Re: [edk2-devel] How to guard CAR's stack overflow

Thu, 22 Sep 2022 14:06:54 -0700

I've never tried protecting the stack like this, so I'm not sure exactly what you're running in to. The documentation says the base address is specified a bit differently for expand-down descriptors.  It needs to point to the page below the stack (lower addresses.)  The descriptors for the other segments would still need to cover the entire address space (limit 0xFFFF, page-granular, expand-up.)  And you'd need to use a different segment selector (and hence a different GDT entry) for the stack segment (ss register) than for the regular data segments.  Most of the early PEI code I've seen uses the same selector for ds, ef, fs, gs, and ss.
Hope that helps.  This is mostly theoretical.  I don't actually have a lot of experience with x86 segment programming. 

Brian J. Johnson
------------------------------------------------------------------------
*From:* Tiger Liu(BJ-RD) [mailto:tiger...@zhaoxin.com]
*Sent:* Wednesday, September 21, 2022, 10:32 PM

*To:* devel@edk2.groups.io <devel@edk2.groups.io>, Ni, Ray 
<ray...@intel.com>, brian.john...@hpe.com <brian.john...@hpe.com>

*Subject:* [edk2-devel] How to guard CAR's stack overflow


Hi, Johnson:

Thanks for your reply!

I tried and found it seemed causing some other problems.

It hang in eary pei stage.


It seems below code could also cause an exception if using expand-down 
mode in CAR phase’s stack established.


mov eax, ss:[ebx]

 mov eax, [ebp]

mov eax, [esp]

Thanks

*发件人:*Brian J. Johnson <brian.john...@hpe.com
*发送时间:*2022年9月20日7:10

*收件人:*devel@edk2.groups.io; ray...@intel.com; Tiger Liu(BJ-RD) 
<tiger...@zhaoxin.com

*抄送:*Fan, Jeff <fanjianf...@byosoft.com.cn
*主题:*Re: [edk2-devel] How to guard CAR's stack overflow


You could also try modifying the Ia32 segment descriptors to mark the 
stack segment as an "expand down" type with a limit set just below the 
low end of the stack area. That should generate a stack-fault 
exception if the stack overflows, and wouldn't require building page 
tables. See sections 5.1 - 5.3 of the Intel SDM, volume 3.


Brian J. Johnson

------------------------------------------------------------------------

*From:*Ni, Ray [mailto:ray...@intel.com <mailto:ray...@intel.com>]

*Sent:*Wednesday, September 14, 2022, 10:25 PM


*To:*devel@edk2.groups.io <devel@edk2.groups.io> 
<mailto:devel@edk2.groups.io>, tiger...@zhaoxin.com 
<tiger...@zhaoxin.com> <mailto:tiger...@zhaoxin.com>



*Cc:*Fan, Jeff <fanjianf...@byosoft.com.cn> 
<mailto:fanjianf...@byosoft.com.cn>


*Subject:*[edk2-devel] How to guard CAR's stack overflow

    It’s doable.

    You need to enable paging and mark the very low 4K area of the
    stack as not-present.

    You could use the UefiCpuPkg/Library/CpuPageTableLib to help you
    create the 1:1 page table with the specific
    4K area as not-present (if you are using x86 processors).

    Thanks,

    Ray

    *From:*devel@edk2.groups.io <devel@edk2.groups.io>
    <mailto:devel@edk2.groups.io> *On Behalf Of *Tiger Liu(BJ-RD)
    *Sent:* Thursday, September 15, 2022 8:50 AM
    *To:* devel@edk2.groups.io
    *Subject:* [edk2-devel] How to guard CAR's stack overflow

    Hi, Experts:

    Usually, we use Cache As Ram to setup stack and heap for C
    language running environment before permanent memory has been
    initialized.

    So, is there a method to guard this phase’s stack overflow?

    Note:

    I find udk has introduced a method to guard stack overflow after
    memory has been initialized and discovered.

    Thanks

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