Re: [PATCH 1/3] sysfs: Fix crash on empty group attributes array

[Rafael J. Wysocki]

On Thu, Feb 22, 2024 at 9:40 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> It turns out that arch/x86/events/intel/core.c makes use of "empty"
> attributes.
>
>         static struct attribute *empty_attrs;
>
>         __init int intel_pmu_init(void)
>         {
>                 struct attribute **extra_skl_attr = &empty_attrs;
>                 struct attribute **extra_attr = &empty_attrs;
>                 struct attribute **td_attr    = &empty_attrs;
>                 struct attribute **mem_attr   = &empty_attrs;
>                 struct attribute **tsx_attr   = &empty_attrs;
>                 ...
>
> That breaks the assumption __first_visible() that expects that if
> grp->attrs is set then grp->attrs[0] must also be set and results in
> backtraces like:
>
>     BUG: kernel NULL pointer dereference, address: 00rnel mode
>     #PF: error_code(0x0000) - not-present ] PREEMPT SMP NOPTI
>     CPU: 1 PID: 1 Comm: swapper/IP: 0010:exra_is_visible+0x14/0x20
>      ? exc_page_fault+0x68/0x190
>      internal_create_groups+0x42/0xa0
>      pmu_dev_alloc+0xc0/0xe0
>      perf_event_sysfs_init+0x580000000000 ]---
>     RIP: 0010:exra_is_visible+0x14/0
>
> Check for non-empty attributes array before calling is_visible().
>
> Reported-by: Pierre-Louis Bossart <pierre-louis.bossart@xxxxxxxxxxxxxxx>
> Closes: https://github.com/thesofproject/linux/pull/4799#issuecomment-1958537212
> Fixes: 70317fd24b41 ("sysfs: Introduce a mechanism to hide static attribute_groups")

This is not in the mainline, so linux-next I suppose?

> Cc: Marc Herbert <marc.herbert@xxxxxxxxx>
> Cc: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>  fs/sysfs/group.c |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/sysfs/group.c b/fs/sysfs/group.c
> index ccb275cdabcb..8c63ba3cfc47 100644
> --- a/fs/sysfs/group.c
> +++ b/fs/sysfs/group.c
> @@ -33,10 +33,10 @@ static void remove_files(struct kernfs_node *parent,
>
>  static umode_t __first_visible(const struct attribute_group *grp, struct kobject *kobj)
>  {
> -       if (grp->attrs && grp->is_visible)
> +       if (grp->attrs && grp->attrs[0] && grp->is_visible)
>                 return grp->is_visible(kobj, grp->attrs[0], 0);
>
> -       if (grp->bin_attrs && grp->is_bin_visible)
> +       if (grp->bin_attrs && grp->bin_attrs[0] && grp->is_bin_visible)
>                 return grp->is_bin_visible(kobj, grp->bin_attrs[0], 0);
>
>         return 0;
>