From mboxrd@z Thu Jan  1 00:00:00 1970
Subject: Access non-SMRAM memory regions from SMM
To: devel@edk2.groups.io
From: asallan3@gmail.com
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Date: Mon, 05 Dec 2022 08:41:23 -0800
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Hi, I'm aware theres been implemented a PCD `gUefiCpuPkgTokenSpaceGuid.PcdC=
puSmmRestrictedMemoryAccess` that blocks access to "any" type of non-SMRAM =
memory after SmmReadyToLock.

But I ran some tests and found out I can access some memory regions outside=
 SMRAM, like address 0xFFFD0 for instance, and dereference a pointer to tha=
t address inside my Mm Handler without any problems, even though this is a =
non-SMRAM address and I have that PCD set to TRUE (Access to non-SMRAM memo=
ry is restricted).

However when I try to dereference a pointer to a memory region used by the =
OS I get an error saying `Access SMM communication forbidden address (0xC60=
0000)!`

So I'm wondering:

1) Is it blocked or not?
2) In what context I *can* access non-SMRAM memory regions from inside my m=
m Handler?
3) Is there any way to read these memory regions used by the OS from inside=
 the mm Handler other than setting PcdCpuSmmRestrictedMemoryAccess to FALSE=
?

Any kind of help would be appreciated. Thanks!

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Hi, I'm aware theres been implemented a PCD `gUefiCpuPkgTokenSpaceGuid.PcdC=
puSmmRestrictedMemoryAccess` that blocks access to "any" type of non-SMRAM =
memory after SmmReadyToLock.<br /><br />But I ran some tests and found out =
I can access some memory regions outside SMRAM, like address 0xFFFD0 for in=
stance, and dereference a pointer to that address inside my Mm Handler with=
out any problems, even though this is a non-SMRAM address and I have that P=
CD set to TRUE (Access to non-SMRAM memory is restricted).<br /><br />Howev=
er when I try to dereference a pointer to a memory region used by the OS I =
get an error saying `Access SMM communication forbidden address (0xC600000)=
!`<br /><br />So I'm wondering:<br /><br />1) Is it blocked or not?<br />2)=
 In what context I <strong>can</strong> access non-SMRAM memory regions fro=
m inside my mm Handler?<br />3) Is there any way to read these memory regio=
ns used by the OS from inside the mm Handler other than setting PcdCpuSmmRe=
strictedMemoryAccess to FALSE?<br /><br />Any kind of help would be appreci=
ated. Thanks!

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