linux-kernel - Re: [PATCH] ceph: fix possible null-pointer dereference in ceph_mdsmap

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Date:   Thu, 5 Aug 2021 20:52:57 +0800
From:   Tuo Li <islituo@...il.com>
To:     Jeff Layton <jlayton@...nel.org>, idryomov@...il.com
Cc:     ceph-devel@...r.kernel.org, linux-kernel@...r.kernel.org,
        baijiaju1990@...il.com, TOTE Robot <oslab@...nghua.edu.cn>
Subject: Re: [PATCH] ceph: fix possible null-pointer dereference in
 ceph_mdsmap_decode()

Thanks for your feedback. It sounds good to put an "if (m->m_info)" around
the for loop to fix this bug, as well as avoid other potential bugs 
caused by calling
ceph_mdsmap_destroy(). I think we can prepare a V2 patch according to 
your advice.


On 2021/8/5 20:29, Jeff Layton wrote:
> On Thu, 2021-08-05 at 05:20 -0700, Tuo Li wrote:
>> kcalloc() is called to allocate memory for m->m_info, and if it fails,
>> ceph_mdsmap_destroy() behind the label out_err will be called:
>>    ceph_mdsmap_destroy(m);
>>
>> In ceph_mdsmap_destroy(), m->m_info is dereferenced through:
>>    kfree(m->m_info[i].export_targets);
>>
>> To fix this possible null-pointer dereference, if memory allocation
>> for m->m_info fails, free m and return -ENOMEM.
>>
>> Reported-by: TOTE Robot <oslab@...nghua.edu.cn>
>> Signed-off-by: Tuo Li <islituo@...il.com>
>> ---
>>   fs/ceph/mdsmap.c | 6 ++++--
>>   1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/ceph/mdsmap.c b/fs/ceph/mdsmap.c
>> index abd9af7727ad..7d73e4b64b12 100644
>> --- a/fs/ceph/mdsmap.c
>> +++ b/fs/ceph/mdsmap.c
>> @@ -166,8 +166,10 @@ struct ceph_mdsmap *ceph_mdsmap_decode(void **p, void *end, bool msgr2)
>>   	m->possible_max_rank = max(m->m_num_active_mds, m->m_max_mds);
>>   
>>   	m->m_info = kcalloc(m->possible_max_rank, sizeof(*m->m_info), GFP_NOFS);
>> -	if (!m->m_info)
>> -		goto nomem;
>> +	if (!m->m_info) {
>> +		kfree(m);
>> +		return ERR_PTR(-ENOMEM);
>> +	}
>>   
>>   	/* pick out active nodes from mds_info (state > 0) */
>>   	for (i = 0; i < n; i++) {
> Good catch. This function is already pretty complex. How about we
> instead fix this in ceph_mdsmap_destroy and make it safe to call that
> with the mdsmap in this state?
>
> Basically, just put an "if (m->m_info)" around the for loop in that
> function. Sound ok?
>