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 information.
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 9 years ago by uwe
- Patch Status changed from there is no patch to patch needs review