File xsa456-3.patch of Package xen.35285

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File xsa456-3.patch of Package xen.35285

# Commit c94e2105924347de0d9f32065370e802a20cc829
# Date 2024-04-09 16:37:30 +0100
# Author Andrew Cooper <andrew.cooper3@citrix.com>
# Committer Andrew Cooper <andrew.cooper3@citrix.com>
x86/tsx: Expose RTM_ALWAYS_ABORT to guests

A TSX Abort is one option mitigate Native-BHI, but a guest kernel doesn't get
to see this if Xen has turned RTM off using MSR_TSX_{CTRL,FORCE_ABORT}.

Therefore, the meaning of RTM_ALWAYS_ABORT has been adjusted to "XBEGIN won't
fault", and it should be exposed to guests so they can make a better decision.

Expose it in the max policy for any RTM-capable system.  Offer it by default
only if RTM has been disabled.

Update test-tsx to account for this new meaning.  While adjusting the logic in
test_guest_policies(), take the opportunity to use feature names (now they're
available) to make the logic easier to follow.

This is part of XSA-456 / CVE-2024-2201.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/cpu-policy.c
+++ b/xen/arch/x86/cpu-policy.c
@@ -430,6 +430,21 @@ static void __init guest_common_feature_
         __set_bit(X86_FEATURE_FB_CLEAR, fs);
         __set_bit(X86_FEATURE_RFDS_CLEAR, fs);
     }
+
+    /*
+     * To mitigate Native-BHI, one option is to use a TSX Abort on capable
+     * systems.  This is safe even if RTM has been disabled for other reasons
+     * via MSR_TSX_{CTRL,FORCE_ABORT}.  However, a guest kernel doesn't get to
+     * know this type of information.
+     *
+     * Therefore the meaning of RTM_ALWAYS_ABORT has been adjusted, to instead
+     * mean "XBEGIN won't fault".  This is enough for a guest kernel to make
+     * an informed choice WRT mitigating Native-BHI.
+     *
+     * If RTM-capable, we can run a VM which has seen RTM_ALWAYS_ABORT.
+     */
+    if ( test_bit(X86_FEATURE_RTM, fs) )
+        __set_bit(X86_FEATURE_RTM_ALWAYS_ABORT, fs);
 }
 
 static void __init calculate_pv_max_policy(void)
--- a/xen/include/public/arch-x86/cpufeatureset.h
+++ b/xen/include/public/arch-x86/cpufeatureset.h
@@ -266,7 +266,7 @@ XEN_CPUFEATURE(AVX512_4VNNIW, 9*32+ 2) /
 XEN_CPUFEATURE(AVX512_4FMAPS, 9*32+ 3) /*A  AVX512 Multiply Accumulation Single Precision */
 XEN_CPUFEATURE(SRBDS_CTRL,    9*32+ 9) /*   MSR_MCU_OPT_CTRL and RNGDS_MITG_DIS. */
 XEN_CPUFEATURE(MD_CLEAR,      9*32+10) /*!A VERW clears microarchitectural buffers */
-XEN_CPUFEATURE(RTM_ALWAYS_ABORT, 9*32+11) /* RTM disabled (but XBEGIN wont fault) */
+XEN_CPUFEATURE(RTM_ALWAYS_ABORT, 9*32+11) /*! RTM disabled (but XBEGIN wont fault) */
 XEN_CPUFEATURE(TSX_FORCE_ABORT, 9*32+13) /* MSR_TSX_FORCE_ABORT.RTM_ABORT */
 XEN_CPUFEATURE(HYBRID,        9*32+15) /*   Heterogeneous platform */
 XEN_CPUFEATURE(CET_IBT,       9*32+20) /*   CET - Indirect Branch Tracking */

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