Re: [PATCH] fs/proc/kcore.c: use probe_kernel_read() instead of memcpy()

[Kees Cook]

On Sat, Dec 2, 2017 at 5:27 AM, Heiko Carstens
<heiko.carstens@xxxxxxxxxx> wrote:
> git commit df04abfd181a ("fs/proc/kcore.c: Add bounce buffer for ktext
> data") added a bounce buffer to avoid hardened usercopy
> checks. Copying to the bounce buffer was implemented with a simple
> memcpy() assuming that it is always valid to read from kernel memory
> iff the kern_addr_valid() check passed.
>
> A simple, but pointless, test case like "dd if=/proc/kcore
> of=/dev/null" now can easily crash the kernel, since the former
> execption handling on invalid kernel addresses now doesn't work
> anymore.
>
> Also adding a kern_addr_valid() implementation wouldn't help
> here. Most architectures simply return 1 here, while a couple
> implemented a page table walk to figure out if something is mapped at
> the address in question.
> With DEBUG_PAGEALLOC active mappings are established and removed all
> the time, so that relying on the result of kern_addr_valid() before
> executing the memcpy() also doesn't work.
>
> Therefore simply use probe_kernel_read() to copy to the bounce
> buffer. This also allows to simplify read_kcore().
>
> At least on s390 this fixes the observed crashes and doesn't introduce
> warnings that were removed with df04abfd181a ("fs/proc/kcore.c: Add
> bounce buffer for ktext data"), even though the generic
> probe_kernel_read() implementation uses uaccess functions.
>
> While looking into this I'm also wondering if kern_addr_valid() could
> be completely removed...(?)
>
> Fixes: df04abfd181a ("fs/proc/kcore.c: Add bounce buffer for ktext data")
> Fixes: f5509cc18daa ("mm: Hardened usercopy")
> Signed-off-by: Heiko Carstens <heiko.carstens@xxxxxxxxxx>

Thanks for the catch! Yeah, this matches what I just sent to Greg for /dev/mem:
https://lkml.org/lkml/2017/12/1/792

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
>  fs/proc/kcore.c | 18 +++++-------------
>  1 file changed, 5 insertions(+), 13 deletions(-)
>
> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> index 4bc85cb8be6a..e8a93bc8285d 100644
> --- a/fs/proc/kcore.c
> +++ b/fs/proc/kcore.c
> @@ -512,23 +512,15 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>                                 return -EFAULT;
>                 } else {
>                         if (kern_addr_valid(start)) {
> -                               unsigned long n;
> -
>                                 /*
>                                  * Using bounce buffer to bypass the
>                                  * hardened user copy kernel text checks.
>                                  */
> -                               memcpy(buf, (char *) start, tsz);
> -                               n = copy_to_user(buffer, buf, tsz);
> -                               /*
> -                                * We cannot distinguish between fault on source
> -                                * and fault on destination. When this happens
> -                                * we clear too and hope it will trigger the
> -                                * EFAULT again.
> -                                */
> -                               if (n) {
> -                                       if (clear_user(buffer + tsz - n,
> -                                                               n))
> +                               if (probe_kernel_read(buf, (void *) start, tsz)) {
> +                                       if (clear_user(buffer, tsz))
> +                                               return -EFAULT;
> +                               } else {
> +                                       if (copy_to_user(buffer, buf, tsz))
>                                                 return -EFAULT;
>                                 }
>                         } else {
> --
> 2.13.5
>



-- 
Kees Cook
Pixel Security