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

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Date:   Wed, 8 Nov 2017 17:14:33 +0100
From:   Denys Vlasenko <dvlasenk@...hat.com>
To:     pbonzini@...hat.com, hpa@...or.com, bp@...e.de, vbabka@...e.cz,
        tony.luck@...el.com, fenghua.yu@...el.com, jpoimboe@...hat.com,
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        ravi.v.shankar@...el.com, ricardo.neri-calderon@...ux.intel.com,
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        mhiramat@...nel.org, luto@...nel.org, dave.hansen@...ux.intel.com,
        ray.huang@....com, mst@...hat.com, linux-kernel@...r.kernel.org,
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        linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/asm] x86/umip: Add emulation code for UMIP instructions

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@...ux.intel.com>
> AuthorDate: Sun, 5 Nov 2017 18:27:52 -0800
> Committer:  Ingo Molnar <mingo@...nel.org>
> 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.

Can we avoid maintain emulation of these isns, by asking Wine to remove their use instead?