linux-kernel - Re: [PATCH] powerpc/mm: Fix null-pointer dereference in pgtable_cache

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Message-ID: <87y1ei2jwa.fsf@mail.lhotse>
Date:   Tue, 28 Nov 2023 22:32:21 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Kunwu Chan <chentao@...inos.cn>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "npiggin@...il.com" <npiggin@...il.com>
Cc:     "kunwu.chan@...mail.com" <kunwu.chan@...mail.com>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] powerpc/mm: Fix null-pointer dereference in
 pgtable_cache_add

Kunwu Chan <chentao@...inos.cn> writes:
> Hi Christophe,
>
> Thanks for your reply.
> It's my bad. According your reply, i read the code in 
> sysfs_do_create_link_sd.There is a null pointer check indeed.
>
> My intention was to check null pointer after memory allocation.
> Whether we can add a comment here for someone like me, the null pointer 
> check is no need here?

I don't mind there being a NULL check for name.

But the code shouldn't silently return if name can't be allocated.
Notice that if we can't create the cache we *panic*. A failure to
allocate name, which causes us to skip the cache creation, needs to also
panic.

cheers

> On 2023/11/24 23:17, Christophe Leroy wrote:
>> 
>> 
>> Le 22/11/2023 à 10:00, Kunwu Chan a écrit :
>>> [Vous ne recevez pas souvent de courriers de chentao@...inos.cn. Découvrez pourquoi ceci est important à https://aka.ms/LearnAboutSenderIdentification ]
>>>
>>> kasprintf() returns a pointer to dynamically allocated memory
>>> which can be NULL upon failure. Ensure the allocation was successful
>>> by checking the pointer validity.
>> 
>> Are you sure this is needed ? Did you check what happens what name is NULL ?
>> 
>> If I followed stuff correctly, I end up in function
>> sysfs_do_create_link_sd() which already handles the NULL name case which
>> a big hammer warning.
>> 
>>>
>>> Signed-off-by: Kunwu Chan <chentao@...inos.cn>
>>> ---
>>>    arch/powerpc/mm/init-common.c | 2 ++
>>>    1 file changed, 2 insertions(+)
>>>
>>> diff --git a/arch/powerpc/mm/init-common.c b/arch/powerpc/mm/init-common.c
>>> index 119ef491f797..0884fc601c46 100644
>>> --- a/arch/powerpc/mm/init-common.c
>>> +++ b/arch/powerpc/mm/init-common.c
>>> @@ -139,6 +139,8 @@ void pgtable_cache_add(unsigned int shift)
>>>
>>>           align = max_t(unsigned long, align, minalign);
>>>           name = kasprintf(GFP_KERNEL, "pgtable-2^%d", shift);
>>> +       if (!name)
>>> +               return;
>>>           new = kmem_cache_create(name, table_size, align, 0, ctor(shift));
>>>           if (!new)
>>>                   panic("Could not allocate pgtable cache for order %d", shift);
>>> --
>>> 2.34.1
>>>