futex: Add robust futex unlock IP range

There will be a VDSO function to unlock robust futexes in user space. The
unlock sequence is racy vs. clearing the list_pending_op pointer in the
tasks robust list head. To plug this race the kernel needs to know the
instruction window. As the VDSO is per MM the addresses are stored in
mm_struct::futex.

Architectures which implement support for this have to update these
addresses when the VDSO is (re)mapped and indicate the pending op pointer
size which is matching the IP.

Arguably this could be resolved by chasing mm->context->vdso->image, but
that's architecture specific and requires to touch quite some cache
lines. Having it in mm::futex reduces the cache line impact and avoids
having yet another set of architecture specific functionality.

To support multi size robust list applications (gaming) this provides two
ranges when COMPAT is enabled.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: André Almeida <andrealmeid@igalia.com>
Link: https://patch.msgid.link/20260602090535.718926819@kernel.org

diff --git a/include/linux/futex.h b/include/linux/futex.h
index 9e6218c2be664666307b285f156cd8246cb364c5..cb2a182547dd9d7457cd57ed3525b296e65e9f3e 100644 (file)
--- a/include/linux/futex.h
+++ b/include/linux/futex.h
@@ -81,11 +81,9 @@ int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)
 #ifdef CONFIG_FUTEX_PRIVATE_HASH
 int futex_hash_allocate_default(void);
 void futex_hash_free(struct mm_struct *mm);
-void futex_mm_init(struct mm_struct *mm);
 #else  /* CONFIG_FUTEX_PRIVATE_HASH */
 static inline int futex_hash_allocate_default(void) { return 0; }
 static inline int futex_hash_free(struct mm_struct *mm) { return 0; }
-static inline void futex_mm_init(struct mm_struct *mm) { }
 #endif /* !CONFIG_FUTEX_PRIVATE_HASH */
 
 #else  /* CONFIG_FUTEX */
@@ -104,7 +102,24 @@ static inline int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsig
 }
 static inline int futex_hash_allocate_default(void) { return 0; }
 static inline int futex_hash_free(struct mm_struct *mm) { return 0; }
-static inline void futex_mm_init(struct mm_struct *mm) { }
 #endif /* !CONFIG_FUTEX */
 
+#ifdef CONFIG_FUTEX_ROBUST_UNLOCK
+void futex_reset_cs_ranges(struct futex_mm_data *fd);
+
+static inline void futex_set_vdso_cs_range(struct futex_mm_data *fd, unsigned int idx,
+                                          unsigned long start, unsigned long end, bool sz32)
+{
+       fd->unlock.cs_ranges[idx].start_ip = start;
+       fd->unlock.cs_ranges[idx].len = end - start;
+       fd->unlock.cs_ranges[idx].pop_size32 = sz32;
+}
+#endif /* CONFIG_FUTEX_ROBUST_UNLOCK */
+
+#if defined(CONFIG_FUTEX_PRIVATE_HASH) || defined(CONFIG_FUTEX_ROBUST_UNLOCK)
+void futex_mm_init(struct mm_struct *mm);
+#else
+static inline void futex_mm_init(struct mm_struct *mm) { }
+#endif
+
 #endif /* _LINUX_FUTEX_H */

diff --git a/include/linux/futex_types.h b/include/linux/futex_types.h
index d41557d2790d555539718e601f87a329a54ca602..d320c0571f0c8c9740cf6f9972a984393097f421 100644 (file)
--- a/include/linux/futex_types.h
+++ b/include/linux/futex_types.h
@@ -55,12 +55,40 @@ struct futex_mm_phash {
 struct futex_mm_phash { };
 #endif /* !CONFIG_FUTEX_ROBUST_UNLOCK */
 
+#ifdef CONFIG_FUTEX_ROBUST_UNLOCK
+/**
+ * struct futex_unlock_cs_range - Range for the VDSO unlock critical section
+ * @start_ip:  The start IP of the robust futex unlock critical section (inclusive)
+ * @len:       The length of the robust futex unlock critical section
+ * @pop_size32:        Pending OP pointer size indicator. 0 == 64-bit, 1 == 32-bit
+ */
+struct futex_unlock_cs_range {
+       unsigned long          start_ip;
+       unsigned int           len;
+       unsigned int           pop_size32;
+};
+
+#define FUTEX_ROBUST_MAX_CS_RANGES     (1 + IS_ENABLED(CONFIG_COMPAT))
+
+/**
+ * struct futex_unlock_cs_ranges - Futex unlock VSDO critical sections
+ * @cs_ranges: Array of critical section ranges
+ */
+struct futex_unlock_cs_ranges {
+       struct futex_unlock_cs_range    cs_ranges[FUTEX_ROBUST_MAX_CS_RANGES];
+};
+#else  /* CONFIG_FUTEX_ROBUST_UNLOCK */
+struct futex_unlock_cs_ranges { };
+#endif /* !CONFIG_FUTEX_ROBUST_UNLOCK */
+
 /**
  * struct futex_mm_data - Futex related per MM data
  * @phash:     Futex private hash related data
+ * @unlock:    Futex unlock VDSO critical sections
  */
 struct futex_mm_data {
        struct futex_mm_phash           phash;
+       struct futex_unlock_cs_ranges   unlock;
 };
 #else  /* CONFIG_FUTEX */
 struct futex_sched_data { };

diff --git a/init/Kconfig b/init/Kconfig
index 2937c4d308aecb993e8bd347b089d1f4fab459e6..165b08e336a853cf840b68b5d9d25a7c98ea57cf 100644 (file)
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1842,6 +1842,12 @@ config FUTEX_MPOL
        depends on FUTEX && NUMA
        default y
 
+config HAVE_FUTEX_ROBUST_UNLOCK
+       bool
+
+config FUTEX_ROBUST_UNLOCK
+       def_bool FUTEX && HAVE_GENERIC_VDSO && GENERIC_IRQ_ENTRY && RSEQ && HAVE_FUTEX_ROBUST_UNLOCK
+
 config EPOLL
        bool "Enable eventpoll support" if EXPERT
        default y

diff --git a/kernel/futex/core.c b/kernel/futex/core.c
index 77ccb7787c349929895cf0980575c74d25c8c0e9..aad6e501fabd38c537c6a4b6cf2237797157a5ba 100644 (file)
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1761,11 +1761,11 @@ static bool futex_ref_is_dead(struct futex_private_hash *fph)
        return atomic_long_read(&mm->futex.phash.atomic) == 0;
 }
 
-void futex_mm_init(struct mm_struct *mm)
+static void futex_hash_init_mm(struct futex_mm_data *fd)
 {
-       memset(&mm->futex, 0, sizeof(mm->futex));
-       mutex_init(&mm->futex.phash.lock);
-       mm->futex.phash.batches = get_state_synchronize_rcu();
+       memset(&fd->phash, 0, sizeof(fd->phash));
+       mutex_init(&fd->phash.lock);
+       fd->phash.batches = get_state_synchronize_rcu();
 }
 
 void futex_hash_free(struct mm_struct *mm)
@@ -1969,19 +1969,47 @@ static int futex_hash_get_slots(void)
                return fph->hash_mask + 1;
        return 0;
 }
+#else  /* CONFIG_FUTEX_PRIVATE_HASH */
+static inline int futex_hash_allocate(unsigned int hslots, unsigned int flags) { return -EINVAL; }
+static inline int futex_hash_get_slots(void) { return 0; }
+static inline void futex_hash_init_mm(struct futex_mm_data *fd) { }
+#endif /* !CONFIG_FUTEX_PRIVATE_HASH */
 
-#else
+#ifdef CONFIG_FUTEX_ROBUST_UNLOCK
+static void futex_invalidate_cs_ranges(struct futex_mm_data *fd)
+{
+       /*
+        * Invalidate start_ip so that the quick check fails for ip >= start_ip
+        * if VDSO is not mapped or the second slot is not available for compat
+        * tasks as they use VDSO32 which does not provide the 64-bit pointer
+        * variant.
+        */
+       for (int i = 0; i < FUTEX_ROBUST_MAX_CS_RANGES; i++)
+               fd->unlock.cs_ranges[i].start_ip = ~0UL;
+}
 
-static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
+void futex_reset_cs_ranges(struct futex_mm_data *fd)
 {
-       return -EINVAL;
+       memset(fd->unlock.cs_ranges, 0, sizeof(fd->unlock.cs_ranges));
+       futex_invalidate_cs_ranges(fd);
 }
 
-static int futex_hash_get_slots(void)
+static void futex_robust_unlock_init_mm(struct futex_mm_data *fd)
 {
-       return 0;
+       /* mm_dup() preserves the range, mm_alloc() clears it */
+       if (!fd->unlock.cs_ranges[0].start_ip)
+               futex_invalidate_cs_ranges(fd);
 }
+#else  /* CONFIG_FUTEX_ROBUST_UNLOCK */
+static inline void futex_robust_unlock_init_mm(struct futex_mm_data *fd) { }
+#endif /* !CONFIG_FUTEX_ROBUST_UNLOCK */
 
+#if defined(CONFIG_FUTEX_PRIVATE_HASH) || defined(CONFIG_FUTEX_ROBUST_UNLOCK)
+void futex_mm_init(struct mm_struct *mm)
+{
+       futex_hash_init_mm(&mm->futex);
+       futex_robust_unlock_init_mm(&mm->futex);
+}
 #endif
 
 int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)