Re: [PATCH] UBI: dereference after kfree in create_vtbl

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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 <[email protected]>
---

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;

-
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