Re: [tip:x86/asm] x86/umip: Add emulation code for UMIP instructions

[Ricardo Neri]

On Wed, Nov 08, 2017 at 05:14:33PM +0100, Denys Vlasenko wrote:
> On 11/08/2017 12:00 PM, tip-bot for Ricardo Neri wrote:
> >Commit-ID:  1e5db223696afa55e6a038fac638f759e1fdcc01
> >Gitweb:     https://git.kernel.org/tip/1e5db223696afa55e6a038fac638f759e1fdcc01
> >Author:     Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> >AuthorDate: Sun, 5 Nov 2017 18:27:52 -0800
> >Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
> >CommitDate: Wed, 8 Nov 2017 11:16:22 +0100
> >
> >x86/umip: Add emulation code for UMIP instructions
> >
> >The feature User-Mode Instruction Prevention present in recent Intel
> >processor prevents a group of instructions (sgdt, sidt, sldt, smsw, and
> >str) from being executed with CPL > 0. Otherwise, a general protection
> >fault is issued.
> 
> This was arguably an oversight on Intel's part - these insns should have been
> protected from the start, as they leak a tiny bit of kernel data.
> 
> >Rather than relaying to the user space the general protection fault caused
> >by the UMIP-protected instructions (in the form of a SIGSEGV signal), it
> >can be trapped and the instruction emulated to provide a dummy result.
> >This allows to both conserve the current kernel behavior and not reveal the
> >system resources that UMIP intends to protect (i.e., the locations of the
> >global descriptor and interrupt descriptor tables, the segment selectors of
> >the local descriptor table, the value of the task state register and the
> >contents of the CR0 register).
> >
> >This emulation is needed because certain applications (e.g., WineHQ and
> >DOSEMU2) rely on this subset of instructions to function.
> 
> I'm surprised. What in the world they need those insns for?
> 
> Wine uses sidt like this, to emulate "mov from r/m to reg" insns:
> 
> static LDT_ENTRY idt[256];
> ...
>     case 0x8a: /* mov Eb, Gb */
>     case 0x8b: /* mov Ev, Gv */
>     {
>         BYTE *data = INSTR_GetOperandAddr(context, instr + 1, long_addr,
>                                           segprefix, &len);
>         unsigned int data_size = (*instr == 0x8b) ? (long_op ? 4 : 2) : 1;
>         struct idtr idtr = get_idtr(); <=============================== HERE
>         unsigned int offset = data - idtr.base;
> 
>         if (offset <= idtr.limit + 1 - data_size)
>         {
>             idt[1].LimitLow = 0x100; /* FIXME */
>             idt[2].LimitLow = 0x11E; /* FIXME */
>             idt[3].LimitLow = 0x500; /* FIXME */
> 
>             switch (*instr)
>             {
>             case 0x8a: store_reg_byte( context, instr[1], (BYTE *)idt + offset ); break;
>             case 0x8b: store_reg_word( context, instr[1], (BYTE *)idt + offset, long_op ); break;
>             }
>             context->Eip += prefixlen + len + 1;
>             return ExceptionContinueExecution;
>         }
>         break;  /* Unable to emulate it */
>     }
> 
> Looks baffling, to say the least... this supports someone who reads
> IDT bytes via those insns, and they need to ensure that the values read
> from idt[1/2/3].LimitLow are as expected. That's it? Pity git history
> doesn't go far enough in the past, and comments are not informative as well...
> 
> I did not find smsw or sgdt in Wine git tree.
> 
> I did not find smsw, sidt or sgdt in dosemu2-devel git tree.

Stas Sergeev investigated what applications use smsw when running dosemu2 in virtual-8086
mode. He found that Windows 3.1 and dos4gw use it [1], [2].

Thanks and BR,
Ricardo

[1]. https://www.winehq.org/pipermail/wine-devel/2017-April/117159.html
[2]. https://marc.info/?l=linux-kernel&m=147876798717927&w=2