[PATCH-for-6.2 v2 0/2] hw/block/fdc: Fix CVE-2021-20196

[PATCH-for-6.2 v2 0/2] hw/block/fdc: Fix CVE-2021-20196

[Philippe Mathieu-Daudé]

I'm not sure what happened to v1 from Prasad, so since we are
at rc2 I took a simpler approach to fix this CVE: create an
empty drive to satisfy the BlockBackend API calls.

Added Alexander's reproducer along.

v1: https://lore.kernel.org/qemu-devel/20210123100345.642933-1-ppandit@redhat.com/

Alexander Bulekov (1):
  tests/qtest/fdc-test: Add a regression test for CVE-2021-20196

Philippe Mathieu-Daudé (1):
  hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196

 hw/block/fdc.c         | 14 +++++++++++++-
 tests/qtest/fdc-test.c | 21 +++++++++++++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)

-- 
2.31.1

[PATCH-for-6.2 v2 1/2] hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196

[Philippe Mathieu-Daudé]

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.

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
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 fa933cd3263..eab17e946d6 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.
+         */
+        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 v2 2/2] tests/qtest/fdc-test: Add a regression test for CVE-2021-20196

[Philippe Mathieu-Daudé]

From: Alexander Bulekov <alxndr@bu.edu>

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

Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
Message-Id: <20210319050906.14875-2-alxndr@bu.edu>
[PMD: Rebased, use global test_image]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 tests/qtest/fdc-test.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/tests/qtest/fdc-test.c b/tests/qtest/fdc-test.c
index 26b69f7c5cd..dfca84c7b96 100644
--- a/tests/qtest/fdc-test.c
+++ b/tests/qtest/fdc-test.c
@@ -546,6 +546,26 @@ static void fuzz_registers(void)
     }
 }
 
+static void test_cve_2021_20196(void)
+{
+    QTestState *s;
+
+    s = qtest_initf("-nographic -m 32M -nodefaults "
+                    "-drive file=%s,format=raw,if=floppy", test_image);
+    qtest_outw(s, 0x3f2, 0x0004);
+    qtest_outw(s, 0x3f4, 0x0200);
+    qtest_outw(s, 0x3f4, 0x0000);
+    qtest_outw(s, 0x3f4, 0x0000);
+    qtest_outw(s, 0x3f4, 0x0000);
+    qtest_outw(s, 0x3f4, 0x0000);
+    qtest_outw(s, 0x3f4, 0x0000);
+    qtest_outw(s, 0x3f4, 0x0000);
+    qtest_outw(s, 0x3f4, 0x0000);
+    qtest_outw(s, 0x3f4, 0x0000);
+    qtest_outw(s, 0x3f2, 0x0001);
+    qtest_quit(s);
+}
+
 int main(int argc, char **argv)
 {
     int fd;
@@ -576,6 +596,7 @@ 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();
 
-- 
2.31.1

Re: [PATCH-for-6.2 v2 1/2] hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196

[Darren Kenny]

Hi Philippe,

A small nit below, but otherwise looks good.

On Thursday, 2021-11-18 at 00:24:21 +01, Philippe Mathieu-Daudé wrote:
> 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.
>
> 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
> 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 fa933cd3263..eab17e946d6 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.
>
The created .... to the controller

Something is missing here, maybe 'device'?

Thanks,

Darren.

> +         */
> +        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

Re: [PATCH-for-6.2 v2 0/2] hw/block/fdc: Fix CVE-2021-20196

[Darren Kenny]

Hi Philippe,

Apart from a nit on patch 1, all looks good, so:

Reviewed-by: Darren Kenny <darren.kenny@oracle.com>

Thanks,

Darren.

On Thursday, 2021-11-18 at 00:24:20 +01, Philippe Mathieu-Daudé wrote:
> I'm not sure what happened to v1 from Prasad, so since we are
> at rc2 I took a simpler approach to fix this CVE: create an
> empty drive to satisfy the BlockBackend API calls.
>
> Added Alexander's reproducer along.
>
> v1: https://lore.kernel.org/qemu-devel/20210123100345.642933-1-ppandit@redhat.com/
>
> Alexander Bulekov (1):
>   tests/qtest/fdc-test: Add a regression test for CVE-2021-20196
>
> Philippe Mathieu-Daudé (1):
>   hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196
>
>  hw/block/fdc.c         | 14 +++++++++++++-
>  tests/qtest/fdc-test.c | 21 +++++++++++++++++++++
>  2 files changed, 34 insertions(+), 1 deletion(-)
>
> -- 
> 2.31.1

Re: [PATCH-for-6.2 v2 1/2] hw/block/fdc: Kludge missing floppy drive to fix CVE-2021-20196

[Philippe Mathieu-Daudé]

On 11/18/21 11:44, Darren Kenny wrote:
> Hi Philippe,
> 
> A small nit below, but otherwise looks good.
> 
> On Thursday, 2021-11-18 at 00:24:21 +01, Philippe Mathieu-Daudé wrote:
>> 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.
>>
>> 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
>> 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 fa933cd3263..eab17e946d6 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.
>>
> The created .... to the controller
> 
> Something is missing here, maybe 'device'?

OK. I forgot to add Cc: qemu-stable@nongnu.org so will respin.

Thanks,

Phil.