Re: [PATCH v7] kobject: Fix global-out-of-bounds in kobject_action_type()

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, May 18, 2023 at 05:16:14PM +0800, Xia Fukun wrote:
> The following c language code can trigger KASAN's global variable
> out-of-bounds access error in kobject_action_type():
> 
> int main() {
>     int fd;
>     char *filename = "/sys/block/ram12/uevent";
>     char str[86] = "offline";
>     int len = 86;
> 
>     fd = open(filename, O_WRONLY);
>     if (fd == -1) {
>         printf("open");
>         exit(1);
>     }
> 
>     if (write(fd, str, len) == -1) {
>         printf("write");
>         exit(1);
>     }
> 
>     close(fd);
>     return 0;
> }
> 
> Function kobject_action_type() receives the input parameters buf and count,
> where count is the length of the string buf.
> 
> In the use case we provided, count is 86, the count_first is 85.
> Buf points to a string with a length of 86, and its first seven characters
> are "offline". In the for loop, kobject_actions[action] is the string
> "offline" with the length of 7,an out-of-boundary access will appear:
> 
> kobject_actions[action][85].
> 
> Use sysfs_match_string() to replace the fragile and convoluted loop.
> This function is well-tested for parsing sysfs inputs. Moreover, this
> modification will not cause any functional changes.
> 
> Fixes: f36776fafbaa ("kobject: support passing in variables for synthetic uevents")
> Signed-off-by: Xia Fukun <xiafukun@huawei.com>
> ---
> v6 -> v7:
> -  Move macro UEVENT_KACT_STRSIZE to the .c file to 
> improve maintainability.
> 
> v5 -> v6:
> - Ensure that the following extensions remain effective:
> https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-uevent
> 
> v4 -> v5:
> - Fixed build errors and warnings, and retested the patch.
> 
> v3 -> v4:
> - Refactor the function to be more obviously correct and readable.
> ---
>  lib/kobject_uevent.c | 38 +++++++++++++++++++++++++-------------
>  1 file changed, 25 insertions(+), 13 deletions(-)
> 
> diff --git a/lib/kobject_uevent.c b/lib/kobject_uevent.c
> index 7c44b7ae4c5c..2171e1648dad 100644
> --- a/lib/kobject_uevent.c
> +++ b/lib/kobject_uevent.c
> @@ -47,6 +47,14 @@ static LIST_HEAD(uevent_sock_list);
>  /* This lock protects uevent_seqnum and uevent_sock_list */
>  static DEFINE_MUTEX(uevent_sock_mutex);
>  
> +/*
> + * The maximum length of the string contained in kobject_actions[].
> + * If there are any actions added or modified, please ensure that
> + * the string length does not exceed the macro, otherwise
> + * should modify the macro definition.
> + */
> +#define UEVENT_KACT_STRSIZE		16

But the biggest size here is not 16, it's 6.  So where did 16 come from?
Why not dynamically determine the biggest size at runtime?

thanks,

greg k-h