kernel/kmod: fix use-after-free of the sub_info structure

Found this in the message log on a s390 system:

    BUG kmalloc-192 (Not tainted): Poison overwritten
    Disabling lock debugging due to kernel taint
    INFO: 0x00000000684761f4-0x00000000684761f7. First byte 0xff instead of 0x6b
    INFO: Allocated in call_usermodehelper_setup+0x70/0x128 age=71 cpu=2 pid=648
     __slab_alloc.isra.47.constprop.56+0x5f6/0x658
     kmem_cache_alloc_trace+0x106/0x408
     call_usermodehelper_setup+0x70/0x128
     call_usermodehelper+0x62/0x90
     cgroup_release_agent+0x178/0x1c0
     process_one_work+0x36e/0x680
     worker_thread+0x2f0/0x4f8
     kthread+0x10a/0x120
     kernel_thread_starter+0x6/0xc
     kernel_thread_starter+0x0/0xc
    INFO: Freed in call_usermodehelper_exec+0x110/0x1b8 age=71 cpu=2 pid=648
     __slab_free+0x94/0x560
     kfree+0x364/0x3e0
     call_usermodehelper_exec+0x110/0x1b8
     cgroup_release_agent+0x178/0x1c0
     process_one_work+0x36e/0x680
     worker_thread+0x2f0/0x4f8
     kthread+0x10a/0x120
     kernel_thread_starter+0x6/0xc
     kernel_thread_starter+0x0/0xc

There is a use-after-free bug on the subprocess_info structure allocated
by the user mode helper.  In case do_execve() returns with an error
____call_usermodehelper() stores the error code to sub_info->retval, but
sub_info can already have been freed.

Regarding UMH_NO_WAIT, the sub_info structure can be freed by
__call_usermodehelper() before the worker thread returns from
do_execve(), allowing memory corruption when do_execve() failed after
exec_mmap() is called.

Regarding UMH_WAIT_EXEC, the call to umh_complete() allows
call_usermodehelper_exec() to continue which then frees sub_info.

To fix this race the code needs to make sure that the call to
call_usermodehelper_freeinfo() is always done after the last store to
sub_info->retval.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 37 insertions, 39 deletions

diff --git a/kernel/kmod.c b/kernel/kmod.c
index 8637e041a247..80f7a6d00519 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -196,12 +196,34 @@ int __request_module(bool wait, const char *fmt, ...)
 EXPORT_SYMBOL(__request_module);
 #endif /* CONFIG_MODULES */
 
+static void call_usermodehelper_freeinfo(struct subprocess_info *info)
+{
+        if (info->cleanup)
+                (*info->cleanup)(info);
+        kfree(info);
+}
+
+static void umh_complete(struct subprocess_info *sub_info)
+{
+        struct completion *comp = xchg(&sub_info->complete, NULL);
+        /*
+         * See call_usermodehelper_exec(). If xchg() returns NULL
+         * we own sub_info, the UMH_KILLABLE caller has gone away
+         * or the caller used UMH_NO_WAIT.
+         */
+        if (comp)
+                complete(comp);
+        else
+                call_usermodehelper_freeinfo(sub_info);
+}
+
 /*
  * This is the task which runs the usermode application
  */
 static int ____call_usermodehelper(void *data)
 {
         struct subprocess_info *sub_info = data;
+        int wait = sub_info->wait & ~UMH_KILLABLE;
         struct cred *new;
         int retval;
 
@@ -221,7 +243,7 @@ static int ____call_usermodehelper(void *data)
         retval = -ENOMEM;
         new = prepare_kernel_cred(current);
         if (!new)
-                goto fail;
+                goto out;
 
         spin_lock(&umh_sysctl_lock);
         new->cap_bset = cap_intersect(usermodehelper_bset, new->cap_bset);
@@ -233,7 +255,7 @@ static int ____call_usermodehelper(void *data)
                 retval = sub_info->init(sub_info, new);
                 if (retval) {
                         abort_creds(new);
-                        goto fail;
+                        goto out;
                 }
         }
 
@@ -242,12 +264,13 @@ static int ____call_usermodehelper(void *data)
         retval = do_execve(getname_kernel(sub_info->path),
                            (const char __user *const __user *)sub_info->argv,
                            (const char __user *const __user *)sub_info->envp);
+out:
+        sub_info->retval = retval;
+        /* wait_for_helper() will call umh_complete if UHM_WAIT_PROC. */
+        if (wait != UMH_WAIT_PROC)
+                umh_complete(sub_info);
         if (!retval)
                 return 0;
-
-        /* Exec failed? */
-fail:
-        sub_info->retval = retval;
         do_exit(0);
 }
 
@@ -258,26 +281,6 @@ static int call_helper(void *data)
         return ____call_usermodehelper(data);
 }
 
-static void call_usermodehelper_freeinfo(struct subprocess_info *info)
-{
-        if (info->cleanup)
-                (*info->cleanup)(info);
-        kfree(info);
-}
-
-static void umh_complete(struct subprocess_info *sub_info)
-{
-        struct completion *comp = xchg(&sub_info->complete, NULL);
-        /*
-         * See call_usermodehelper_exec(). If xchg() returns NULL
-         * we own sub_info, the UMH_KILLABLE caller has gone away.
-         */
-        if (comp)
-                complete(comp);
-        else
-                call_usermodehelper_freeinfo(sub_info);
-}
-
 /* Keventd can't block, but this (a child) can. */
 static int wait_for_helper(void *data)
 {
@@ -336,18 +339,8 @@ static void __call_usermodehelper(struct work_struct *work)
                 kmod_thread_locker = NULL;
         }
 
-        switch (wait) {
-        case UMH_NO_WAIT:
-                call_usermodehelper_freeinfo(sub_info);
-                break;
-
-        case UMH_WAIT_PROC:
-                if (pid > 0)
-                        break;
-                /* FALLTHROUGH */
-        case UMH_WAIT_EXEC:
-                if (pid < 0)
-                        sub_info->retval = pid;
+        if (pid < 0) {
+                sub_info->retval = pid;
                 umh_complete(sub_info);
         }
 }
@@ -588,7 +581,12 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
                 goto out;
         }
 
-        sub_info->complete = &done;
+        /*
+         * Set the completion pointer only if there is a waiter.
+         * This makes it possible to use umh_complete to free
+         * the data structure in case of UMH_NO_WAIT.
+         */
+        sub_info->complete = (wait == UMH_NO_WAIT) ? NULL : &done;
         sub_info->wait = wait;
 
         queue_work(khelper_wq, &sub_info->work);