Re: [x86/asm/entry] BUG: unable to handle kernel paging request

[Andy Lutomirski]

On Sun, Mar 8, 2015 at 12:49 PM, Denys Vlasenko
<vda.linux@xxxxxxxxxxxxxx> wrote:
> On Sun, Mar 8, 2015 at 8:13 PM, Denys Vlasenko <vda.linux@xxxxxxxxxxxxxx> wrote:
>>> /*
>>>  * The below -8 is to reserve 8 bytes on top of the ring0 stack.
>>>  * This is necessary to guarantee that the entire "struct pt_regs"
>>>  * is accessible even if the CPU haven't stored the SS/ESP registers
>>>  * on the stack (interrupt gate does not save these registers
>>>  * when switching to the same priv ring).
>>>  * Therefore beware: accessing the ss/esp fields of the
>>>  * "struct pt_regs" is possible, but they may contain the
>>>  * completely wrong values.
>>>  */
>>> #define task_pt_regs(task)                                             \
>>> ({                                                                     \
>>>        struct pt_regs *__regs__;                                       \
>>>        __regs__ = (struct pt_regs *)(KSTK_TOP(task_stack_page(task))-8); \
>>>        __regs__ - 1;                                                   \
>>> })
>>>
>>> I'm confused about multiple things:
>>>
>>> 1. I don't understand this comment.
>>
>> Comment says that in 32-bit x86, interrupts and exceptions
>> in ring 0 do not push SS,ESP - they only save EFLAGS,CS,EIP
>> in iret frame. (This happens because CPL doesn't
>> change, not beacuse ot is zero).
>>
>> IRET insn likewise does not restore SS,ESP if it detects
>> that RPL(stack_CS) = RPL(CS).
>
> It seems that whoever wrote that code were afraid of this behavior
> and they added this 8-byte area to ensure that pt_regs->sp
> and pt_regs->ss always can be accessed.
>
> They were wrong.
>
> tss.sp0 will only be used on *inter-CPL* interrupts/exceptions,
> and those *always* push SS,ESP.
>
> If interrupt/exception happens while we are in CPL0,
> it will _not_ use tss.sp0 - it will not switch stacks
> since it is already on CPL0-stack. Therefore,
> the scenario where SS,ESP are "missing" and must not
> be accessed via pt_regs->esp in fear of touching
> not-present page is impossible.
>
> Let's just remove this "-8" thingy.

Sounds good to me.  If you do that, please make sure you don't break
task_pt_regs :)

Also, if you ever look at vm86, prepare to be scared.  It does what
appears to be horrible, awful things to sp0.  I'm really glad that
x86_64 doesn't support vm86.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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