Il 31/03/2014 08:16, Wu, Feng ha scritto:
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/*
* If CPL < 3, SMAP protections are disabled if EFLAGS.AC = 1.
*
* If CPL = 3, SMAP applies to all supervisor-mode data accesses
* (these are implicit supervisor accesses) regardless of the value
* of EFLAGS.AC.
*
* This computes (cpl < 3) && (rflags & X86_EFLAGS_AC), leaving
* the result in X86_EFLAGS_AC. We then insert it in place of
* the PFERR_RSVD_MASK bit; this bit will always be zero in pfec,
* but it will be one in index if SMAP checks are being overridden.
* It is important to keep this branchless.
*/
smap = (cpl - 3) & (rflags & X86_EFLAGS_AC);
index =
(pfec >> 1) +
(smap >> (X86_EFLAGS_AC_BIT - PFERR_RSVD_BIT + 1));
return (mmu->permissions[index] >> pte_access) & 1;
The direction of PFERR_RSVD_MASK is the opposite compared to your code.
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Correct.
I am a little confused about some points of the above code:
1. "smap = (cpl - 3) & (rflags & X86_EFLAGS_AC);"
"smap" equals 1 when it is overridden and it is 0 when being enforced.
Actually, smap equals X86_EFLAGS_AC when it is overridden. Perhaps this
is the source of the confusion. Note that I'm using &, not &&.
So "index"
will be (pfec >> 1) when SMAP is enforced, but in my understanding of this case, we
should use the index with PFERR_RSVD_MASK bit being 1 in mmu-> permissions[]
to check the fault.
2. " smap >> (X86_EFLAGS_AC_BIT - PFERR_RSVD_BIT + 1)"
I am not quite understand this line. BTW, I cannot find the definition of "PFERR_RSVD_BIT",
Do you mean PFERR_RSVD_BIT equals 3?
Yes. You can similarly add PFERR_PRESENT_BIT (equal to 0) etc.
I think the basic idea is using group 0 to check permission faults when !((cpl - 3) & (rflags & X86_EFLAGS_AC)), that is SMAP is overridden
while using group 1 to check faults when (cpl - 3) & (rflags & X86_EFLAGS_AC), that is SMAP is enforced.
Here is the code base on your proposal in my understanding:
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smap = !((cpl - 3) & (rflags & X86_EFLAGS_AC));
index =
(pfec >> 1) + (smap << (PFERR_RSVD_BIT - 1)); /*assuming PFERR_RSVD_BIT == 3*/
return (mmu->permissions[index] >> pte_access) & 1;
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Could you please have a look at it? Appreciate your help! :)
It is faster if you avoid the "!" and shift right from the AC bit into
position PFERR_RSVD_BIT - 1. In update_permission_bitmask you can
invert the direction of the bit when you extract it from pfec.
Paolo
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