[PATCH v2 4/5] x86/ucode: Refine the boundary checks for Entrysign

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

After initial publication, the SB-7033 / CVE-2024-36347 bulletin was updated
to list Zen5 CPUs as vulnerable.  Use Fam1ah as an upper bound, and adjust the
command line documentation.

When the Zen6 (also Fam1ah processors) model numbers are known, they'll want
excluding from the family ranges.

Fixes: 630e8875ab36 ("x86/ucode: Perform extra SHA2 checks on AMD Fam17h/19h 
microcode")
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

v2:
 * New
---
 docs/misc/xen-command-line.pandoc | 7 ++++---
 xen/arch/x86/cpu/microcode/amd.c  | 9 +++++++--
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
index 28a98321c762..34004ce282be 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2826,9 +2826,10 @@ stop_machine context. In NMI handler, even NMIs are 
blocked, which is
 considered safer. The default value is `true`.
 
 The `digest-check=` option is active by default and controls whether to
-perform additional authenticity checks.  Collisions in the signature algorithm
-used by AMD Fam17h/19h processors have been found.  Xen contains a table of
-digests of microcode patches with known-good provenance, and will block
+perform additional authenticity checks.  The Entrysign vulnerability (AMD
+SB-7033, CVE-2024-36347) on Zen1-5 processors allows forging the signature on
+arbitrary microcode such that it is accepted by the CPU.  Xen contains a table
+of digests of microcode patches with known-good provenance, and will block
 loading of patches that do not match.
 
 ### unrestricted_guest (Intel)
diff --git a/xen/arch/x86/cpu/microcode/amd.c b/xen/arch/x86/cpu/microcode/amd.c
index ba03401c24c5..f331d9dfee6e 100644
--- a/xen/arch/x86/cpu/microcode/amd.c
+++ b/xen/arch/x86/cpu/microcode/amd.c
@@ -125,7 +125,7 @@ static bool check_digest(const struct container_microcode 
*mc)
      * microcode updates.  Mitigate by checking the digest of the patch
      * against a list of known provenance.
      */
-    if ( boot_cpu_data.family < 0x17 ||
+    if ( boot_cpu_data.family < 0x17 || boot_cpu_data.family > 0x1a ||
          !opt_digest_check )
         return true;
 
@@ -571,7 +571,12 @@ static const struct microcode_ops __initconst_cf_clobber 
amd_ucode_ops = {
 
 void __init ucode_probe_amd(struct microcode_ops *ops)
 {
-    if ( !opt_digest_check && boot_cpu_data.family >= 0x17 )
+    /*
+     * The Entrysign vulnerability (SB-7033, CVE-2024-36347) affects Zen1-5
+     * CPUs.  Taint Xen if digest checking is turned off.
+     */
+    if ( boot_cpu_data.family >= 0x17 && boot_cpu_data.family <= 0x1a &&
+         !opt_digest_check )
     {
         printk(XENLOG_WARNING
                "Microcode patch additional digest checks disabled\n");
-- 
2.39.5