Re: [Xen-devel] [PATCH 2/4] x86: suppress SMAP and SMEP while running 32-bit PV guest code

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 09/03/16 12:27, Wu, Feng wrote:
>
>> -----Original Message-----
>> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
>> Sent: Wednesday, March 9, 2016 6:46 PM
>> To: Wu, Feng <feng.wu@xxxxxxxxx>; Jan Beulich <JBeulich@xxxxxxxx>; xen-
>> devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
>> Cc: Keir Fraser <keir@xxxxxxx>
>> Subject: Re: [PATCH 2/4] x86: suppress SMAP and SMEP while running 32-bit PV
>> guest code
>>
>> On 09/03/16 08:09, Wu, Feng wrote:
>>
>>>> --- a/xen/arch/x86/x86_64/entry.S
>>>> +++ b/xen/arch/x86/x86_64/entry.S
>>>> @@ -434,6 +434,7 @@ ENTRY(dom_crash_sync_extable)
>>>>
>>>>  ENTRY(common_interrupt)
>>>>          SAVE_ALL CLAC
>>>> +        SMEP_SMAP_RESTORE
>>>>          movq %rsp,%rdi
>>>>          callq do_IRQ
>>>>          jmp ret_from_intr
>>>> @@ -454,13 +455,64 @@ ENTRY(page_fault)
>>>>  GLOBAL(handle_exception)
>>>>          SAVE_ALL CLAC
>>>>  handle_exception_saved:
>>>> +        GET_CURRENT(%rbx)
>>>>          testb $X86_EFLAGS_IF>>8,UREGS_eflags+1(%rsp)
>>>>          jz    exception_with_ints_disabled
>>>> -        sti
>>>> +
>>>> +.Lsmep_smap_orig:
>>>> +        jmp   0f
>>>> +        .if 0 // GAS bug (affecting at least 2.22 ... 2.26)
>>>> +        .org .Lsmep_smap_orig + (.Lsmep_smap_alt_end - .Lsmep_smap_alt),
>> 0xcc
>>>> +        .else
>>>> +        // worst case: rex + opcode + modrm + 4-byte displacement
>>>> +        .skip (1 + 1 + 1 + 4) - 2, 0xcc
>>>> +        .endif
>>>> +        .pushsection .altinstr_replacement, "ax"
>>>> +.Lsmep_smap_alt:
>>>> +        mov   VCPU_domain(%rbx),%rax
>>>> +.Lsmep_smap_alt_end:
>>>> +        .section .altinstructions, "a"
>>>> +        altinstruction_entry .Lsmep_smap_orig, .Lsmep_smap_alt, \
>>>> +                             X86_FEATURE_SMEP, \
>>>> +                             (.Lsmep_smap_alt_end - .Lsmep_smap_alt), \
>>>> +                             (.Lsmep_smap_alt_end - .Lsmep_smap_alt)
>>>> +        altinstruction_entry .Lsmep_smap_orig, .Lsmep_smap_alt, \
>>>> +                             X86_FEATURE_SMAP, \
>>>> +                             (.Lsmep_smap_alt_end - .Lsmep_smap_alt), \
>>>> +                             (.Lsmep_smap_alt_end - .Lsmep_smap_alt)
>>>> +        .popsection
>>>> +
>>>> +        testb $3,UREGS_cs(%rsp)
>>>> +        jz    0f
>>>> +        cmpb  $0,DOMAIN_is_32bit_pv(%rax)
>>>> +        je    0f
>>>> +        call  cr4_smep_smap_restore
>>>> +        /*
>>>> +         * An NMI or #MC may occur between clearing CR4.SMEP and
>> CR4.SMAP in
>>> Do you mean "before" when you typed "between" above?
>> The meaning is "between (clearing CR4.SMEP and CR4.SMAP in
>> compat_restore_all_guest) and (it actually returning to guest)"
>>
>> Nested lists in English are a source of confusion, even to native speakers.
> Oh, thanks for the clarification! Do you know how "An NMI or #MC may occur
> between clearing CR4.SMEP and CR4.SMAP in compat_restore_all_guest and
> it actually returning to guest context, in which case the guest would run with
> the two features enabled. " can happen? Especially how the guest can run
> with the two features enabled?

NMIs and MCEs can occur at any point, even if interrupts are disabled.

The bad situation is this sequence:

* Xen is returning to the guest and disables CR4.SMEP/SMAP
* NMI occurs while still in Xen
* NMI exit path sees it is returning to Xen and re-enabled CR4.SMEP/SMAP
* Xen ends up returning to guest with CR4.SMEP/SMAP enabled.

~Andrew

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