Skip to content

x86/coco: Require seeding RNG with RDRAND on CoCo systems

Lenny Szubowicz requested to merge lszubowi/centos-stream-9:rhel-37269 into main 
JIRA: https://issues.redhat.com/browse/RHEL-37269  
CVE: CVE-2024-35875  
  
Conflicts:  
    arch/x86/coco/core.c  
    - Context difference because RHEL9 does not have upstream 1c811d403afd  
      ("x86/sev: Fix position dcc_set_mask(ependent variable references in  
      startup code") which, among other things, moves cc_set_mask() from  
      arch/x86/coco/core.c to a static inline in arch/x86/include/asm/coco.h  
  
commit 99485c4c026f024e7cb82da84c7951dbe3deb584  
Author: Jason A. Donenfeld <Jason@zx2c4.com>  
Date:   Tue Mar 26 17:07:35 2024 +0100  
  
    x86/coco: Require seeding RNG with RDRAND on CoCo systems  
  
    There are few uses of CoCo that don't rely on working cryptography and  
    hence a working RNG. Unfortunately, the CoCo threat model means that the  
    VM host cannot be trusted and may actively work against guests to  
    extract secrets or manipulate computation. Since a malicious host can  
    modify or observe nearly all inputs to guests, the only remaining source  
    of entropy for CoCo guests is RDRAND.  
  
    If RDRAND is broken -- due to CPU hardware fault -- the RNG as a whole  
    is meant to gracefully continue on gathering entropy from other sources,  
    but since there aren't other sources on CoCo, this is catastrophic.  
    This is mostly a concern at boot time when initially seeding the RNG, as  
    after that the consequences of a broken RDRAND are much more  
    theoretical.  
  
    So, try at boot to seed the RNG using 256 bits of RDRAND output. If this  
    fails, panic(). This will also trigger if the system is booted without  
    RDRAND, as RDRAND is essential for a safe CoCo boot.  
  
    Add this deliberately to be "just a CoCo x86 driver feature" and not  
    part of the RNG itself. Many device drivers and platforms have some  
    desire to contribute something to the RNG, and add_device_randomness()  
    is specifically meant for this purpose.  
  
    Any driver can call it with seed data of any quality, or even garbage  
    quality, and it can only possibly make the quality of the RNG better or  
    have no effect, but can never make it worse.  
  
    Rather than trying to build something into the core of the RNG, consider  
    the particular CoCo issue just a CoCo issue, and therefore separate it  
    all out into driver (well, arch/platform) code.  
  
      [ bp: Massage commit message. ]  
  
    Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>  
    Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>  
    Reviewed-by: Elena Reshetova <elena.reshetova@intel.com>  
    Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>  
    Reviewed-by: Theodore Ts'o <tytso@mit.edu>  
    Cc: stable@vger.kernel.org  
    Link: https://lore.kernel.org/r/20240326160735.73531-1-Jason@zx2c4.com  
  
Signed-off-by: Lenny Szubowicz <lszubowi@redhat.com>
Edited by Lenny Szubowicz

Merge request reports