linux-kernel - Re: [PATCH] mpi: Avoid using freed pointer in mpi_lshift

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Message-ID: <CALLzPKbAGKL-o66UmWJ4uGSmA1TymdCQA0CfBajHLVYzt_60UA@mail.gmail.com>
Date:	Tue, 13 Mar 2012 14:34:32 +0200
From:	"Kasatkin, Dmitry" <dmitry.kasatkin@...el.com>
To:	Jesper Juhl <jj@...osbits.net>
Cc:	linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	James Morris <jmorris@...ei.org>
Subject: Re: [PATCH] mpi: Avoid using freed pointer in mpi_lshift_limbs()

On Thu, Mar 1, 2012 at 11:33 PM, Jesper Juhl <jj@...osbits.net> wrote:
>
> Ping?
>
> On Mon, 6 Feb 2012, Jesper Juhl wrote:
>
>> At the start of the function we assign 'a->d' to 'ap'. Then we use the
>> RESIZE_IF_NEEDED macro on 'a' - this may free 'a->d' and replace it
>> with newly allocaetd storage. In that case, we'll be operating on
>> freed memory further down in the function when we index into 'ap[]'.
>> Since we don't actually need 'ap' until after the use of the
>> RESIZE_IF_NEEDED macro we can just delay the assignment to it until
>> after we've potentially resized, thus avoiding the issue.
>>
>> While I was there anyway I also changed the integer variable 'n' to be
>> const. It might as well be since we only assign to it once and use it
>> as a constant, and then the compiler will tell us if we ever assign to
>> it in the future.
>>
>> Signed-off-by: Jesper Juhl <jj@...osbits.net>
>> ---
>>  lib/mpi/mpi-bit.c |    5 +++--
>>  1 files changed, 3 insertions(+), 2 deletions(-)
>>
>>  only compile tested.
>>
>> diff --git a/lib/mpi/mpi-bit.c b/lib/mpi/mpi-bit.c
>> index 2f52662..0c50536 100644
>> --- a/lib/mpi/mpi-bit.c
>> +++ b/lib/mpi/mpi-bit.c
>> @@ -177,8 +177,8 @@ int mpi_rshift(MPI x, MPI a, unsigned n)
>>   */
>>  int mpi_lshift_limbs(MPI a, unsigned int count)
>>  {
>> -     mpi_ptr_t ap = a->d;
>> -     int n = a->nlimbs;
>> +     const int n = a->nlimbs;
>> +     mpi_ptr_t ap;
>>       int i;
>>
>>       if (!count || !n)
>> @@ -187,6 +187,7 @@ int mpi_lshift_limbs(MPI a, unsigned int count)
>>       if (RESIZE_IF_NEEDED(a, n + count) < 0)
>>               return -ENOMEM;
>>
>> +     ap = a->d;
>>       for (i = n - 1; i >= 0; i--)
>>               ap[i + count] = ap[i];
>>       for (i = 0; i < count; i++)
>>
>
> --
> Jesper Juhl <jj@...osbits.net>       http://www.chaosbits.net/
> Don't top-post http://www.catb.org/jargon/html/T/top-post.html
> Plain text mails only, please.
>
> --
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Hello,

I was travelling last week.
I will review the patch now..

Thanks,
Dmitry
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