Subject: rwlock: introduce support for blocking speculation into critical regions
From: Roger Pau Monné <[email-masked]>

Introduce inline wrappers as required and add direct calls to
block_lock_speculation() in order to prevent speculation into the rwlock
protected critical regions.

Note the rwlock primitives are adjusted to use the non speculation safe variants
of the spinlock handlers, as a speculation barrier is added in the rwlock
calling wrappers.

trylock variants are protected by using lock_evaluate_nospec().

This is part of XSA-453 / CVE-2024-2193

Signed-off-by: Roger Pau Monné <[email-masked]>
Reviewed-by: Jan Beulich <[email-masked]>
(cherry picked from commit a1fb15f61692b1fa9945fc51f55471ace49cdd59)

diff --git a/xen/common/rwlock.c b/xen/common/rwlock.c
index dadab372b5e1..2464f745485d 100644
--- a/xen/common/rwlock.c
+++ b/xen/common/rwlock.c
@@ -34,8 +34,11 @@ void queue_read_lock_slowpath(rwlock_t *lock)
 
     /*
      * Put the reader into the wait queue.
+     *
+     * Use the speculation unsafe helper, as it's the caller responsibility to
+     * issue a speculation barrier if required.
      */
-    spin_lock(&lock->lock);
+    _spin_lock(&lock->lock);
 
     /*
      * At the head of the wait queue now, wait until the writer state
@@ -64,8 +67,13 @@ void queue_write_lock_slowpath(rwlock_t *lock)
 {
     u32 cnts;
 
-    /* Put the writer into the wait queue. */
-    spin_lock(&lock->lock);
+    /*
+     * Put the writer into the wait queue.
+     *
+     * Use the speculation unsafe helper, as it's the caller responsibility to
+     * issue a speculation barrier if required.
+     */
+    _spin_lock(&lock->lock);
 
     /* Try to acquire the lock directly if no reader is present. */
     if ( !atomic_read(&lock->cnts) &&
diff --git a/xen/include/xen/rwlock.h b/xen/include/xen/rwlock.h
index 0cc9167715b3..fd0458be94ae 100644
--- a/xen/include/xen/rwlock.h
+++ b/xen/include/xen/rwlock.h
@@ -247,27 +247,49 @@ static inline int _rw_is_write_locked(rwlock_t *lock)
     return (atomic_read(&lock->cnts) & _QW_WMASK) == _QW_LOCKED;
 }
 
-#define read_lock(l)                  _read_lock(l)
-#define read_lock_irq(l)              _read_lock_irq(l)
+static always_inline void read_lock(rwlock_t *l)
+{
+    _read_lock(l);
+    block_lock_speculation();
+}
+
+static always_inline void read_lock_irq(rwlock_t *l)
+{
+    _read_lock_irq(l);
+    block_lock_speculation();
+}
+
 #define read_lock_irqsave(l, f)                                 \
     ({                                                          \
         BUILD_BUG_ON(sizeof(f) != sizeof(unsigned long));       \
         ((f) = _read_lock_irqsave(l));                          \
+        block_lock_speculation();                               \
     })
 
 #define read_unlock(l)                _read_unlock(l)
 #define read_unlock_irq(l)            _read_unlock_irq(l)
 #define read_unlock_irqrestore(l, f)  _read_unlock_irqrestore(l, f)
-#define read_trylock(l)               _read_trylock(l)
+#define read_trylock(l)               lock_evaluate_nospec(_read_trylock(l))
+
+static always_inline void write_lock(rwlock_t *l)
+{
+    _write_lock(l);
+    block_lock_speculation();
+}
+
+static always_inline void write_lock_irq(rwlock_t *l)
+{
+    _write_lock_irq(l);
+    block_lock_speculation();
+}
 
-#define write_lock(l)                 _write_lock(l)
-#define write_lock_irq(l)             _write_lock_irq(l)
 #define write_lock_irqsave(l, f)                                \
     ({                                                          \
         BUILD_BUG_ON(sizeof(f) != sizeof(unsigned long));       \
         ((f) = _write_lock_irqsave(l));                         \
+        block_lock_speculation();                               \
     })
-#define write_trylock(l)              _write_trylock(l)
+#define write_trylock(l)              lock_evaluate_nospec(_write_trylock(l))
 
 #define write_unlock(l)               _write_unlock(l)
 #define write_unlock_irq(l)           _write_unlock_irq(l)