[xen staging] x86/viridian: protect concurrent modification of the reference TSC page

[patchbot@xxxxxxx]

commit 45729a510a352e1a973e4023bd0067cddb1bcaf8
Author:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
AuthorDate: Thu Jul 3 13:09:03 2025 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Sep 9 14:12:00 2025 +0200

    x86/viridian: protect concurrent modification of the reference TSC page
    
    The reference TSC page is shared between all vCPUs, and the data stored in
    the domain struct.  However the handlers to set and clear it are not safe
    against concurrent accesses.  It's possible for two (or more) vCPUs to call
    HV_X64_MSR_REFERENCE_TSC at the same time and cause the in-use reference
    TSC page to be freed, while still being on the p2m.  This creates a use-
    after-free bug, where the page can end up mapped in another domain or used
    by Xen while still being part of the original domain's p2m.
    
    It's also possible to underflow the reference counter, as multiple
    concurrent writes to HV_X64_MSR_REFERENCE_TSC can create an imbalance on
    the number of put_page_and_type() calls.
    
    Introduce a lock to protect the reference TSC domain field, thus
    serializing concurrent vCPU accesses.
    
    This is CVE-2025-58143 / part of XSA-472.
    
    Fixes: 386b3365221d ('viridian: use viridian_map/unmap_guest_page() for 
reference tsc page')
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/hvm/viridian/time.c        | 4 ++++
 xen/arch/x86/hvm/viridian/viridian.c    | 2 ++
 xen/arch/x86/include/asm/hvm/viridian.h | 1 +
 3 files changed, 7 insertions(+)

diff --git a/xen/arch/x86/hvm/viridian/time.c b/xen/arch/x86/hvm/viridian/time.c
index ca6d526f46..9311858d63 100644
--- a/xen/arch/x86/hvm/viridian/time.c
+++ b/xen/arch/x86/hvm/viridian/time.c
@@ -108,8 +108,10 @@ static void time_ref_count_thaw(const struct domain *d)
 
     trc->off = (int64_t)trc->val - trc_val(d, 0);
 
+    spin_lock(&vd->lock);
     if ( vd->reference_tsc.msr.enabled )
         update_reference_tsc(d, false);
+    spin_unlock(&vd->lock);
 }
 
 static uint64_t time_ref_count(const struct domain *d)
@@ -331,6 +333,7 @@ int viridian_time_wrmsr(struct vcpu *v, uint32_t idx, 
uint64_t val)
         if ( !(viridian_feature_mask(d) & HVMPV_reference_tsc) )
             return X86EMUL_EXCEPTION;
 
+        spin_lock(&vd->lock);
         viridian_unmap_guest_page(&vd->reference_tsc);
         vd->reference_tsc.msr.raw = val;
         viridian_dump_guest_page(v, "REFERENCE_TSC", &vd->reference_tsc);
@@ -339,6 +342,7 @@ int viridian_time_wrmsr(struct vcpu *v, uint32_t idx, 
uint64_t val)
             viridian_map_guest_page(d, &vd->reference_tsc);
             update_reference_tsc(d, true);
         }
+        spin_unlock(&vd->lock);
         break;
 
     case HV_X64_MSR_TIME_REF_COUNT:
diff --git a/xen/arch/x86/hvm/viridian/viridian.c 
b/xen/arch/x86/hvm/viridian/viridian.c
index 7ea6c90168..c0be24bd22 100644
--- a/xen/arch/x86/hvm/viridian/viridian.c
+++ b/xen/arch/x86/hvm/viridian/viridian.c
@@ -494,6 +494,8 @@ int viridian_domain_init(struct domain *d)
     if ( !d->arch.hvm.viridian )
         return -ENOMEM;
 
+    spin_lock_init(&d->arch.hvm.viridian->lock);
+
     rc = viridian_synic_domain_init(d);
     if ( rc )
         goto fail;
diff --git a/xen/arch/x86/include/asm/hvm/viridian.h 
b/xen/arch/x86/include/asm/hvm/viridian.h
index 4c8ff6e80b..47c9d13841 100644
--- a/xen/arch/x86/include/asm/hvm/viridian.h
+++ b/xen/arch/x86/include/asm/hvm/viridian.h
@@ -71,6 +71,7 @@ struct viridian_domain
     DECLARE_BITMAP(hypercall_flags, _HCALL_nr);
     struct viridian_time_ref_count time_ref_count;
     struct viridian_page reference_tsc;
+    spinlock_t lock;
 };
 
 void cpuid_viridian_leaves(const struct vcpu *v, uint32_t leaf,
--
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