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

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Message-ID: <5586e0f0-8e4f-4096-8383-bf3f80fae6f7@kylinos.cn>
Date:   Thu, 30 Nov 2023 16:45:22 +0800
From:   Kunwu Chan <chentao@...inos.cn>
To:     Michael Ellerman <mpe@...erman.id.au>,
        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

Thanks for your reply.

Ok, I know what you mean, when name is NULL. The process should be 
aborted and the specific reason for the error should be printed, not 
just return.

I will update v2 patch with "panic".

Thanks again,
Kunwu

On 2023/11/28 19:32, Michael Ellerman wrote:
> 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
>>>>