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

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Date:	Thu, 12 Apr 2012 00:31:59 +0200 (CEST)
From:	Jesper Juhl <jj@...osbits.net>
To:	"Kasatkin, Dmitry" <dmitry.kasatkin@...el.com>
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 Tue, 13 Mar 2012, Kasatkin, Dmitry wrote:

> 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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> > Please read the FAQ at  http://www.tux.org/lkml/
> 
> Hello,
> 
> I was travelling last week.
> I will review the patch now..
> 

Hi.

Did you get a chance to review the patch?  Did you find it acceptable or 
flawed?

If it is OK, what do I need to from this point forward to get it merged?


-- 
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.