Re: [PATCH] fs/proc/kcore: reinstate bounce buffer for KCORE_TEXT regions

[David Hildenbrand <david@redhat.com>]

On 31.07.23 23:50, Lorenzo Stoakes wrote:
> Some architectures do not populate the entire range categorised by
> KCORE_TEXT, so we must ensure that the kernel address we read from is
> valid.
> 
> Unfortunately there is no solution currently available to do so with a
> purely iterator solution so reinstate the bounce buffer in this instance so
> we can use copy_from_kernel_nofault() in order to avoid page faults when
> regions are unmapped.
> 
> This change partly reverts commit 2e1c0170771e ("fs/proc/kcore: avoid
> bounce buffer for ktext data"), reinstating the bounce buffer, but adapts
> the code to continue to use an iterator.
> 
> Fixes: 2e1c0170771e ("fs/proc/kcore: avoid bounce buffer for ktext data")
> Reported-by: Jiri Olsa <olsajiri@gmail.com>
> Closes: https://lore.kernel.org/all/ZHc2fm+9daF6cgCE@krava
> Cc: stable@vger.kernel.org
> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> ---
>   fs/proc/kcore.c | 26 +++++++++++++++++++++++++-
>   1 file changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> index 9cb32e1a78a0..3bc689038232 100644
> --- a/fs/proc/kcore.c
> +++ b/fs/proc/kcore.c
> @@ -309,6 +309,8 @@ static void append_kcore_note(char *notes, size_t *i, const char *name,
>   
>   static ssize_t read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter)
>   {
> +	struct file *file = iocb->ki_filp;
> +	char *buf = file->private_data;
>   	loff_t *fpos = &iocb->ki_pos;
>   	size_t phdrs_offset, notes_offset, data_offset;
>   	size_t page_offline_frozen = 1;
> @@ -554,11 +556,22 @@ static ssize_t read_kcore_iter(struct kiocb *iocb, struct iov_iter *iter)
>   			fallthrough;
>   		case KCORE_VMEMMAP:
>   		case KCORE_TEXT:
> +			/*
> +			 * Sadly we must use a bounce buffer here to be able to
> +			 * make use of copy_from_kernel_nofault(), as these
> +			 * memory regions might not always be mapped on all
> +			 * architectures.
> +			 */
> +			if (copy_from_kernel_nofault(buf, (void *)start, tsz)) {
> +				if (iov_iter_zero(tsz, iter) != tsz) {
> +					ret = -EFAULT;
> +					goto out;
> +				}
>   			/*
>   			 * We use _copy_to_iter() to bypass usermode hardening
>   			 * which would otherwise prevent this operation.
>   			 */

Having a comment at this indentation level looks for the else case looks 
kind of weird.

(does that comment still apply?)


-- 
Cheers,

David / dhildenb