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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.03.16 at 11:45, <andrew.cooper3@xxxxxxxxxx> wrote:
> 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.

I've switched the first "and" to / to avoid the ambiguity.

Jan


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