL); + dev->conf.blk = blk_create_empty_drive(); ret = blk_attach_dev(dev->conf.blk, qdev); assert(ret == 0); From 1ab95af033a419e7a64e2d58e67dd96b20af5233 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Philippe=20Mathieu-Daud=C3=A9?= Date: Wed, 24 Nov 2021 17:15:35 +0100 Subject: [PATCH 2/3] hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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) Reviewed-by: Darren Kenny Reviewed-by: Hanna Reitz Signed-off-by: Philippe Mathieu-Daudé Message-id: 20211124161536.631563-3-philmd@redhat.com BugLink: https://bugs.launchpad.net/qemu/+bug/1912780 Resolves: https://gitlab.com/qemu-project/qemu/-/issues/338 Reviewed-by: Darren Kenny Reviewed-by: Hanna Reitz Signed-off-by: Philippe Mathieu-Daudé Signed-off-by: John Snow --- 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 1dbf3f6028..21d18ac2e3 100644 --- a/hw/block/fdc.c +++ b/hw/block/fdc.c @@ -1166,7 +1166,19 @@ 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) */ From cc20926e9b8077bff6813efc8dcdeae90d1a3b10 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Philippe=20Mathieu-Daud=C3=A9?= Date: Wed, 24 Nov 2021 17:15:36 +0100 Subject: [PATCH 3/3] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Without the previous commit, when running 'make check-qtest-i386' with QEMU configured with '--enable-sanitizers' we get: AddressSanitizer:DEADLYSIGNAL ================================================================= ==287878==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000344 ==287878==The signal is caused by a WRITE memory access. ==287878==Hint: address points to the zero page. #0 0x564b2e5bac27 in blk_inc_in_flight block/block-backend.c:1346:5 #1 0x564b2e5bb228 in blk_pwritev_part block/block-backend.c:1317:5 #2 0x564b2e5bcd57 in blk_pwrite block/block-backend.c:1498:11 #3 0x564b2ca1cdd3 in fdctrl_write_data hw/block/fdc.c:2221:17 #4 0x564b2ca1b2f7 in fdctrl_write hw/block/fdc.c:829:9 #5 0x564b2dc49503 in portio_write softmmu/ioport.c:201:9 Add the reproducer for CVE-2021-20196. Suggested-by: Alexander Bulekov Reviewed-by: Darren Kenny Signed-off-by: Philippe Mathieu-Daudé Message-id: 20211124161536.631563-4-philmd@redhat.com Signed-off-by: John Snow --- tests/qtest/fdc-test.c | 38 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 38 insertions(+) diff --git a/tests/qtest/fdc-test.c b/tests/qtest/fdc-test.c index 26b69f7c5c..8f6eee84a4 100644 --- a/tests/qtest/fdc-test.c +++ b/tests/qtest/fdc-test.c @@ -32,6 +32,9 @@ /* TODO actually test the results and get rid of this */ #define qmp_discard_response(...) qobject_unref(qmp(__VA_ARGS__)) +#define DRIVE_FLOPPY_BLANK \ + "-drive if=floppy,file=null-co://,file.read-zeroes=on,format=raw,size=1440k" + #define TEST_IMAGE_SIZE 1440 * 1024 #define FLOPPY_BASE 0x3f0 @@ -546,6 +549,40 @@ static void fuzz_registers(void) } } +static bool qtest_check_clang_sanitizer(void) +{ +#if defined(__SANITIZE_ADDRESS__) || __has_feature(address_sanitizer) + return true; +#else + g_test_skip("QEMU not configured using --enable-sanitizers"); + return false; +#endif +} +static void test_cve_2021_20196(void) +{ + QTestState *s; + + if (!qtest_check_clang_sanitizer()) { + return; + } + + s = qtest_initf("-nographic -m 32M -nodefaults " DRIVE_FLOPPY_BLANK); + + qtest_outw(s, 0x3f4, 0x0500); + qtest_outb(s, 0x3f5, 0x00); + qtest_outb(s, 0x3f5, 0x00); + qtest_outw(s, 0x3f4, 0x0000); + qtest_outb(s, 0x3f5, 0x00); + qtest_outw(s, 0x3f1, 0x0400); + qtest_outw(s, 0x3f4, 0x0000); + qtest_outw(s, 0x3f4, 0x0000); + qtest_outb(s, 0x3f5, 0x00); + qtest_outb(s, 0x3f5, 0x01); + qtest_outw(s, 0x3f1, 0x0500); + qtest_outb(s, 0x3f5, 0x00); + qtest_quit(s); +} + int main(int argc, char **argv) { int fd; @@ -576,6 +613,7 @@ int main(int argc, char **argv) qtest_add_func("/fdc/read_no_dma_18", test_read_no_dma_18); qtest_add_func("/fdc/read_no_dma_19", test_read_no_dma_19); qtest_add_func("/fdc/fuzz-registers", fuzz_registers); + qtest_add_func("/fdc/fuzz/cve_2021_20196", test_cve_2021_20196); ret = g_test_run();