Re: [PATCH 15/18] vfs, fdtable: prevent bounds-check bypass via speculative execution

[Sergei Shtylyov]

On 1/6/2018 4:11 AM, Dan Williams wrote:

> Expectedly, static analysis reports that 'fd' is a user controlled value
> that is used as a data dependency to read from the 'fdt->fd' array.  In
> order to avoid potential leaks of kernel memory values, block
> speculative execution of the instruction stream that could issue reads
> based on an invalid 'file *' returned from __fcheck_files.
>
> Based on an original patch by Elena Reshetova.
>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
> Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> ---
>   include/linux/fdtable.h |    5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
> index 1c65817673db..4a147c5c2533 100644
> --- a/include/linux/fdtable.h
> +++ b/include/linux/fdtable.h
> @@ -81,9 +81,10 @@ struct dentry;
>   static inline struct file *__fcheck_files(struct files_struct *files, unsigned int fd)
>   {
>   	struct fdtable *fdt = rcu_dereference_raw(files->fdt);
> +	struct file __rcu **fdp;
>   
> -	if (fd < fdt->max_fds)
> -		return rcu_dereference_raw(fdt->fd[fd]);
> +	if ((fdp = nospec_array_ptr(fdt->fd, fd, fdt->max_fds)))

   And here...

> +		return rcu_dereference_raw(*fdp);
>   	return NULL;
>   }
>   

MMR, Sergei