Re: [RFC PATCH v2] hw/i386/vmport: Allow QTest use without crashing

[Paolo Bonzini <pbonzini@redhat.com>]

On 18/05/20 12:31, Philippe Mathieu-Daudé wrote:
> Trying libFuzzer on the vmport device, we get:
> 
>   AddressSanitizer:DEADLYSIGNAL
>   =================================================================
>   ==29476==ERROR: AddressSanitizer: SEGV on unknown address 0x000000008840 (pc 0x56448bec4d79 bp 0x7ffeec9741b0 sp 0x7ffeec9740e0 T0)
>   ==29476==The signal is caused by a READ memory access.
>     #0 0x56448bec4d78 in vmport_ioport_read (qemu-fuzz-i386+0x1260d78)
>     #1 0x56448bb5f175 in memory_region_read_accessor (qemu-fuzz-i386+0xefb175)
>     #2 0x56448bb30c13 in access_with_adjusted_size (qemu-fuzz-i386+0xeccc13)
>     #3 0x56448bb2ea27 in memory_region_dispatch_read1 (qemu-fuzz-i386+0xecaa27)
>     #4 0x56448bb2e443 in memory_region_dispatch_read (qemu-fuzz-i386+0xeca443)
>     #5 0x56448b961ab1 in flatview_read_continue (qemu-fuzz-i386+0xcfdab1)
>     #6 0x56448b96336d in flatview_read (qemu-fuzz-i386+0xcff36d)
>     #7 0x56448b962ec4 in address_space_read_full (qemu-fuzz-i386+0xcfeec4)
> 
> This is easily reproducible using:
> 
>   $ echo inb 0x5658 | qemu-system-i386 -M isapc,accel=qtest -qtest stdio
>   [I 1589796572.009763] OPENED
>   [R +0.008069] inb 0x5658
>   Segmentation fault (core dumped)
> 
>   $ coredumpctl gdb -q
>   Program terminated with signal SIGSEGV, Segmentation fault.
>   #0  0x00005605b54d0f21 in vmport_ioport_read (opaque=0x5605b7531ce0, addr=0, size=4) at hw/i386/vmport.c:77
>   77          eax = env->regs[R_EAX];
>   (gdb) p cpu
>   $1 = (X86CPU *) 0x0
>   (gdb) bt
>   #0  0x00005605b54d0f21 in vmport_ioport_read (opaque=0x5605b7531ce0, addr=0, size=4) at hw/i386/vmport.c:77
>   #1  0x00005605b53db114 in memory_region_read_accessor (mr=0x5605b7531d80, addr=0, value=0x7ffc9d261a30, size=4, shift=0, mask=4294967295, attrs=...) at memory.c:434
>   #2  0x00005605b53db5d4 in access_with_adjusted_size (addr=0, value=0x7ffc9d261a30, size=1, access_size_min=4, access_size_max=4, access_fn=
>       0x5605b53db0d2 <memory_region_read_accessor>, mr=0x5605b7531d80, attrs=...) at memory.c:544
>   #3  0x00005605b53de156 in memory_region_dispatch_read1 (mr=0x5605b7531d80, addr=0, pval=0x7ffc9d261a30, size=1, attrs=...) at memory.c:1396
>   #4  0x00005605b53de228 in memory_region_dispatch_read (mr=0x5605b7531d80, addr=0, pval=0x7ffc9d261a30, op=MO_8, attrs=...) at memory.c:1424
>   #5  0x00005605b537c80a in flatview_read_continue (fv=0x5605b7650290, addr=22104, attrs=..., ptr=0x7ffc9d261b4b, len=1, addr1=0, l=1, mr=0x5605b7531d80) at exec.c:3200
>   #6  0x00005605b537c95d in flatview_read (fv=0x5605b7650290, addr=22104, attrs=..., buf=0x7ffc9d261b4b, len=1) at exec.c:3239
>   #7  0x00005605b537c9e6 in address_space_read_full (as=0x5605b5f74ac0 <address_space_io>, addr=22104, attrs=..., buf=0x7ffc9d261b4b, len=1) at exec.c:3252
>   #8  0x00005605b53d5a5d in address_space_read (len=1, buf=0x7ffc9d261b4b, attrs=..., addr=22104, as=0x5605b5f74ac0 <address_space_io>) at include/exec/memory.h:2401
>   #9  0x00005605b53d5a5d in cpu_inb (addr=22104) at ioport.c:88
> 
> X86CPU is NULL because QTest accelerator does not use CPU.
> Fix by returning default values when QTest accelerator is used.
> 
> Reported-by: Clang AddressSanitizer
> Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
> ---
> v2: Removed Red Hat authorship ¯\_(ツ)_/¯
> 
>  hw/i386/vmport.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/i386/vmport.c b/hw/i386/vmport.c
> index 1aaaab691a..4ec95f8778 100644
> --- a/hw/i386/vmport.c
> +++ b/hw/i386/vmport.c
> @@ -24,6 +24,7 @@
>  #include "qemu/osdep.h"
>  #include "hw/isa/isa.h"
>  #include "sysemu/hw_accel.h"
> +#include "sysemu/qtest.h"
>  #include "qemu/log.h"
>  #include "vmport.h"
>  #include "cpu.h"
> @@ -64,10 +65,14 @@ static uint64_t vmport_ioport_read(void *opaque, hwaddr addr,
>      VMPortState *s = opaque;
>      CPUState *cs = current_cpu;
>      X86CPU *cpu = X86_CPU(cs);
> -    CPUX86State *env = &cpu->env;
> +    CPUX86State *env;
>      unsigned char command;
>      uint32_t eax;
>  
> +    if (qtest_enabled()) {
> +        return -1;
> +    }
> +    env = &cpu->env;
>      cpu_synchronize_state(cs);
>  
>      eax = env->regs[R_EAX];
> @@ -90,6 +95,9 @@ static void vmport_ioport_write(void *opaque, hwaddr addr,
>  {
>      X86CPU *cpu = X86_CPU(current_cpu);
>  
> +    if (qtest_enabled()) {
> +        return;
> +    }
>      cpu->env.regs[R_EAX] = vmport_ioport_read(opaque, addr, 4);
>  }
>  
> @@ -97,6 +105,9 @@ static uint32_t vmport_cmd_get_version(void *opaque, uint32_t addr)
>  {
>      X86CPU *cpu = X86_CPU(current_cpu);
>  
> +    if (qtest_enabled()) {
> +        return -1;
> +    }
>      cpu->env.regs[R_EBX] = VMPORT_MAGIC;
>      return 6;
>  }
> @@ -105,6 +116,9 @@ static uint32_t vmport_cmd_ram_size(void *opaque, uint32_t addr)
>  {
>      X86CPU *cpu = X86_CPU(current_cpu);
>  
> +    if (qtest_enabled()) {
> +        return -1;
> +    }
>      cpu->env.regs[R_EBX] = 0x1177;
>      return ram_size;
>  }
> 

Queued, thanks.

Paolo