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

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

[Martin Schwidefsky]

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.

For UMH_NO_WAIT and UMH_WAIT_EXEC the call to kernel_thread() in
__call_usermodehelper() can return while do_execve() is still running.
For UMH_NO_WAIT __call_usermodehelper() frees the sub_info structure
directly, for 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>
---
 kernel/kmod.c | 74 ++++++++++++++++++++++++++---------------------------------
 1 file changed, 33 insertions(+), 41 deletions(-)

diff --git a/kernel/kmod.c b/kernel/kmod.c
index 8637e04..641c44b 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -196,12 +196,33 @@ 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.
+	 */
+	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 +242,7 @@ 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 +254,7 @@ static int ____call_usermodehelper(void *data)
 		retval = sub_info->init(sub_info, new);
 		if (retval) {
 			abort_creds(new);
-			goto fail;
+			goto out;
 		}
 	}
 
@@ -242,13 +263,14 @@ 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);
-	if (!retval)
-		return 0;
-
-	/* Exec failed? */
-fail:
+out:
 	sub_info->retval = retval;
-	do_exit(0);
+	if (wait != UMH_WAIT_PROC)
+		/* For UMH_WAIT_PROC wait_for_helper calls umh_complete */
+		umh_complete(sub_info);
+	if (retval)
+		do_exit(0);
+	return 0;
 }
 
 static int call_helper(void *data)
@@ -258,26 +280,6 @@ 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 +338,8 @@ 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 +580,7 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
 		goto out;
 	}
 
-	sub_info->complete = &done;
+	sub_info->complete = (wait == UMH_NO_WAIT) ? NULL : &done;
 	sub_info->wait = wait;
 
 	queue_work(khelper_wq, &sub_info->work);
-- 
1.8.5.5

Re: [PATCH] kernel/kmod: fix use-after-free of the sub_info structure

[Tetsuo Handa]

Martin Schwidefsky wrote:
> Found this in the message log on a s390 system:
> 
> BUG kmalloc-192 (Not tainted): Poison overwritten

The use-after-free location you are suspecting is

----------
        retval = do_execve(getname_kernel(sub_info->path),
                           (const char __user *const __user *)sub_info->argv,
                           (const char __user *const __user *)sub_info->envp);
        if (!retval)
                return 0;

        /* Exec failed? */
fail:
        sub_info->retval = retval;
        do_exit(0);
}
----------

, isn't it?

I could not interpret the

> For UMH_NO_WAIT __call_usermodehelper() frees the sub_info structure
> directly, for UMH_WAIT_EXEC the call to umh_complete() allows
> call_usermodehelper_exec() to continue which then frees sub_info.

lines.

For both UMH_NO_WAIT and UMH_WAIT_EXEC cases,

  kernel_thread(call_helper, sub_info, CLONE_VFORK | SIGCHLD)

in __call_usermodehelper() waits for do_execve() to succeed or do_exit(),
doesn't it? What is wrong with assigning sub_info->retval in
____call_usermodehelper() when do_execve() did not succeed?
Which function/thread can free sub_info before assigning sub_info->retval?

Regards.

Re: [PATCH] kernel/kmod: fix use-after-free of the sub_info structure

[Oleg Nesterov]

On 10/17, Tetsuo Handa wrote:
>
> For both UMH_NO_WAIT and UMH_WAIT_EXEC cases,
>
>   kernel_thread(call_helper, sub_info, CLONE_VFORK | SIGCHLD)
>
> in __call_usermodehelper() waits for do_execve() to succeed or do_exit(),

Well, not really. kernel_thread(CLONE_VFORK) waits for do_exit() or
exec_mmap(), and exec can fail after that.

Oleg.

Re: [PATCH] kernel/kmod: fix use-after-free of the sub_infostructure

[Tetsuo Handa]

Oleg Nesterov wrote:
> On 10/17, Tetsuo Handa wrote:
> >
> > For both UMH_NO_WAIT and UMH_WAIT_EXEC cases,
> >
> >   kernel_thread(call_helper, sub_info, CLONE_VFORK | SIGCHLD)
> >
> > in __call_usermodehelper() waits for do_execve() to succeed or do_exit(),
> 
> Well, not really. kernel_thread(CLONE_VFORK) waits for do_exit() or
> exec_mmap(), and exec can fail after that.

Ah, I see. Here is a draft of an updated patch.

Can we rewrite

Martin Schwidefsky wrote:
> for UMH_WAIT_EXEC the call to umh_complete() allows
> call_usermodehelper_exec() to continue which then frees sub_info.

lines?

By the way, it seems to me that nothing prevents

        if (info->cleanup)
                (*info->cleanup)(info);

>from crashing when info->cleanup points to a function in a loadable kernel
module and the loadable kernel module got unloaded before the worker thread
calls call_usermodehelper_freeinfo().
----------
[PATCH] kernel/kmod: fix use-after-free of the sub_info structure

A use-after-free bug was found on the subprocess_info structure allocated
by the user mode helper.

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
-----

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, we need to make sure that the call to
call_usermodehelper_freeinfo() in call_usermodehelper_exec() is
always made after the last store to sub_info->retval.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
---
 kernel/kmod.c |   66 +++++++++++++++++++++++++--------------------------------
 1 files changed, 29 insertions(+), 37 deletions(-)

diff --git a/kernel/kmod.c b/kernel/kmod.c
index 8637e04..378b47f 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -196,12 +196,33 @@ 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 caller has gone away.
+	 */
+	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;
 
@@ -242,12 +263,13 @@ 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);
-	if (!retval)
-		return 0;
-
-	/* Exec failed? */
 fail:
 	sub_info->retval = retval;
+	/* wait_for_helper() will call umh_complete() if UMH_WAIT_PROC. */
+	if (wait != UMH_WAIT_PROC)
+		umh_complete(sub_info);
+	if (!retval)
+		return 0;
 	do_exit(0);
 }
 
@@ -258,26 +280,6 @@ 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 +338,8 @@ 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 +580,7 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
 		goto out;
 	}
 
-	sub_info->complete = &done;
+	sub_info->complete = (wait == UMH_NO_WAIT) ? NULL : &done;
 	sub_info->wait = wait;
 
 	queue_work(khelper_wq, &sub_info->work);
-- 
1.7.1

Re: [PATCH] kernel/kmod: fix use-after-free of the sub_infostructure

[Oleg Nesterov]

On 10/17, Tetsuo Handa wrote:
>
> Ah, I see. Here is a draft of an updated patch.

Do you mean this part

	>  	sub_info->retval = retval;
	> +	/* wait_for_helper() will call umh_complete() if UMH_WAIT_PROC. */
	> +	if (wait != UMH_WAIT_PROC)
	> +		umh_complete(sub_info);
	> +	if (!retval)
	> +		return 0;
	>  	do_exit(0);
	>  }

?

Personally I agree, this looks a bit better to me. But this is cosmetic
and subjective, I leave this to Martin ;)

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.

> By the way, it seems to me that nothing prevents
>
>         if (info->cleanup)
>                 (*info->cleanup)(info);
>
> from crashing when info->cleanup points to a function in a loadable kernel
> module and the loadable kernel module got unloaded before the worker thread
> calls call_usermodehelper_freeinfo().

Just don't do this? I mean, in this case the caller of call_usermodehelper()
is obviously buggy? Or I missed your point?

Oleg.

Re: [PATCH] kernel/kmod: fix use-after-free of the sub_infostructure

[Martin Schwidefsky]

On Thu, 16 Oct 2014 23:58:34 +0200
Oleg Nesterov <oleg@redhat.com> wrote:

> On 10/17, Tetsuo Handa wrote:
> >
> > Ah, I see. Here is a draft of an updated patch.
> 
> Do you mean this part
> 
> 	>  	sub_info->retval = retval;
> 	> +	/* wait_for_helper() will call umh_complete() if UMH_WAIT_PROC. */
> 	> +	if (wait != UMH_WAIT_PROC)
> 	> +		umh_complete(sub_info);
> 	> +	if (!retval)
> 	> +		return 0;
> 	>  	do_exit(0);
> 	>  }
> 
> ?
> 
> Personally I agree, this looks a bit better to me. But this is cosmetic
> and subjective, I leave this to Martin ;)

I don't mind the different code order. As you seem to prefer the above I
use your version.
 
> 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.

Ok, this part is a bit tricky and deserves a comment. I'll create one
and send v2.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

Re: [PATCH] kernel/kmod: fix use-after-free of the sub_infostructure

[Martin Schwidefsky]

On Thu, 16 Oct 2014 23:58:34 +0200
Oleg Nesterov <oleg@redhat.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@de.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@de.ibm.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Cc: Tetsuo Handa <penguin-kernel@i-love.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

Re: [PATCH] kernel/kmod: fix use-after-free of the sub_info structure

[Tetsuo Handa]

Martin Schwidefsky wrote:
> Oleg Nesterov <oleg@redhat.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.

OK. I understood that the

| For UMH_NO_WAIT __call_usermodehelper() frees the sub_info structure
| directly, for UMH_WAIT_EXEC the call to umh_complete() allows
| call_usermodehelper_exec() to continue which then frees sub_info.

lines described the following sequence.

----------
Race window for call_usermodehelper_exec(UMH_WAIT_EXEC) case is shown below.

(1) The caller enters into sleep at wait_for_completion()
    in call_usermodehelper_exec().
(2) The khelper thread calls __call_usermodehelper() and enters into
    sleep at kthread_create(CLONE_VFORK).
(3) The worker thread calls do_execve() in ____call_usermodehelper()
    called from call_helper().
(4) complete_vfork_done() called from mm_release() called from exec_mmap()
    called from flush_old_exec() called from binfmt handler wakes up
    the khelper thread.
(5) The khelper thread calls umh_complete() and wakes up the caller.
(6) The caller fetches sub_info->retval (which is 0) and calls
    call_usermodehelper_freeinfo().
(7) do_execve() returns a negative value due to an unexpected failure
    after complete_vfork_done() was called.
(8) The worker thread tries to store sub_info->retval (which is a negative
    value).

When hitting this race window, the caller fails to know that do_execve()
failed and the worker thread triggers use-after-free memory corruption.

The race window for call_usermodehelper_exec(UMH_NO_WAIT) case is similar
except that the caller does not enter into sleep at (1) and the khelper
thread calls call_usermodehelper_freeinfo() at (5).
----------

do_execve() can fail for the various reasons. But this race unlikely happens
because do_execve() seldom fails after complete_vfork_done() was called.

Re: [PATCH] kernel/kmod: fix use-after-free of the sub_infostructure

[Oleg Nesterov]

On 10/17, Martin Schwidefsky wrote:
>
> On Thu, 16 Oct 2014 23:58:34 +0200
> Oleg Nesterov <oleg@redhat.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.

Thanks!

> 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.

Yes. But if it fails before exec_mmap() (which in particular calls
mm_release()->complete_vfork_done()) we are safe; sub_info can't go away
because the parent sleeps in sys_wait4() if UMH_WAIT_PROC, or it sleeps
in wait_for_vfork_done() otherwise.

But this is minor, and this only relates to the changelog. So still/again

Reviewed-by: Oleg Nesterov <oleg@redhat.com>

[PATCH 0/2] (Was: kernel/kmod: fix use-after-free of the sub_infostructure)

[Oleg Nesterov]

On 10/17, Martin Schwidefsky wrote:
>
> Version 2 of the patch.

On top of this patch.

Tetsuo, can you review?

Oleg.

 kernel/kmod.c |   43 +++++--------------------------------------
 1 files changed, 5 insertions(+), 38 deletions(-)

[PATCH 1/2] usermodehelper: don't use CLONE_VFORK for ____call_usermodehelper()

[Oleg Nesterov]

After the previous fix CLONE_VFORK in __call_usermodehelper() buys
nothing, we rely on on umh_complete() in ____call_usermodehelper()
anyway.

Remove it. This also eliminates the unnecessary sleep/wakeup in the
likely case, and this allows the next change.

While at it, kill the "int wait" locals in ____call_usermodehelper()
and __call_usermodehelper(), they can safely use sub_info->wait.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/kmod.c |   12 +++---------
 1 files changed, 3 insertions(+), 9 deletions(-)

diff --git a/kernel/kmod.c b/kernel/kmod.c
index 80f7a6d..4621771 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -223,7 +223,6 @@ static void umh_complete(struct subprocess_info *sub_info)
 static int ____call_usermodehelper(void *data)
 {
 	struct subprocess_info *sub_info = data;
-	int wait = sub_info->wait & ~UMH_KILLABLE;
 	struct cred *new;
 	int retval;
 
@@ -267,7 +266,7 @@ static int ____call_usermodehelper(void *data)
 out:
 	sub_info->retval = retval;
 	/* wait_for_helper() will call umh_complete if UHM_WAIT_PROC. */
-	if (wait != UMH_WAIT_PROC)
+	if (!(sub_info->wait & UMH_WAIT_PROC))
 		umh_complete(sub_info);
 	if (!retval)
 		return 0;
@@ -323,18 +322,13 @@ static void __call_usermodehelper(struct work_struct *work)
 {
 	struct subprocess_info *sub_info =
 		container_of(work, struct subprocess_info, work);
-	int wait = sub_info->wait & ~UMH_KILLABLE;
 	pid_t pid;
 
-	/* CLONE_VFORK: wait until the usermode helper has execve'd
-	 * successfully We need the data structures to stay around
-	 * until that is done.  */
-	if (wait == UMH_WAIT_PROC)
+	if (sub_info->wait & UMH_WAIT_PROC)
 		pid = kernel_thread(wait_for_helper, sub_info,
 				    CLONE_FS | CLONE_FILES | SIGCHLD);
 	else {
-		pid = kernel_thread(call_helper, sub_info,
-				    CLONE_VFORK | SIGCHLD);
+		pid = kernel_thread(call_helper, sub_info, SIGCHLD);
 		/* Worker thread stopped blocking khelper thread. */
 		kmod_thread_locker = NULL;
 	}
-- 
1.5.5.1

[PATCH 2/2] usermodehelper: kill the kmod_thread_locker logic

[Oleg Nesterov]

Now that we do not call kernel_thread(CLONE_VFORK) from the worker
thread we can not deadlock if do_execve() in turn triggers another
call_usermodehelper(), we can remove the kmod_thread_locker code.

Note: we should probably kill khelper_wq and simply use one of the
global workqueues, say, system_unbound_wq, this special wq for umh
buys nothing nowadays.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/kmod.c |   33 +++------------------------------
 1 files changed, 3 insertions(+), 30 deletions(-)

diff --git a/kernel/kmod.c b/kernel/kmod.c
index 4621771..2777f40 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -47,13 +47,6 @@ extern int max_threads;
 
 static struct workqueue_struct *khelper_wq;
 
-/*
- * kmod_thread_locker is used for deadlock avoidance.  There is no explicit
- * locking to protect this global - it is private to the singleton khelper
- * thread and should only ever be modified by that thread.
- */
-static const struct task_struct *kmod_thread_locker;
-
 #define CAP_BSET	(void *)1
 #define CAP_PI		(void *)2
 
@@ -273,13 +266,6 @@ out:
 	do_exit(0);
 }
 
-static int call_helper(void *data)
-{
-	/* Worker thread started blocking khelper thread. */
-	kmod_thread_locker = current;
-	return ____call_usermodehelper(data);
-}
-
 /* Keventd can't block, but this (a child) can. */
 static int wait_for_helper(void *data)
 {
@@ -327,11 +313,9 @@ static void __call_usermodehelper(struct work_struct *work)
 	if (sub_info->wait & UMH_WAIT_PROC)
 		pid = kernel_thread(wait_for_helper, sub_info,
 				    CLONE_FS | CLONE_FILES | SIGCHLD);
-	else {
-		pid = kernel_thread(call_helper, sub_info, SIGCHLD);
-		/* Worker thread stopped blocking khelper thread. */
-		kmod_thread_locker = NULL;
-	}
+	else
+		pid = kernel_thread(____call_usermodehelper, sub_info,
+				    SIGCHLD);
 
 	if (pid < 0) {
 		sub_info->retval = pid;
@@ -565,17 +549,6 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
 		goto out;
 	}
 	/*
-	 * Worker thread must not wait for khelper thread at below
-	 * wait_for_completion() if the thread was created with CLONE_VFORK
-	 * flag, for khelper thread is already waiting for the thread at
-	 * wait_for_completion() in do_fork().
-	 */
-	if (wait != UMH_NO_WAIT && current == kmod_thread_locker) {
-		retval = -EBUSY;
-		goto out;
-	}
-
-	/*
 	 * 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.
-- 
1.5.5.1

Re: [PATCH 0/2] (Was: kernel/kmod: fix use-after-free of thesub_infostructure)

[Tetsuo Handa]

Oleg Nesterov wrote:
> On 10/17, Martin Schwidefsky wrote:
> >
> > Version 2 of the patch.
> 
> On top of this patch.
> 
> Tetsuo, can you review?
> 
> Oleg.
> 
>  kernel/kmod.c |   43 +++++--------------------------------------
>  1 files changed, 5 insertions(+), 38 deletions(-)
> 
Looks OK to me.

Reviewed-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

Re: [PATCH] kernel/kmod: fix use-after-free of the sub_infostructure

[Martin Schwidefsky]

On Fri, 17 Oct 2014 06:30:29 +0900
Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:

> 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, we need to make sure that the call to
> call_usermodehelper_freeinfo() in call_usermodehelper_exec() is
> always made after the last store to sub_info->retval.

I like this improved description for the UMH_NO_WAIT and UMH_WAIT_EXEC
cases. I mix it with parts of the original description. 

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

Re: [PATCH] kernel/kmod: fix use-after-free of the sub_info structure

[Oleg Nesterov]

On 10/16, Martin Schwidefsky wrote:
>
> 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.

Hmm, yes... do_execve() can fail after mm_release(). CLONE_VFORK doesn't
help in this case.

> @@ -242,13 +263,14 @@ 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);
> -	if (!retval)
> -		return 0;
> -
> -	/* Exec failed? */
> -fail:
> +out:
>  	sub_info->retval = retval;
> -	do_exit(0);
> +	if (wait != UMH_WAIT_PROC)
> +		/* For UMH_WAIT_PROC wait_for_helper calls umh_complete */
> +		umh_complete(sub_info);
> +	if (retval)
> +		do_exit(0);
> +	return 0;
>  }

OK... I am wondering if __call_usermodehelper() still needs CLONE_VFORK
with this patch.

> @@ -588,7 +580,7 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
>  		goto out;
>  	}
>  
> -	sub_info->complete = &done;
> +	sub_info->complete = (wait == UMH_NO_WAIT) ? NULL : &done;

This probably needs a comment, and the comment in umh_complete() should
be updated,

	- we own sub_info, the UMH_KILLABLE caller has gone away.
	+ we own sub_info, the UMH_KILLABLE caller has gone away
	+ or the caller used UMH_NO_WAIT.

The patch looks correct at first glance. I'll try to re-read it later
once again.

Thanks!

Oleg.

Re: [PATCH] kernel/kmod: fix use-after-free of the sub_info structure

[Oleg Nesterov]

On 10/16, Oleg Nesterov wrote:
>
> OK... I am wondering if __call_usermodehelper() still needs CLONE_VFORK
> with this patch.

Yes, looks like it doesn't, but this needs another patch.

> > @@ -588,7 +580,7 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
> >  		goto out;
> >  	}
> >
> > -	sub_info->complete = &done;
> > +	sub_info->complete = (wait == UMH_NO_WAIT) ? NULL : &done;
>
> This probably needs a comment, and the comment in umh_complete() should
> be updated,
>
> 	- we own sub_info, the UMH_KILLABLE caller has gone away.
> 	+ we own sub_info, the UMH_KILLABLE caller has gone away
> 	+ or the caller used UMH_NO_WAIT.
>
> The patch looks correct at first glance. I'll try to re-read it later
> once again.

Reviewed-by: Oleg Nesterov <oleg@redhat.com>