[PATCH] md: fix missing check on list iterator

[PATCH] md: fix missing check on list iterator

[Xiaomeng Tong]

The bug is here:
    bypass_pg(m, pg, bypassed);

The list iterator 'pg' will point to a bogus position containing
HEAD if the list is empty or no element is found. This case must
be checked before any use of the iterator, otherwise it will lead
to a invalid memory access.

To fix this bug, run bypass_pg(m, pg, bypassed); and return 0
when found, otherwise return -EINVAL.

Cc: stable@vger.kernel.org
Fixes: ^1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
---
 drivers/md/dm-mpath.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
index f4719b65e5e3..6ba8f1133564 100644
--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -1496,12 +1496,13 @@ static int bypass_pg_num(struct multipath *m, const char *pgstr, bool bypassed)
 	}
 
 	list_for_each_entry(pg, &m->priority_groups, list) {
-		if (!--pgnum)
-			break;
+		if (!--pgnum) {
+			bypass_pg(m, pg, bypassed);
+			return 0;
+		}
 	}
 
-	bypass_pg(m, pg, bypassed);
-	return 0;
+	return -EINVAL;
 }
 
 /*
-- 
2.17.1

Re: md: fix missing check on list iterator

[Mike Snitzer]

On Sun, Mar 27 2022 at  1:37P -0400,
Xiaomeng Tong <xiam0nd.tong@gmail.com> wrote:

> The bug is here:
>     bypass_pg(m, pg, bypassed);
> 
> The list iterator 'pg' will point to a bogus position containing
> HEAD if the list is empty or no element is found. This case must
> be checked before any use of the iterator, otherwise it will lead
> to a invalid memory access.
> 
> To fix this bug, run bypass_pg(m, pg, bypassed); and return 0
> when found, otherwise return -EINVAL.
> 
> Cc: stable@vger.kernel.org
> Fixes: ^1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
> ---
>  drivers/md/dm-mpath.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/md/dm-mpath.c b/drivers/md/dm-mpath.c
> index f4719b65e5e3..6ba8f1133564 100644
> --- a/drivers/md/dm-mpath.c
> +++ b/drivers/md/dm-mpath.c
> @@ -1496,12 +1496,13 @@ static int bypass_pg_num(struct multipath *m, const char *pgstr, bool bypassed)
>  	}
>  
>  	list_for_each_entry(pg, &m->priority_groups, list) {
> -		if (!--pgnum)
> -			break;
> +		if (!--pgnum) {
> +			bypass_pg(m, pg, bypassed);
> +			return 0;
> +		}
>  	}
>  
> -	bypass_pg(m, pg, bypassed);
> -	return 0;
> +	return -EINVAL;
>  }
>  
>  /*
> -- 
> 2.17.1
> 

Did you acually hit a bug (invalid memory access)?

I cannot see how given the checks prior to iterating m->priority_groups:

        if (!pgstr || (sscanf(pgstr, "%u%c", &pgnum, &dummy) != 1) || !pgnum ||
            !m->nr_priority_groups || (pgnum > m->nr_priority_groups)) {
                DMWARN("invalid PG number supplied to bypass_pg");
                return -EINVAL;
        }

So I have _not_ taken your "fix".

Re: md: fix missing check on list iterator

[Xiaomeng Tong]

On Fri, 1 Apr 2022 10:47:18 -0400, Mike Snitzer wrote:
> Did you acually hit a bug (invalid memory access)?
> 
> I cannot see how given the checks prior to iterating m->priority_groups:
> 
>         if (!pgstr || (sscanf(pgstr, "%u%c", &pgnum, &dummy) != 1) || !pgnum ||
>             !m->nr_priority_groups || (pgnum > m->nr_priority_groups)) {
>                 DMWARN("invalid PG number supplied to bypass_pg");
>                 return -EINVAL;
>         }
> 
> So I have _not_ taken your "fix".

Yes, you are correct. It has been checked before, thus not a bug and
no need to fix. And I have sent a PATCH v2 to use list iterator only
inside the loop, please check it. Thank you very much.

--
Xiaomeng Tong