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

Sergei Shtylyov
2018-01-06T10:05:43Z

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@zeniv.linux.org.uk>
> Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> ---
> 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