From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from an-out-0708.google.com ([209.85.132.247]) by canuck.infradead.org with esmtp (Exim 4.63 #1 (Red Hat Linux)) id 1Hk77K-0007ln-6w for linux-mtd@lists.infradead.org; Fri, 04 May 2007 19:23:20 -0400 Received: by an-out-0708.google.com with SMTP id d40so2573836and for ; Fri, 04 May 2007 16:23:12 -0700 (PDT) Message-ID: <463BC019.40305@gmail.com> Date: Fri, 04 May 2007 19:22:01 -0400 From: Florin Malita MIME-Version: 1.0 To: Satyam Sharma Subject: Re: [PATCH] UBI: dereference after kfree in create_vtbl References: <463A04A5.5030103@gmail.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: linux-mtd@lists.infradead.org, Linux Kernel Mailing List List-Id: Linux MTD discussion mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Hi Satyam, Satyam Sharma wrote: > Eeks ... no, wait. You found a (two, actually) bug alright, but fixed > it wrong. When we fail a write, we *must* add it to the corrupted list > and _then_ attempt to retry. So, the "if (++tries <= 5)" applies to > "if (!err) goto retry;" and not to the ubi_scan_add_to_list(). The > difference is quite subtle here ... Not being familiar with the code, I was specifically trying to preserve the old semantics and only address the use-after-free issue. So if there was another bug... well, I guess I succeeded at preserving it ;) > The correct fix should actually be as follows: (Artem, this is diffed > on the original vtbl.c) [snip] > + err = ubi_scan_add_to_list(si, new_seb->pnum, new_seb->ec, > &si->corr); > + kfree(new_seb); > + if (++tries <= 5) > if (!err) > goto retry; There's a side effect to this change: by unconditionally overwriting err we lose the original error code. Then if we're exceeding the number of retries we'll end up returning 0 which is probably not what you want. Return code aside, it seems the only thing ubi_scan_add_to_list() is doing is allocate a new struct ubi_scan_leb, initialize some fields with values passed from new_seb and then add it to the desired list. But copying new_seb to a newly allocated structure and then immediately freeing the old one seems redundant - why not just add new_seb to the corrupted list and be done? Then we don't have to deal with allocation failures in an error path anymore - something like this (diff against the original code): Signed-off-by: Florin Malita --- diff --git a/drivers/mtd/ubi/vtbl.c b/drivers/mtd/ubi/vtbl.c index b6fd6bb..2ad2d59 100644 --- a/drivers/mtd/ubi/vtbl.c +++ b/drivers/mtd/ubi/vtbl.c @@ -317,14 +317,10 @@ retry: return err; write_error: - kfree(new_seb); - /* May be this physical eraseblock went bad, try to pick another one */ - if (++tries <= 5) { - err = ubi_scan_add_to_list(si, new_seb->pnum, new_seb->ec, - &si->corr); - if (!err) - goto retry; - } + /* Maybe this physical eraseblock went bad, try to pick another one */ + list_add_tail(&new_seb->u.list, &si->corr); + if (++tries <= 5) + goto retry; out_free: ubi_free_vid_hdr(ubi, vid_hdr); return err;