Re: [PATCH bpf-next v3 06/17] HID: allow to change the report descriptor from an eBPF program

[Benjamin Tissoires]

On Fri, Mar 18, 2022 at 10:10 PM Song Liu <song@xxxxxxxxxx> wrote:
>
> On Fri, Mar 18, 2022 at 9:17 AM Benjamin Tissoires
> <benjamin.tissoires@xxxxxxxxxx> wrote:
> >
> > Make use of BPF_HID_ATTACH_RDESC_FIXUP so we can trigger an rdesc fixup
> > in the bpf world.
> >
> > Whenever the program gets attached/detached, the device is reconnected
> > meaning that userspace will see it disappearing and reappearing with
> > the new report descriptor.
> >
> > Signed-off-by: Benjamin Tissoires <benjamin.tissoires@xxxxxxxxxx>
> >
> > ---
> >
> > changes in v3:
> > - ensure the ctx.size is properly bounded by allocated size
> > - s/link_attached/post_link_attach/
> > - removed the switch statement with only one case
> >
> > changes in v2:
> > - split the series by bpf/libbpf/hid/selftests and samples
> > ---
> >  drivers/hid/hid-bpf.c  | 62 ++++++++++++++++++++++++++++++++++++++++++
> >  drivers/hid/hid-core.c |  3 +-
> >  include/linux/hid.h    |  6 ++++
> >  3 files changed, 70 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/hid/hid-bpf.c b/drivers/hid/hid-bpf.c
> > index 5060ebcb9979..45c87ff47324 100644
> > --- a/drivers/hid/hid-bpf.c
> > +++ b/drivers/hid/hid-bpf.c
> > @@ -50,6 +50,14 @@ static struct hid_device *hid_bpf_fd_to_hdev(int fd)
> >         return hdev;
> >  }
> >
> > +static int hid_reconnect(struct hid_device *hdev)
> > +{
> > +       if (!test_and_set_bit(ffs(HID_STAT_REPROBED), &hdev->status))
> > +               return device_reprobe(&hdev->dev);
> > +
> > +       return 0;
> > +}
> > +
> >  static int hid_bpf_pre_link_attach(struct hid_device *hdev, enum bpf_hid_attach_type type)
> >  {
> >         int err = 0;
> > @@ -92,6 +100,12 @@ static int hid_bpf_pre_link_attach(struct hid_device *hdev, enum bpf_hid_attach_
> >         return err;
> >  }
> >
> > +static void hid_bpf_post_link_attach(struct hid_device *hdev, enum bpf_hid_attach_type type)
> > +{
> > +       if (type == BPF_HID_ATTACH_RDESC_FIXUP)
> > +               hid_reconnect(hdev);
> > +}
> > +
> >  static void hid_bpf_array_detach(struct hid_device *hdev, enum bpf_hid_attach_type type)
> >  {
> >         switch (type) {
> > @@ -99,6 +113,9 @@ static void hid_bpf_array_detach(struct hid_device *hdev, enum bpf_hid_attach_ty
> >                 kfree(hdev->bpf.device_data);
> >                 hdev->bpf.device_data = NULL;
> >                 break;
> > +       case BPF_HID_ATTACH_RDESC_FIXUP:
> > +               hid_reconnect(hdev);
> > +               break;
> >         default:
> >                 /* do nothing */
> >                 break;
> > @@ -116,6 +133,9 @@ static int hid_bpf_run_progs(struct hid_device *hdev, struct hid_bpf_ctx_kern *c
> >         case HID_BPF_DEVICE_EVENT:
> >                 type = BPF_HID_ATTACH_DEVICE_EVENT;
> >                 break;
> > +       case HID_BPF_RDESC_FIXUP:
> > +               type = BPF_HID_ATTACH_RDESC_FIXUP;
> > +               break;
> >         default:
> >                 return -EINVAL;
> >         }
> > @@ -155,11 +175,53 @@ u8 *hid_bpf_raw_event(struct hid_device *hdev, u8 *data, int *size)
> >         return ctx.data;
> >  }
> >
> > +u8 *hid_bpf_report_fixup(struct hid_device *hdev, u8 *rdesc, unsigned int *size)
> > +{
> > +       int ret;
> > +       struct hid_bpf_ctx_kern ctx = {
> > +               .type = HID_BPF_RDESC_FIXUP,
> > +               .hdev = hdev,
> > +               .size = *size,
> > +       };
> > +
> > +       if (bpf_hid_link_empty(&hdev->bpf, BPF_HID_ATTACH_RDESC_FIXUP))
>
> Do we need to lock bpf_hid_mutex before calling bpf_hid_link_empty()?
> (or maybe we
> already did?)

The mutex is not locked before this call, indeed.

However, bpf_hid_link_empty() is an inlined function that just calls
in the end list_empty(). Given that all the list heads are created
just once for the entire life of the HID device, I *think* this is
thread safe and does not require mutex locking.

(I might be wrong)

So when first plugging in the device, if there is a fighting process
that attempts to add a program, if the program managed to insert
itself before we enter this code, then the list won't be empty and we
will execute BPF_PROG_RUN_ARRAY(), and if not, well, we ignore it and
wait for reconnect().

But now I am starting to wonder if I need to also protect
BPF_PROG_RUN_ARRAY() under bpf_hid_mutex...

Cheers,
Benjamin

>
>
> > +               goto ignore_bpf;
> > +
> > +       ctx.data = kmemdup(rdesc, HID_MAX_DESCRIPTOR_SIZE, GFP_KERNEL);
> > +       if (!ctx.data)
> > +               goto ignore_bpf;
> > +
> > +       ctx.allocated_size = HID_MAX_DESCRIPTOR_SIZE;
> > +
> > +       ret = hid_bpf_run_progs(hdev, &ctx);
> > +       if (ret)
> > +               goto ignore_bpf;
> > +
> > +       if (ctx.size > ctx.allocated_size)
> > +               goto ignore_bpf;
> > +
> > +       *size = ctx.size;
> > +
> > +       if (*size) {
> > +               rdesc = krealloc(ctx.data, *size, GFP_KERNEL);
> > +       } else {
> > +               rdesc = NULL;
> > +               kfree(ctx.data);
> > +       }
> > +
> > +       return rdesc;
> > +
> > + ignore_bpf:
> > +       kfree(ctx.data);
> > +       return kmemdup(rdesc, *size, GFP_KERNEL);
> > +}
> > +
> >  int __init hid_bpf_module_init(void)
> >  {
> >         struct bpf_hid_hooks hooks = {
> >                 .hdev_from_fd = hid_bpf_fd_to_hdev,
> >                 .pre_link_attach = hid_bpf_pre_link_attach,
> > +               .post_link_attach = hid_bpf_post_link_attach,
> >                 .array_detach = hid_bpf_array_detach,
> >         };
> >
> > diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
> > index 937fab7eb9c6..3182c39db006 100644
> > --- a/drivers/hid/hid-core.c
> > +++ b/drivers/hid/hid-core.c
> > @@ -1213,7 +1213,8 @@ int hid_open_report(struct hid_device *device)
> >                 return -ENODEV;
> >         size = device->dev_rsize;
> >
> > -       buf = kmemdup(start, size, GFP_KERNEL);
> > +       /* hid_bpf_report_fixup() ensures we work on a copy of rdesc */
> > +       buf = hid_bpf_report_fixup(device, start, &size);
> >         if (buf == NULL)
> >                 return -ENOMEM;
> >
> > diff --git a/include/linux/hid.h b/include/linux/hid.h
> > index 8fd79011f461..66d949d10b78 100644
> > --- a/include/linux/hid.h
> > +++ b/include/linux/hid.h
> > @@ -1213,10 +1213,16 @@ do {                                                                    \
> >
> >  #ifdef CONFIG_BPF
> >  u8 *hid_bpf_raw_event(struct hid_device *hdev, u8 *rd, int *size);
> > +u8 *hid_bpf_report_fixup(struct hid_device *hdev, u8 *rdesc, unsigned int *size);
> >  int hid_bpf_module_init(void);
> >  void hid_bpf_module_exit(void);
> >  #else
> >  static inline u8 *hid_bpf_raw_event(struct hid_device *hdev, u8 *rd, int *size) { return rd; }
> > +static inline u8 *hid_bpf_report_fixup(struct hid_device *hdev, u8 *rdesc,
> > +                                      unsigned int *size)
> > +{
> > +       return kmemdup(rdesc, *size, GFP_KERNEL);
> > +}
> >  static inline int hid_bpf_module_init(void) { return 0; }
> >  static inline void hid_bpf_module_exit(void) {}
> >  #endif
> > --
> > 2.35.1
> >
>