Disable SMM on K8 platforms
|Reported by:||stepan||Owned by:||stepan|
|Patch Status:||patch needs review|
This was a while ago, but it got forgotten.
Loïc Duflot, security engineer and researcher for the scientific
division of the french Central Directorate for Information Systems
Security ("french version of the NSA"), gives some insight on fun that
can be had with the system management mode (SMM) of x86 CPUs.
See http://www.securityfocus.com/print/columnists/402 for more
While, as Loïc writes later in his article, the whole issue of
exploiting SMM is pretty pointless in Linux as the super user can
conquer ring 0 without further effort, the idea of fixing what we
can fix on the bios level seems worthwhile.
If something seems as simple as setting the D_LCK bit of SMM, we should
definitely do it.. It will at least be a marketable feature against
other upcoming firmware implementations.
Carl-Daniel Hailfinger said:
I believe that setting D_LCK will mitigate a few attacks but I strongly doubt that it cannot be cleared during system operation. Yes, the manual specifies it, but manuals have been underspecified before. Since we don't use SMM for anything, we might as well * clear D_OPEN * set D_CLOSE * clear "Enable" * set D_LCK. So, by all means, do it now. Until somebody figures out a way to disable D_LCK again we offer a much higher degree of security than everybody else.
Ok, D_LCK/D_OPEN/D_CLOSE is intel vocabulary. There is no such thing on
AMD. They call it SMMLOCK in their BKDG:
6.11.6 Locking SMM
The SMM registers can be locked by setting the SMMLOCK (HWCR, bit 0).
Once set, the SMM_BASE, the SMM_ADDR, all but the two close bits of
SMM_MASK and the RSMSPCYCDIS, SMISPCYCDIS, and SMMLOCK bits of HWCR are
locked and cannot be changed. The only way to unlock the SMM registers
is to assert reset. This provides security to the SMM mechanism. The
BIOS can lock the SMM environment after setting it up so that it can not
be tampered with.
So I propose the following patch for LinuxBIOS to fix the SMM problem
for all supported AMD K8 mainboards:
Set SMMLOCK on K8 to avoid exploits messing with SMM Signed-off-by: Stefan Reinauer <firstname.lastname@example.org> Index: src/cpu/amd/model_fxx/model_fxx_init.c =================================================================== --- src/cpu/amd/model_fxx/model_fxx_init.c (revision 2302) +++ src/cpu/amd/model_fxx/model_fxx_init.c (working copy) @@ -454,6 +454,12 @@ k8_errata(); + /* Set SMMLOCK to avoid exploits messing with SMM */ + msr = rdmsr(HWCR_MSR); + msr.lo |= (1 << 0); + wrmsr(HWCR_MSR, msr); + + enable_cache(); /* Enable the local cpu apics */
Change History (9)
comment:7 Changed 7 years ago by uwe
- Patch Status changed from there is no patch to patch needs review