Re: [PATCH v1] sysfs: fix sysfs_kf_seq_show null pointer dereference

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Tue, Jun 14, 2022 at 05:24:01PM +0000, Will McVicker wrote:
> When the kobj->ktype is null,

How can that happen?  What in-tree code does that?

> sysfs_file_ops() returns a NULL pointer
> for the sysfs_ops. When this happens, we hit a kernel panic in
> sysfs_kf_seq_show() by dereferencing ops to check if ->show is NULL.
> Based on commit 820879ee1865 ("sysfs: simplify sysfs_kf_seq_show"), it
> sounds like we won't hit this often, but I have randomly hit this on my
> Pixel 6 with 5.19-rc1. Refer to the crash stack below:
> 
>    Unable to handle kernel paging request at virtual address ...
>    Internal error: Oops: 96000004 [#1] PREEMPT SMP
>    Hardware name: Oriole EVT 1.0 (DT)
>    pc : sysfs_kf_seq_show+0x3c/0x160
>    lr : kernfs_seq_show+0x54/0xa0
>    Call trace:
>     sysfs_kf_seq_show+0x3c/0x160
>     kernfs_seq_show+0x54/0xa0
>     seq_read_iter+0x17c/0x638
>     kernfs_fop_read_iter+0x70/0x1f4
>     vfs_read+0x240/0x36c
>     ksys_read+0x7c/0xf0
>     __arm64_sys_read+0x20/0x30
>     invoke_syscall+0x60/0x150
>     el0_svc_common+0xb8/0x100
>     do_el0_svc+0x30/0xd4
>     el0_svc+0x30/0xc0
>     el0t_64_sync_handler+0x88/0xf8
>     el0t_64_sync+0x1a0/0x1a4
> 
> Fixes: 820879ee1865 ("sysfs: simplify sysfs_kf_seq_show")
> Signed-off-by: Will McVicker <willmcvicker@google.com>
> ---
>  fs/sysfs/file.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/sysfs/file.c b/fs/sysfs/file.c
> index a12ac0356c69..f09f86f10410 100644
> --- a/fs/sysfs/file.c
> +++ b/fs/sysfs/file.c
> @@ -45,7 +45,7 @@ static int sysfs_kf_seq_show(struct seq_file *sf, void *v)
>  	ssize_t count;
>  	char *buf;
>  
> -	if (WARN_ON_ONCE(!ops->show))
> +	if (WARN_ON_ONCE(!ops || !ops->show))
>  		return -EINVAL;

Seems reasonable, but I want to track down how ops can be NULL here
under normal operation.

thanks,

greg k-h