Messages in this thread | | | Subject | Re: [tip:x86/asm] x86/umip: Add emulation code for UMIP instructions | From | Denys Vlasenko <> | Date | Wed, 8 Nov 2017 17:14:33 +0100 |
| |
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@linux.intel.com> > AuthorDate: Sun, 5 Nov 2017 18:27:52 -0800 > Committer: Ingo Molnar <mingo@kernel.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?
| |