Assertion failure in fifo8_push_all() through am53c974

[Cheolwoo,Myung]

Hello,

Using hypervisor fuzzer, hyfuzz, I found an assertion failure through am53c974 emulator.

A malicious guest user/process could use this flaw to abort the QEMU process on the host, resulting in a denial of service.

This was found in version 5.2.0 (master, 3f8d1885e4)

```
qemu-system-i386: ../util/fifo8.c:43: fifo8_push_all: Assertion `fifo->num + num <= fifo->capacity' failed.

#0 0x00007ffff0218fb7 in __GI_raise (sig=sig@entry=0x6) at ../sysdeps/unix/sysv/linux/raise.c:51
#1 0x00007ffff021a921 in __GI_abort () at abort.c:79
#2 0x00007ffff020a48a in __assert_fail_base (fmt=0x7ffff0391750 "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n", assertion=assertion@entry=0x555558ed2400 "fifo->num + num <= fifo->capacity", file=file@entry=0x555558ed2380 "../util/fifo8.c", line=line@entry=0x2b, function=function@entry=0x555558ed2560 <__PRETTY_FUNCTION__.16583> "fifo8_push_all")
    at assert.c:92
#3 0x00007ffff020a502 in __GI___assert_fail (assertion=assertion@entry=0x555558ed2400 "fifo->num + num <= fifo->capacity", file=file@entry=0x555558ed2380 "../util/fifo8.c", line=line@entry=0x2b, function=function@entry=0x555558ed2560 <__PRETTY_FUNCTION__.16583> "fifo8_push_all") at assert.c:101
#4 0x00005555587749c4 in fifo8_push_all (fifo=fifo@entry=0x61f000005200, data=data@entry=0x7fff72bfa640 "", num=num@entry=0x24) at ../util/fifo8.c:43
#5 0x00005555572bd13e in esp_do_dma (s=s@entry=0x61f000005088) at ../hw/scsi/esp.c:577
#6 0x00005555572bfc8f in handle_ti (s=0x61f000005088) at ../hw/scsi/esp.c:845
#7 0x00005555572c419c in esp_reg_write (s=0x61f000005088, saddr=saddr@entry=0x3, val=<optimized out>)
    at ../hw/scsi/esp.c:987
#8 0x0000555557bb916a in esp_pci_io_write (opaque=0x61f000004680, addr=<optimized out>, val=<optimized out>, size=<optimized out>) at ../hw/scsi/esp-pci.c:214
#9 0x000055555817ea28 in memory_region_write_accessor (mr=0x61f000004f70, addr=<optimized out>, value=<optimized out>, size=<optimized out>, shift=<optimized out>, mask=<optimized out>, attrs=...) at ../softmmu/memory.c:491
#10 0x0000555558176671 in access_with_adjusted_size (addr=addr@entry=0xc, value=value@entry=0x7fff72bfb2a8, size=size@entry=0x1, access_size_min=<optimized out>, access_size_max=<optimized out>, access_fn=
    0x55555817e7c0 <memory_region_write_accessor>, mr=0x61f000004f70, attrs=...) at ../softmmu/memory.c:552
#11 0x00005555581892aa in memory_region_dispatch_write (mr=mr@entry=0x61f000004f70, addr=<optimized out>, data=<optimized out>, data@entry=0xffffff90, op=op@entry=MO_8, attrs=..., attrs@entry=...) at ../softmmu/memory.c:1508
#12 0x0000555558024b66 in address_space_stb (as=<optimized out>, addr=<optimized out>, val=<optimized out>, attrs=..., result=0x0) at /home/cwmyung/prj/hyfuzz/src/qemu-master/memory_ldst.c.inc:382
#13 0x00007fff9323641c in code_gen_buffer ()
#14 0x0000555557e793bb in cpu_tb_exec (tb_exit=<optimized out>, itb=<optimized out>, cpu=0x62e0000004b4)
    at ../accel/tcg/cpu-exec.c:190
#15 0x0000555557e793bb in cpu_loop_exec_tb (tb_exit=<optimized out>, last_tb=<optimized out>, tb=<optimized out>, cpu=0x62e0000004b4) at ../accel/tcg/cpu-exec.c:673
#16 0x0000555557e793bb in cpu_exec (cpu=cpu@entry=0x62e000000400) at ../accel/tcg/cpu-exec.c:798
#17 0x0000555557f5fc5a in tcg_cpus_exec (cpu=cpu@entry=0x62e000000400) at ../accel/tcg/tcg-accel-ops.c:68
#18 0x00005555582260af in mttcg_cpu_thread_fn (arg=arg@entry=0x62e000000400) at ../accel/tcg/tcg-accel-ops-mttcg.c:70
#19 0x0000555558777b05 in qemu_thread_start (args=<optimized out>) at ../util/qemu-thread-posix.c:521
#20 0x00007ffff05d26db in start_thread (arg=0x7fff72bff700) at pthread_create.c:463
#21 0x00007ffff02fb71f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
```

To reproduce the assertion failure, please run the QEMU with the following command line.

```

$ ./qemu-system-i386 -m 512 -drive file=./hyfuzz.img,index=0,media=disk,format=raw -device am53c974,id=scsi -device scsi-hd,drive=SysDisk -drive id=SysDisk,if=none,file=./disk.img

```

Please let me know if I can provide any further info.

Thank you.

- Cheolwoo, Myung (Seoul National University)

[Mark Cave-Ayland (mark-cave-ayland)]

Thanks for the test case - looks like the problem occurs because a command hasn't been submitted before initiating a DMA transfer, and TC is set to a value higher than the size of cmdfifo. Can you confirm that the following fix works for you?

diff --git a/hw/scsi/esp.c b/hw/scsi/esp.c
index 507ab363bc..0a26ee1dfd 100644
--- a/hw/scsi/esp.c
+++ b/hw/scsi/esp.c
@@ -573,6 +573,7 @@ static void esp_do_dma(ESPState *s)
         cmdlen = fifo8_num_used(&s->cmdfifo);
         trace_esp_do_dma(cmdlen, len);
         if (s->dma_memory_read) {
+ len = MIN(len, fifo8_num_free(&s->cmdfifo));
             s->dma_memory_read(s->dma_opaque, buf, len);
             fifo8_push_all(&s->cmdfifo, buf, len);
         } else {

ATB,

Mark.

[Alexander Bulekov (a1xndr)]

QTest Reproducer:
/*
 * Autogenerated Fuzzer Test Case
 *
 * This work is licensed under the terms of the GNU GPL, version 2 or
 * later. See the COPYING file in the top-level directory.
 */

#include "qemu/osdep.h"

#include "libqos/libqtest.h"

/*
 * cat << EOF | ./qemu-system-i386 -display none -machine accel=qtest, \
 * -m 512M -device am53c974,id=scsi -device scsi-hd,drive=disk0 -drive \
 * id=disk0,if=none,file=null-co://,format=raw -nodefaults -qtest stdio
 * outl 0xcf8 0x80001004
 * outw 0xcfc 0x01
 * outl 0xcf8 0x8000100e
 * outl 0xcfc 0x0e000000
 * outl 0xe40 0x03
 * outl 0xe0b 0x4100
 * outl 0xe0b 0x9000
 * EOF
 */
static void test_fuzz(void)
{
    QTestState *s = qtest_init(
        "-display none , -m 512M -device am53c974,id=scsi -device "
        "scsi-hd,drive=disk0 -drive "
        "id=disk0,if=none,file=null-co://,format=raw -nodefaults");
    qtest_outl(s, 0xcf8, 0x80001004);
    qtest_outw(s, 0xcfc, 0x01);
    qtest_outl(s, 0xcf8, 0x8000100e);
    qtest_outl(s, 0xcfc, 0x0e000000);
    qtest_outl(s, 0xe40, 0x03);
    qtest_outl(s, 0xe0b, 0x4100);
    qtest_outl(s, 0xe0b, 0x9000);
    qtest_quit(s);
}
int main(int argc, char **argv)
{
    const char *arch = qtest_get_arch();

    g_test_init(&argc, &argv, NULL);

    if (strcmp(arch, "i386") == 0) {
        qtest_add_func("fuzz/test_fuzz", test_fuzz);
    }

    return g_test_run();
}

[Cheolwoo,Myung (cwmyung)]

Hello Mark,

I tested on fixed version, and checked that it does not trigger the assertion failure.

Thanks,
- Cheolwoo Myung

[Mark Cave-Ayland (mark-cave-ayland)]

Thank you both for the reproducers. Please see the proposed patchset here:

https://lists.gnu.org/archive/html/qemu-devel/2021-03/msg06063.html

[Mauro Matteo Cascella (mauro-cascella)]

This is fixed now, thank you Mark.

Patchset v4:
https://lists.gnu.org/archive/html/qemu-devel/2021-04/msg01000.html

Upstream commits:
https://git.qemu.org/?p=qemu.git;a=commit;h=0db895361b8a82e1114372ff9f48
https://git.qemu.org/?p=qemu.git;a=commit;h=e392255766071c8cac480da3a9ae
https://git.qemu.org/?p=qemu.git;a=commit;h=e5455b8c1c6170c788f3c0fd577c
https://git.qemu.org/?p=qemu.git;a=commit;h=c5fef9112b15c4b5494791cdf8bb
https://git.qemu.org/?p=qemu.git;a=commit;h=7b320a8e67a534925048cbabfa51
https://git.qemu.org/?p=qemu.git;a=commit;h=99545751734035b76bd372c4e721
https://git.qemu.org/?p=qemu.git;a=commit;h=fa7505c154d4d00ad89a747be2ed
https://git.qemu.org/?p=qemu.git;a=commit;h=fbc6510e3379fa8f8370bf71198f
https://git.qemu.org/?p=qemu.git;a=commit;h=0ebb5fd80589835153a0c2baa1b8
https://git.qemu.org/?p=qemu.git;a=commit;h=324c8809897c8c53ad05c3a7147d
https://git.qemu.org/?p=qemu.git;a=commit;h=607206948cacda4a80be5b976dba

[Mauro Matteo Cascella (mauro-cascella)]

I'm not able to change the status of this bug anymore. It should have been closed as "Fix committed" - QEMU 6.0.0 is not yet released.