Re: [Qemu-devel] sparc-softmmu uninitialized memory read?

[Andreas Färber]

Am 06.05.2012 18:44, schrieb Blue Swirl:
> On Sun, May 6, 2012 at 2:02 PM, Andreas Färber <address@hidden> wrote:
>> Am 06.05.2012 13:32, schrieb Blue Swirl:
>>> On Sat, May 5, 2012 at 3:37 PM, Andreas Färber <address@hidden> wrote:
>>>> Hello Blue,
>>>>
>>>> Testing a potential AREG0 fix for ppc host by malc I got an error
>>>> running `./sparc-softmmu/sparc-softmmu` (same with CD/kernel):
>>>>
>>>> qemu: fatal: Trap 0x07 while interrupts disabled, Error state
>>>> pc: 00005e0c  npc: 00005e10
>>>> General Registers:
>>>> %g0-7: 00000000 00000001 babababa 00000000 00000020 07ffff08 07ffe000
>>>> babababa
>>>>
>>>> Current Register Window:
>>>> %o0-7: 00000000 00000000 00000000 00000000 00000000 00000000 00000000
>>>> 00000000
>>>> %l0-7: 00000000 00000000 00000000 00000000 00000000 00000000 00000000
>>>> 00000000
>>>> %i0-7: 00000000 00000000 00000000 00000000 00000000 00000000 00000000
>>>> 00000000
>>>>
>>>> Floating Point Registers:
>>>> %f00: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>>>> %f08: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>>>> %f16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>>>> %f24: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
>>>> psr: 048000c0 (icc: N--- SPE: SP-) wim: 00000001
>>>> fsr: 00000000 y: 00000020
>>>> Abgebrochen
>>>>
>>>> The 0xbabababa in %g2 and %g7 is a signature I've seen in uninitialized
>>>> memory on openSUSE 12.1 Betas. So I ran valgrind, and the following
>>>> caught my eye on both ppc and x86_64:
>>>>
>>>> ==18801== Command: ./sparc-softmmu/qemu-system-sparc
>>>> ==18801==
>>>> ==18801== Thread 2:
>>>> ==18801== Conditional jump or move depends on uninitialised value(s)
>>>> ==18801==    at 0x25C5AF: compute_all_logic (cc_helper.c:37)
>>>> ==18801==    by 0x25C648: helper_compute_psr (cc_helper.c:470)
>>>> ==18801==    by 0x8CD0981: ???
>>>> ==18801==  Uninitialised value was created by a heap allocation
>>>> ==18801==    at 0x4C27CE8: memalign (in
>>>> /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
>>>> ==18801==    by 0x4C27D97: posix_memalign (in
>>>> /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
>>>> ==18801==    by 0x1F2101: qemu_memalign (oslib-posix.c:93)
>>>> ==18801==    by 0x1F21A9: qemu_vmalloc (oslib-posix.c:126)
>>>> ==18801==    by 0x2665F6: qemu_ram_alloc_from_ptr (exec.c:2647)
>>>> ==18801==    by 0x286D76: memory_region_init_ram (memory.c:954)
>>>> ==18801==    by 0x297FFD: ram_init1 (sun4m.c:757)
>>>> ==18801==    by 0x204DAE: qdev_init (qdev.c:151)
>>>> ==18801==    by 0x204EEC: qdev_init_nofail (qdev.c:258)
>>>> ==18801==    by 0x298845: ram_init.constprop.7 (sun4m.c:783)
>>>> ==18801==    by 0x298980: sun4m_hw_init (sun4m.c:862)
>>>> ==18801==    by 0x2994A2: ss5_init (sun4m.c:1289)
>>>>
>>>> This is at 8f473dd104f0937ce98523fa6f9de0bd845aebbe, and cc_helper.c:37
>>>> is int32_t dst argument of get_NZ_icc(), which is always called with
>>>> CC_DST, i.e. env->cc_dst.
>>>>
>>>> This seems to indicate that a read from uninitialized memory occurred,
>>>> from which cc_dst is being initialized?
>>>
>>> This should happen in target-sparc/cpu.c:45
>>>     memset(env, 0, offsetof(CPUSPARCState, breakpoints));
>>>
>>> cc_dst is between structure start and CPU_COMMON.
>>>
>>> 89aaf60dedbe0e6415acfe816e02b538e5c54e68 fixed a bug relating to reset 
>>> recently.
>>
>> The still-current master commit above includes that fix though, and
>> that's no explanation for the uninitialized memory stemming from sun4m
>> RAM as opposed to QOM object_new(). Somewhere a read is happening,
>> possibly in OpenBIOS, from uninitialized memory that is then stored into
>> the CPUSPARCState after that has been zero-initialized, IIUC.
> 
> Ok, I see it now. OpenBIOS assumes that the Sparc32 SMP table is valid
> when the valid field is nonzero, indicating secondary processor setup
> so OpenBIOS jumps to the location indicated with the SMP table. With
> 0xbabababa in memory, this fragile logic fails and there is the early
> crash.
> https://tracker.coreboot.org/trac/openbios/browser/trunk/openbios-devel/arch/sparc32/entry.S#L132
> https://tracker.coreboot.org/trac/openbios/browser/trunk/openbios-devel/arch/sparc32/entry.S#L157

Great! I have tested the following workaround:

diff --git a/hw/sun4m.c b/hw/sun4m.c
index 34088ad..55d5bdc 100644
--- a/hw/sun4m.c
+++ b/hw/sun4m.c
@@ -755,6 +755,7 @@ static int ram_init1(SysBusDevice *dev)
     RamDevice *d = FROM_SYSBUS(RamDevice, dev);

     memory_region_init_ram(&d->ram, "sun4m.ram", d->size);
+    memset(memory_region_get_ram_ptr(&d->ram), 0, d->size);
     vmstate_register_ram_global(&d->ram);
     sysbus_init_mmio(dev, &d->ram);
     return 0;

This makes sparc32 work on ppc with malc's attached patch (and doesn't
break on x86_64).

Andreas

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