Re: x86 SGDT emulation for Wine

[Sean Christopherson <seanjc@google.com>]

On Wed, Dec 27, 2023, H. Peter Anvin wrote:
> On December 27, 2023 2:20:37 PM PST, Elizabeth Figura <zfigura@codeweavers.com> wrote:
> >Hello all,
> >
> >There is a Windows 98 program, a game called Nuclear Strike, which wants to do 
> >some amount of direct VGA access. Part of this is port I/O, which naturally 
> >throws SIGILL that we can trivially catch and emulate in Wine. The other part 
> >is direct access to the video memory at 0xa0000, which in general isn't a 
> >problem to catch and virtualize as well.
> >
> >However, this program is a bit creative about how it accesses that memory; 
> >instead of just writing to 0xa0000 directly, it looks up a segment descriptor 
> >whose base is at 0xa0000 and then uses the %es override to write bytes. In 
> >pseudo-C, what it does is:

...

> >Currently we emulate IDT access. On a read fault, we execute sidt ourselves, 
> >check if the read address falls within the IDT, and return some dummy data 
> >from the exception handler if it does [1]. We can easily enough implement GDT 
> >access as well this way, and there is even an out-of-tree patch written some 
> >years ago that does this, and helps the game run.
> >
> >However, there are two problems that I have observed or anticipated:
> >
> >(1) On systems with UMIP, the kernel emulates sgdt instructions and returns a 
> >consistent address which we can guarantee is invalid. However, it also returns 
> >a size of zero. The program doesn't expect this (cf. the way the loop is 
> >written above) and I believe will effectively loop forever in that case, or 
> >until it finds the VGA selector or hits invalid memory.
> >
> >    I see two obvious ways to fix this: either adjust the size of the fake 
> >kernel GDT, or provide a switch to stop emulating and let Wine handle it. The 
> >latter may very well a more sustainable option in the long term (although I'll 
> >admit I can't immediately come up with a reason why, other than "we might need 
> >to raise the size yet again".)
> >
> >    Does anyone have opinions on this particular topic? I can look into 
> >writing a patch but I'm not sure what the best approach is.
> >
> >(2) On 64-bit systems without UMIP, sgdt returns a truncated address when in 
> >32-bit mode. This truncated address in practice might point anywhere in the 
> >address space, including to valid memory.
> >
> >    In order to fix this, we would need the kernel to guarantee that the GDT 
> >base points to an address whose bottom 32 bits we can guarantee are 
> >inaccessible. This is relatively easy to achieve ourselves by simply mapping 
> >those pages as noaccess, but it also means that those pages can't overlap 
> >something we need; we already go to pains to make sure that certain parts of 
> >the address space are free. Broadly anything above the 2G boundary *should* be 
> >okay though. Is this feasible?
> >
> >    We could also just decide we don't care about systems without UMIP, but 
> >that seems a bit unfortunate; it's not that old of a feature. But I also have 
> >no idea how hard it would be to make this kind of a guarantee on the kernel 
> >side.
> >
> >    This is also, theoretically, a problem for the IDT, except that on the 
> >machines I've tested, the IDT is always at 0xfffffe0000000000. That's not 
> >great either (it's certainly caused some weirdness and confusion when 
> >debugging, when we unexpectedly catch an unrelated null pointer access) but it 
> >seems to work in practice.
> >
> >--Zeb
> >
> >[1] https://source.winehq.org/git/wine.git/blob/HEAD:/dlls/krnl386.exe16/
> >instr.c#l702
> >
> >
> 
> A prctl() to set the UMIP-emulated return values or disable it (giving
> SIGILL) would be easy enough.
> 
> For the non-UMIP case, and probably for a lot of other corner cases like
> relying on certain magic selector values and what not, the best option really
> would be to wrap the code in a lightweight KVM container. I do *not* mean
> running the Qemu user space part of KVM; instead have Wine interface with
> /dev/kvm directly.

+1.  Pivoting to KVM would require quite a bit of work up front, but I suspect
the payoff would be worthwhile in the end.

See also https://github.com/dosemu2/dosemu2/tree/devel/src/base/emu-i386.