Re: [PATCH] proc/kcore: fix invalid memory access in multi-page read optimization

[Omar Sandoval <osandov@osandov.com>]

On Wed, Aug 29, 2018 at 06:04:07AM +0200, Dominique Martinet wrote:
> The 'm' kcore_list item can point to kclist_head, and it is incorrect to
> look at m->addr / m->size in this case.
> There is no choice but to run through the list of entries for every address
> if we did not find any entry in the previous iteration
> 
> Fixes: bf991c2231117 ("proc/kcore: optimize multiple page reads")
> Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
> ---
> 
> I guess now I'm looking at bf991c2231117 again that it would be slightly
> more efficient to remove the !m check and initialize m to point to
> kclist_head like this:
>  m = list_entry(&kclist_head, struct kcore_list, list);
> but it feels a bit forced to me; deferring the choice to others.

Good catch! Sorry I missed this last week, Google decided this was spam
for some reason. How about fixing it like this? One less conditional in
the common case, no hacky list_entry :)

diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
index ad72261ee3fe..578926032880 100644
--- a/fs/proc/kcore.c
+++ b/fs/proc/kcore.c
@@ -464,6 +464,7 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
 				ret = -EFAULT;
 				goto out;
 			}
+			m = NULL;
 		} else if (m->type == KCORE_VMALLOC) {
 			vread(buf, (char *)start, tsz);
 			/* we have to zero-fill user buffer even if no read */

>  fs/proc/kcore.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/proc/kcore.c b/fs/proc/kcore.c
> index ad72261ee3fe..50036f6e1f52 100644
> --- a/fs/proc/kcore.c
> +++ b/fs/proc/kcore.c
> @@ -451,7 +451,8 @@ read_kcore(struct file *file, char __user *buffer, size_t buflen, loff_t *fpos)
>  		 * If this is the first iteration or the address is not within
>  		 * the previous entry, search for a matching entry.
>  		 */
> -		if (!m || start < m->addr || start >= m->addr + m->size) {
> +		if (!m || &m->list == &kclist_head || start < m->addr ||
> +		    start >= m->addr + m->size) {
>  			list_for_each_entry(m, &kclist_head, list) {
>  				if (start >= m->addr &&
>  				    start < m->addr + m->size)
> -- 
> 2.17.1
>