Re: Regression in 2.6.25-rc3: s2ram segfaults before suspending

H. Peter Anvin
2008-03-01T19:54:51Z

Klaus S. Madsen wrote:
> On Fri, Feb 29, 2008 at 22:26:54 +0100, Ingo Molnar wrote:
>> * H. Peter Anvin <hpa@zytor.com> wrote:
>>
>>> Klaus S. Madsen wrote:
>>>> open("/dev/mem", O_RDWR) = 5
>>>> mmap2(NULL, 1282, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_SHARED|MAP_FIXED, 5, 0) = 0
>>>> mmap2(0xa0000, 393216, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_FIXED, 5, 0xa0) = 0xa0000
>>> ^^^^^^^^^^^^^^^^^^^^
>>>> close(5) = 0
>>>> ioperm(0, 0x400, 0x1) = 0
>>>> iopl(0x3) = 0
>>>> access("/sys/bus/pci", R_OK) = 0
>>>> write(1, "Calling get_mode\n", 17) = 17
>>>> vm86(0x1, 0xb7f14ccc, 0xb7f14830, 0xc000, 0x18b6 <unfinished ...>
>>>> --- SIGSEGV (Segmentation fault) @ 0 (0) ---
>>>> +++ killed by SIGSEGV (core dumped) +++
>>> This is the VGA BIOS being mapped, it's mapped PROT_READ|PROT_WRITE, but
>>> no PROT_EXEC; if the kernel is NX-capable it *should* segfault trying to
>>> execute out of this area, which is exactly what will happen when vm86
>>> executes INT 10h.
>>>
>>> If we can find that mmap() in the s2ram source code and add PROT_EXEC
>>> to it, it would be interesting.
>> Klaus, could you send your .config as well? Lets make sure that NX is
>> even relevant in this context.
> Allright. The mmap in question is in the x86-common.c file in libx86,
> and adding PROT_EXEC to it solves the problem.
>
> I have attached my .config.
>
> The only thing I don't understand is why this is suddenly a problem with
> 2.6.25, and not with 2.6.24? Is there a bug in 2.6.24 and previously
> that allows real-mode execution of non-executable pages?
>

One wonders, especially since the checkin it was bisected to it had
nothing to do with NX. I suspect there is either a bug in the NX logic,
which this checkin inadvertently fixed(!), or there is an ad hoc hack
that should probably never have existed. More investigation necessary,
but now we know a lot more.

-hpa