Date:   Thu, 2 Feb 2023 22:18:18 +0800
From:   Coly Li <colyli@...e.de>
To:     Zheng Hacker <hackerzheng666@...il.com>
Cc:     Zheng Wang <zyytlz.wz@....com>,
        Kent Overstreet <kent.overstreet@...il.com>,
        linux-bcache@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        alex000young@...il.com
Subject: Re: [PATCH] bcache: Fix a NULL or wild pointer dereference in
 btree_split



> 2023年2月2日 22:11，Zheng Hacker <hackerzheng666@...il.com> 写道：
> 
> Coly Li <colyli@...e.de> 于2023年2月2日周四 20:22写道：
> 
>> Hmm, there should be something to be fixed, but not the non-exist NULL dereference.
>> 
>> If you look inside btree_node_alloc_replacement(), you may find __bch_btree_node_alloc() which does the real work indeed. And yes, I would suggest you to improve a bit inside __bch_btree_node_alloc().
>> 
>> 1088 struct btree *__bch_btree_node_alloc(struct cache_set *c, struct btree_op *op,
>> [snipped]
>> 1093         struct btree *b = ERR_PTR(-EAGAIN);
>> 1094
>> 1095         mutex_lock(&c->bucket_lock);
>> 1096 retry:
>> 1097         if (__bch_bucket_alloc_set(c, RESERVE_BTREE, &k.key, wait))
>> 1098                 goto err;
>> [snipped]
>> 1102
>> 1103         b = mca_alloc(c, op, &k.key, level);
>> 1104         if (IS_ERR(b))
>> 1105                 goto err_free;
>> 1106
>> 1107         if (!b) {
>> 1108                 cache_bug(c,
>> 1109                         "Tried to allocate bucket that was in btree cache");
>> 1110                 goto retry;
>> 1111         }
>> 1112
>> 
>> From the above code, at line 1097 if __bch_bucket_alloc_set() returns non-zero value, the code will jump to label err: and returns ERR_PTR(-EAGAIN). But if the code fails at line 1103 and b is set to NULL, at line 1110 the code will jump back to label retry: and calls __bch_bucket_alloc_set() again. If the second time __bch_bucket_alloc_set() returns non-zero value and the code jumps to label err:, a NULL pointer other than ERR_PTR(-EAGAIN) will be returned. This inconsistent return value on same location at line 1097 seems incorrect, where ERR_PTR(-EAGAIN) should be returned IMHO.
>> 
>> Therefore I feel that “b = ERR_PTR(-EAGAIN)” should be moved to the location after label retry:, then btree_node_alloc_replacement() will only return error code, and no NULL pointer returned.
>> 
>> After this change, all following IS_ERR_OR_NULL() checks to btree_node_alloc_replacement() are unnecessary and IS_ERR() just enough, because no NULL will be returned.
>> 
>> This is a nice catch. I’d like to have it to be fixed. I do appreciate if you want to compose two patches for the fix. Otherwise I can handle it myself.
>> 
> Hi Coly,
> 
> Thanks for your reply and detailed explaination! As you explain, I
> found __bch_btree_node_alloc may return NULL in some situation. So I
> add some more check in upper code.
> Your suggestion is more constructive. It'll make the function more
> clear for other developer. I'd like to help with the patch. And you
> have kindly pointed the right way to fix.
> May I merge fix it in one patch with the commit msg "refactor
> __bch_btree_node_alloc to avoid poential NULL dereference"? Because I
> think if __bch_btree_node_alloc returns
> NULL to bch_btree_node_alloc, the function
> btree_node_alloc_replacement will strill return NULL to n1 in
> btree_split. I think the possibility is low, if it's not proper,
> please feel free
> to let me know.

This is not a refactor indeed, just a simple fix to __bch_btree_node_alloc() to make the failure behavior of calling __bch_bucket_alloc_set() at line 1097 to be consistent. A.K.A always returning ERR_PTR(-EAGAIN) when it returns failure.

Another optional patch is to change the unnecessary IS_ERR_OR_NULL() to IS_ERR() in proper locations, because after the first fix, NULL won’t be returned indeed. And extra code comments on why IS_ERR() is sufficient might be preferred IMHO.

Thanks.

Coly Li

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