robust futex thread exit race

Calling handle_futex_death in exit_robust_list for the different robust
mutexes of a thread basically frees the mutex.  Another thread might grab
the lock immediately which updates the next pointer of the mutex.
fetch_robust_entry over the next pointer might therefore branch into the
robust mutex list of a different thread.  This can cause two problems: 1)
some mutexes held by the dead thread are not getting freed and 2) some
mutexs held by a different thread are freed.

The next point need to be read before calling handle_futex_death.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/futex.c b/kernel/futex.c
index e8935b1..fcc94e7 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1943,9 +1943,10 @@ static inline int fetch_robust_entry(struct robust_list __user **entry,
 void exit_robust_list(struct task_struct *curr)
 {
        struct robust_list_head __user *head = curr->robust_list;
-       struct robust_list __user *entry, *pending;
-       unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
+       struct robust_list __user *entry, *next_entry, *pending;
+       unsigned int limit = ROBUST_LIST_LIMIT, pi, next_pi, pip;
        unsigned long futex_offset;
+       int rc;
 
        /*
         * Fetch the list head (which was registered earlier, via
@@ -1965,12 +1966,14 @@ void exit_robust_list(struct task_struct *curr)
        if (fetch_robust_entry(&pending, &head->list_op_pending, &pip))
                return;
 
-       if (pending)
-               handle_futex_death((void __user *)pending + futex_offset,
-                                  curr, pip);
-
+       next_entry = NULL;      /* avoid warning with gcc */
        while (entry != &head->list) {
                /*
+                * Fetch the next entry in the list before calling
+                * handle_futex_death:
+                */
+               rc = fetch_robust_entry(&next_entry, &entry->next, &next_pi);
+               /*
                 * A pending lock might already be on the list, so
                 * don't process it twice:
                 */
@@ -1978,11 +1981,10 @@ void exit_robust_list(struct task_struct *curr)
                        if (handle_futex_death((void __user *)entry + futex_offset,
                                                curr, pi))
                                return;
-               /*
-                * Fetch the next entry in the list:
-                */
-               if (fetch_robust_entry(&entry, &entry->next, &pi))
+               if (rc)
                        return;
+               entry = next_entry;
+               pi = next_pi;
                /*
                 * Avoid excessively long or circular lists:
                 */
@@ -1991,6 +1993,10 @@ void exit_robust_list(struct task_struct *curr)
 
                cond_resched();
        }
+
+       if (pending)
+               handle_futex_death((void __user *)pending + futex_offset,
+                                  curr, pip);
 }
 
 long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,

diff --git a/kernel/futex_compat.c b/kernel/futex_compat.c
index 7e52eb0..2c2e295 100644 (file)
--- a/kernel/futex_compat.c
+++ b/kernel/futex_compat.c
@@ -38,10 +38,11 @@ fetch_robust_entry(compat_uptr_t *uentry, struct robust_list __user **entry,
 void compat_exit_robust_list(struct task_struct *curr)
 {
        struct compat_robust_list_head __user *head = curr->compat_robust_list;
-       struct robust_list __user *entry, *pending;
-       unsigned int limit = ROBUST_LIST_LIMIT, pi, pip;
-       compat_uptr_t uentry, upending;
+       struct robust_list __user *entry, *next_entry, *pending;
+       unsigned int limit = ROBUST_LIST_LIMIT, pi, next_pi, pip;
+       compat_uptr_t uentry, next_uentry, upending;
        compat_long_t futex_offset;
+       int rc;
 
        /*
         * Fetch the list head (which was registered earlier, via
@@ -61,11 +62,16 @@ void compat_exit_robust_list(struct task_struct *curr)
        if (fetch_robust_entry(&upending, &pending,
                               &head->list_op_pending, &pip))
                return;
-       if (pending)
-               handle_futex_death((void __user *)pending + futex_offset, curr, pip);
 
+       next_entry = NULL;      /* avoid warning with gcc */
        while (entry != (struct robust_list __user *) &head->list) {
                /*
+                * Fetch the next entry in the list before calling
+                * handle_futex_death:
+                */
+               rc = fetch_robust_entry(&next_uentry, &next_entry,
+                       (compat_uptr_t __user *)&entry->next, &next_pi);
+               /*
                 * A pending lock might already be on the list, so
                 * dont process it twice:
                 */
@@ -74,12 +80,11 @@ void compat_exit_robust_list(struct task_struct *curr)
                                                curr, pi))
                                return;
 
-               /*
-                * Fetch the next entry in the list:
-                */
-               if (fetch_robust_entry(&uentry, &entry,
-                                      (compat_uptr_t __user *)&entry->next, &pi))
+               if (rc)
                        return;
+               uentry = next_uentry;
+               entry = next_entry;
+               pi = next_pi;
                /*
                 * Avoid excessively long or circular lists:
                 */
@@ -88,6 +93,9 @@ void compat_exit_robust_list(struct task_struct *curr)
 
                cond_resched();
        }
+       if (pending)
+               handle_futex_death((void __user *)pending + futex_offset,
+                                  curr, pip);
 }
 
 asmlinkage long