Re: [PATCH v8 1/4] fs/proc/kcore: avoid bounce buffer for ktext data

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Mar 23, 2023 at 10:15:16AM +0000, Lorenzo Stoakes wrote:
> Commit df04abfd181a ("fs/proc/kcore.c: Add bounce buffer for ktext data")
> introduced the use of a bounce buffer to retrieve kernel text data for
> /proc/kcore in order to avoid failures arising from hardened user copies
> enabled by CONFIG_HARDENED_USERCOPY in check_kernel_text_object().
> 
> We can avoid doing this if instead of copy_to_user() we use _copy_to_user()
> which bypasses the hardening check. This is more efficient than using a
> bounce buffer and simplifies the code.
> 
> We do so as part an overall effort to eliminate bounce buffer usage in the
> function with an eye to converting it an iterator read.
> 
> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>

hi,
sorry for late feedback, but looks like this one breaks reading
/proc/kcore with objdump for me:

  # cat /proc/kallsyms | grep ksys_read
  ffffffff8150ebc0 T ksys_read
  # objdump -d  --start-address=0xffffffff8150ebc0 --stop-address=0xffffffff8150ebd0 /proc/kcore 

  /proc/kcore:     file format elf64-x86-64

  objdump: Reading section load1 failed because: Bad address

reverting this makes it work again

thanks,
jirka

> ---
>  fs/proc/kcore.c | 17 +++++------------
>  1 file changed, 5 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> index 71157ee35c1a..556f310d6aa4 100644
> --- a/fs/proc/kcore.c
> +++ b/fs/proc/kcore.c
> @@ -541,19 +541,12 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  		case KCORE_VMEMMAP:
>  		case KCORE_TEXT:
>  			/*
> -			 * Using bounce buffer to bypass the
> -			 * hardened user copy kernel text checks.
> +			 * We use _copy_to_user() to bypass usermode hardening
> +			 * which would otherwise prevent this operation.
>  			 */
> -			if (copy_from_kernel_nofault(buf, (void *)start, tsz)) {
> -				if (clear_user(buffer, tsz)) {
> -					ret = -EFAULT;
> -					goto out;
> -				}
> -			} else {
> -				if (copy_to_user(buffer, buf, tsz)) {
> -					ret = -EFAULT;
> -					goto out;
> -				}
> +			if (_copy_to_user(buffer, (char *)start, tsz)) {
> +				ret = -EFAULT;
> +				goto out;
>  			}
>  			break;
>  		default:
> -- 
> 2.39.2
>