From: Zijun Hu <quic_zijuhu@quicinc.com>

class_dev_iter_init(struct class_dev_iter *iter, struct class *class, ...)
has return type void, but it does not initialize its output parameter @iter
when suffers class_to_subsys(@class) error, so caller can not detect the
error and call API class_dev_iter_next(@iter) which will dereference wild
pointers of @iter's members as shown by below typical usage:

// @iter's members are wild pointers
struct class_dev_iter iter;

// No change in @iter when the error happens.
class_dev_iter_init(&iter, ...);

// dereference these wild member pointers here.
while (dev = class_dev_iter_next(&iter)) { ... }.

Actually, all callers of the API have such usage pattern in kernel tree.
Fix by memset() @iter in API *_init() and error checking @iter in *_next().

Fixes: 7b884b7f24b4 ("driver core: class.c: convert to only use class_to_subsys")
Cc: stable@vger.kernel.org
Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>

---
Alternative fix solutions ever thought about:

1) Use BUG_ON(!sp) instead of error return in class_dev_iter_init().
2) Change class_dev_iter_init()'s type to int, lots of jobs to do.
---
 drivers/base/class.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/base/class.c b/drivers/base/class.c
index cb5359235c70..b331dda002e3 100644
--- a/drivers/base/class.c
+++ b/drivers/base/class.c
@@ -323,8 +323,11 @@ void class_dev_iter_init(struct class_dev_iter *iter, const struct class *class,
 	struct subsys_private *sp = class_to_subsys(class);
 	struct klist_node *start_knode = NULL;
 
-	if (!sp)
+	memset(iter, 0, sizeof(*iter));
+	if (!sp) {
+		pr_crit("%s: the class was not registered yet\n", __func__);
 		return;
+	}
 
 	if (start)
 		start_knode = &start->p->knode_class;
@@ -351,6 +354,9 @@ struct device *class_dev_iter_next(struct class_dev_iter *iter)
 	struct klist_node *knode;
 	struct device *dev;
 
+	if (!iter->sp)
+		return NULL;
+
 	while (1) {
 		knode = klist_next(&iter->ki);
 		if (!knode)

-- 
2.34.1

On Tue, Nov 05, 2024 at 08:20:22AM +0800, Zijun Hu wrote:
> From: Zijun Hu <quic_zijuhu@quicinc.com>
> 
> class_dev_iter_init(struct class_dev_iter *iter, struct class *class, ...)
> has return type void, but it does not initialize its output parameter @iter
> when suffers class_to_subsys(@class) error, so caller can not detect the
> error and call API class_dev_iter_next(@iter) which will dereference wild
> pointers of @iter's members as shown by below typical usage:
> 
> // @iter's members are wild pointers
> struct class_dev_iter iter;
> 
> // No change in @iter when the error happens.
> class_dev_iter_init(&iter, ...);
> 
> // dereference these wild member pointers here.
> while (dev = class_dev_iter_next(&iter)) { ... }.
> 
> Actually, all callers of the API have such usage pattern in kernel tree.
> Fix by memset() @iter in API *_init() and error checking @iter in *_next().
> 
> Fixes: 7b884b7f24b4 ("driver core: class.c: convert to only use class_to_subsys")
> Cc: stable@vger.kernel.org

There is no in-kernel broken users of this from what I can tell, right?
Otherwise things would have blown up by now, so why is this needed in
stable kernels?

thanks,

greg k-h

On 2024/11/12 19:43, Greg Kroah-Hartman wrote:
> On Tue, Nov 05, 2024 at 08:20:22AM +0800, Zijun Hu wrote:
>> From: Zijun Hu <quic_zijuhu@quicinc.com>
>>
>> class_dev_iter_init(struct class_dev_iter *iter, struct class *class, ...)
>> has return type void, but it does not initialize its output parameter @iter
>> when suffers class_to_subsys(@class) error, so caller can not detect the
>> error and call API class_dev_iter_next(@iter) which will dereference wild
>> pointers of @iter's members as shown by below typical usage:
>>
>> // @iter's members are wild pointers
>> struct class_dev_iter iter;
>>
>> // No change in @iter when the error happens.
>> class_dev_iter_init(&iter, ...);
>>
>> // dereference these wild member pointers here.
>> while (dev = class_dev_iter_next(&iter)) { ... }.
>>
>> Actually, all callers of the API have such usage pattern in kernel tree.
>> Fix by memset() @iter in API *_init() and error checking @iter in *_next().
>>
>> Fixes: 7b884b7f24b4 ("driver core: class.c: convert to only use class_to_subsys")
>> Cc: stable@vger.kernel.org
> 
> There is no in-kernel broken users of this from what I can tell, right?
> Otherwise things would have blown up by now, so why is this needed in
> stable kernels?
> 

For all callers of the API in current kernel tree, the class should have
been registered successfully when the API is invoking.

so, could you remove both Fix and stable tag directly?

> thanks,
> 
> greg k-h

On Tue, Nov 12, 2024 at 10:46:27PM +0800, Zijun Hu wrote:
> On 2024/11/12 19:43, Greg Kroah-Hartman wrote:
> > On Tue, Nov 05, 2024 at 08:20:22AM +0800, Zijun Hu wrote:
> >> From: Zijun Hu <quic_zijuhu@quicinc.com>
> >>
> >> class_dev_iter_init(struct class_dev_iter *iter, struct class *class, ...)
> >> has return type void, but it does not initialize its output parameter @iter
> >> when suffers class_to_subsys(@class) error, so caller can not detect the
> >> error and call API class_dev_iter_next(@iter) which will dereference wild
> >> pointers of @iter's members as shown by below typical usage:
> >>
> >> // @iter's members are wild pointers
> >> struct class_dev_iter iter;
> >>
> >> // No change in @iter when the error happens.
> >> class_dev_iter_init(&iter, ...);
> >>
> >> // dereference these wild member pointers here.
> >> while (dev = class_dev_iter_next(&iter)) { ... }.
> >>
> >> Actually, all callers of the API have such usage pattern in kernel tree.
> >> Fix by memset() @iter in API *_init() and error checking @iter in *_next().
> >>
> >> Fixes: 7b884b7f24b4 ("driver core: class.c: convert to only use class_to_subsys")
> >> Cc: stable@vger.kernel.org
> > 
> > There is no in-kernel broken users of this from what I can tell, right?
> > Otherwise things would have blown up by now, so why is this needed in
> > stable kernels?
> > 
> 
> For all callers of the API in current kernel tree, the class should have
> been registered successfully when the API is invoking.

Great, so the existing code is just fine :)

> so, could you remove both Fix and stable tag directly?

Nope, sorry.  Asking a maintainer that gets hundreds of patches to
hand-edit them does not scale.

But really, as all in-kernel users are just fine, why add additional
code if it's not needed?  THat's just going to increase our maintance
burden for the next 40+ years for no good reason.

thanks,

greg k-h

On 2024/11/12 22:57, Greg Kroah-Hartman wrote:
> On Tue, Nov 12, 2024 at 10:46:27PM +0800, Zijun Hu wrote:
>> On 2024/11/12 19:43, Greg Kroah-Hartman wrote:
>>> On Tue, Nov 05, 2024 at 08:20:22AM +0800, Zijun Hu wrote:
>>>> From: Zijun Hu <quic_zijuhu@quicinc.com>
>>>>
>>>> class_dev_iter_init(struct class_dev_iter *iter, struct class *class, ...)
>>>> has return type void, but it does not initialize its output parameter @iter
>>>> when suffers class_to_subsys(@class) error, so caller can not detect the
>>>> error and call API class_dev_iter_next(@iter) which will dereference wild
>>>> pointers of @iter's members as shown by below typical usage:
>>>>
>>>> // @iter's members are wild pointers
>>>> struct class_dev_iter iter;
>>>>
>>>> // No change in @iter when the error happens.
>>>> class_dev_iter_init(&iter, ...);
>>>>
>>>> // dereference these wild member pointers here.
>>>> while (dev = class_dev_iter_next(&iter)) { ... }.
>>>>
>>>> Actually, all callers of the API have such usage pattern in kernel tree.
>>>> Fix by memset() @iter in API *_init() and error checking @iter in *_next().
>>>>
>>>> Fixes: 7b884b7f24b4 ("driver core: class.c: convert to only use class_to_subsys")
>>>> Cc: stable@vger.kernel.org
>>>
>>> There is no in-kernel broken users of this from what I can tell, right?
>>> Otherwise things would have blown up by now, so why is this needed in
>>> stable kernels?
>>>
>>
>> For all callers of the API in current kernel tree, the class should have
>> been registered successfully when the API is invoking.
> 
> Great, so the existing code is just fine :)
> 
>> so, could you remove both Fix and stable tag directly?
> 
> Nope, sorry.  Asking a maintainer that gets hundreds of patches to
> hand-edit them does not scale.
> 
> But really, as all in-kernel users are just fine, why add additional
> code if it's not needed?  THat's just going to increase our maintance
> burden for the next 40+ years for no good reason.
> 

Hi Greg.

This fix is very worthy and necessary since it fixes the APIs self issue
and the issue is irrelevant with how various API users (in-tree or
out-of-tree) use it as below inference shown:

API class_dev_iter_init() has checks for class_to_subsys(class) error
-> the error may happen
-> once the error happens
-> wild pointers dereference must happen within API class_dev_iter_next()
-> how terrible for such issue, and the error handling also have no
prompt messages.

API users must not like silent wild pointer dereference once the error
happen.

> thanks,
> 
> greg k-h

On 2024/11/12 22:57, Greg Kroah-Hartman wrote:
> On Tue, Nov 12, 2024 at 10:46:27PM +0800, Zijun Hu wrote:
>> On 2024/11/12 19:43, Greg Kroah-Hartman wrote:
>>> On Tue, Nov 05, 2024 at 08:20:22AM +0800, Zijun Hu wrote:
>>>> From: Zijun Hu <quic_zijuhu@quicinc.com>
>>>>
>>>> class_dev_iter_init(struct class_dev_iter *iter, struct class *class, ...)
>>>> has return type void, but it does not initialize its output parameter @iter
>>>> when suffers class_to_subsys(@class) error, so caller can not detect the
>>>> error and call API class_dev_iter_next(@iter) which will dereference wild
>>>> pointers of @iter's members as shown by below typical usage:
>>>>
>>>> // @iter's members are wild pointers
>>>> struct class_dev_iter iter;
>>>>
>>>> // No change in @iter when the error happens.
>>>> class_dev_iter_init(&iter, ...);
>>>>
>>>> // dereference these wild member pointers here.
>>>> while (dev = class_dev_iter_next(&iter)) { ... }.
>>>>
>>>> Actually, all callers of the API have such usage pattern in kernel tree.
>>>> Fix by memset() @iter in API *_init() and error checking @iter in *_next().
>>>>
>>>> Fixes: 7b884b7f24b4 ("driver core: class.c: convert to only use class_to_subsys")
>>>> Cc: stable@vger.kernel.org
>>>
>>> There is no in-kernel broken users of this from what I can tell, right?
>>> Otherwise things would have blown up by now, so why is this needed in
>>> stable kernels?
>>>
>>
>> For all callers of the API in current kernel tree, the class should have
>> been registered successfully when the API is invoking.
> 
> Great, so the existing code is just fine :)
> 
>> so, could you remove both Fix and stable tag directly?
> 
> Nope, sorry.  Asking a maintainer that gets hundreds of patches to
> hand-edit them does not scale.
>
okay, let me send a updated revision now.

> But really, as all in-kernel users are just fine, why add additional
> code if it's not needed?  THat's just going to increase our maintance
> burden for the next 40+ years for no good reason.
> 

IMO, this fix is very necessary for the API.

> thanks,
> 
> greg k-h