Re: [PATCH] Revert "ACPI: custom_method: fix memory leaks"

[Greg Kroah-Hartman]

On Sun, May 02, 2021 at 10:23:26AM -0700, Kees Cook wrote:
> This reverts commit 03d1571d9513369c17e6848476763ebbd10ec2cb.
> 
> While /sys/kernel/debug/acpi/custom_method is already a privileged-only
> API providing proxied arbitrary write access to kernel memory[1][2],
> with existing race conditions[3] in buffer allocation and use that could
> lead to memory leaks and use-after-free conditions, the above commit
> appears to accidentally make the use-after-free conditions even easier
> to accomplish. ("buf" is a global variable and prior kfree()s would set
> buf back to NULL.)
> 
> This entire interface needs to be reworked (if not entirely removed).
> 
> [1] https://lore.kernel.org/lkml/20110222193250.GA23913@xxxxxxxxxxx/
> [2] https://lore.kernel.org/lkml/201906221659.B618D83@keescook/
> [3] https://lore.kernel.org/lkml/20170109231323.GA89642@beast/
> 
> Cc: Wenwen Wang <wenwen@xxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  drivers/acpi/custom_method.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/drivers/acpi/custom_method.c b/drivers/acpi/custom_method.c
> index 7b54dc95d36b..36d95a02cd30 100644
> --- a/drivers/acpi/custom_method.c
> +++ b/drivers/acpi/custom_method.c
> @@ -53,10 +53,8 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
>  	if ((*ppos > max_size) ||
>  	    (*ppos + count > max_size) ||
>  	    (*ppos + count < count) ||
> -	    (count > uncopied_bytes)) {
> -		kfree(buf);
> +	    (count > uncopied_bytes))
>  		return -EINVAL;
> -	}
>  
>  	if (copy_from_user(buf + (*ppos), user_buf, count)) {
>  		kfree(buf);
> @@ -76,7 +74,6 @@ static ssize_t cm_write(struct file *file, const char __user * user_buf,
>  		add_taint(TAINT_OVERRIDDEN_ACPI_TABLE, LOCKDEP_NOW_UNRELIABLE);
>  	}
>  
> -	kfree(buf);
>  	return count;
>  }
>  
> -- 
> 2.25.1
> 

Thanks for the revert, I'll queue it up on my larger "umn.edu reverts"
branch that I'll be sending out for review in a day or so.

greg k-h