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Date: Fri, 17 Oct 2014 09:36:53 +0200
From: Martin Schwidefsky <schwidefsky@...ibm.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
akpm@...ux-foundation.org
Subject: Re: [PATCH] kernel/kmod: fix use-after-free of the
sub_infostructure
On Thu, 16 Oct 2014 23:58:34 +0200
Oleg Nesterov <oleg@...hat.com> wrote:
> I also agree that the changelog could mention exec_mmap. Plus a comment
> about UMH_NO_WAIT && sub_info->complete == NULL. So yes, perhaps v2 makes
> sense if Martin agrees.
Version 2 of the patch. All change requests have gone in except for the
mention of exec_mmap. I don't quite get the relevance of it, do_execve
can fail for the various reasons.
---
Subject: [PATCH] kernel/kmod: fix use-after-free of the sub_info structure
From: Martin Schwidefsky <schwidefsky@...ibm.com>
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@...ibm.com>
Reviewed-by: Oleg Nesterov <oleg@...hat.com>
Cc: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
---
kernel/kmod.c | 76 +++++++++++++++++++++++++++++------------------------------
1 file changed, 37 insertions(+), 39 deletions(-)
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 8637e04..80f7a6d 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);
--
1.8.5.5
--
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