Re: [PATCH riscv64] kobject: fix WARNING in input_register_device

Edward Adam Davis
2024-02-13T00:43:36Z

On Thu, 8 Feb 2024 12:25:10 +0000, Greg KH wrote:
> On Thu, Feb 08, 2024 at 07:37:56PM +0800, Edward Adam Davis wrote:
> > On Thu, 8 Feb 2024 10:56:00, Greg KH wrote:
> > > > The input_add_uevent_modalias_var()->input_print_modalias() will add 1684 bytes
> > > > of data to env, which will result in insufficient memory allocated to the buf
> > > > members of env.
> > >
> > > What is "env"? And can you wrap your lines at 72 columns please?
> > env is an instance of struct kobj_uevent_env.
>
> Ok, be specific please in your changelog text, otherwise we can't really
> understand what is happening.
>
> > > > Reported-and-tested-by: syzbot+8e41bb0c055b209ebbf4@syzkaller.appspotmail.com
> > > > Signed-off-by: Edward Adam Davis <eadavis@qq.com>
> > > > ---
> > > > include/linux/kobject.h | 2 +-
> > > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/include/linux/kobject.h b/include/linux/kobject.h
> > > > index c30affcc43b4..74b37b6459cd 100644
> > > > --- a/include/linux/kobject.h
> > > > +++ b/include/linux/kobject.h
> > > > @@ -30,7 +30,7 @@
> > > >
> > > > #define UEVENT_HELPER_PATH_LEN 256
> > > > #define UEVENT_NUM_ENVP 64 /* number of env pointers */
> > > > -#define UEVENT_BUFFER_SIZE 2048 /* buffer for the variables */
> > > > +#define UEVENT_BUFFER_SIZE 2560 /* buffer for the variables */
> > >
> > > That's an odd number, why that? Why not just a page? What happens if
> > > some other path wants more?
> > An increase of 512 bytes is sufficient for the current issue. Do not consider
> > the problem of hypothetical existence.
>
> Why is this 512 bytes sufficient now? What changed to cause this?
There is the following code in input_print_modalias():

drivers/input/input.c
1 len += input_print_modalias_bits(buf + len, size - len,
1403 'k', id->keybit, KEY_MIN_INTERESTING, KEY_MAX);
This code will add up to 2608 bytes of data to env at most.
(KEY_MAX - KEY_MIN_INTERESTING) * 4 = (256 * 3 - 1 - 113 ) * 4 = (765 - 113) * 4 = 652 * 4 = 2608 bytes。
Note: In the expression, 4 represents 3 bytes of hexadecimal data and 1 byte of comma.

include/uapi/linux/input-event-codes.h
188 #define KEY_MUTE 113
807 #define KEY_MIN_INTERESTING KEY_MUTE
808 #define KEY_MAX 0x2ff
During my actual testing process, I found that a total of 1684 bytes were
contributed in input_print_modalias().
>
> And how can we detect this automatically in the future? Shouldn't we
> just be truncating the buffer instead of having an overflow?
>
> > > And what's causing the input stack to have so many variables all of a
> > > sudden, what changed to cause this? Is this a bugfix for a specific
> > > commit that needs to be backported to older kernels? Why did this
> > > buffer size all of a sudden be too small?
> > The result of my analysis is that several members of struct input_dev are too
> > large, such as its member keybit.
>
> And when did that change? What commit id? What prevents it from
> growing again and us needing to change this again?
The code that caused this issue has been introduced for a long time, and it is
speculated that it was due to the fact that the warning in add_uevent_var() was
returned directly to ENOMEM without being taken seriously.

lib/kobject_uevent.c
2 if (len >= (sizeof(env->buf) - env->buflen)) {
1 WARN(1, KERN_ERR "add_uevent_var: buffer size too small\n");
672 return -ENOMEM;
1 }

I believe that this issue was introduced by:
7eff2e7a8b65 - Driver core: change add_ueventvar to use a struct.

thanks,
edward.