linux-kernel - Re: [Outreachy kernel] [PATCH v2] staging: android: Replace strcpy with strlcpy

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Date:   Sat, 11 Mar 2017 22:34:48 +0100 (CET)
From:   Julia Lawall <julia.lawall@...6.fr>
To:     SIMRAN SINGHAL <singhalsimran0@...il.com>
cc:     Greg KH <gregkh@...uxfoundation.org>, arve@...roid.com,
        riandrews@...roid.com, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org, outreachy-kernel@...glegroups.com
Subject: Re: [Outreachy kernel] [PATCH v2] staging: android: Replace strcpy
 with strlcpy



On Sun, 12 Mar 2017, SIMRAN SINGHAL wrote:

> On Sun, Mar 12, 2017 at 2:53 AM, Julia Lawall <julia.lawall@...6.fr> wrote:
> >
> >
> > On Sun, 12 Mar 2017, simran singhal wrote:
> >
> >> Replace strcpy with strlcpy as strcpy does not check for buffer
> >> overflow.
> >> This is found using Flawfinder.
> >>
> >> Signed-off-by: simran singhal <singhalsimran0@...il.com>
> >> ---
> >>
> >>  v2:
> >>    -Correcting the place of the parenthesis
> >>
> >>  drivers/staging/android/ashmem.c | 3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
> >> index 7cbad0d..eb2f4ef 100644
> >> --- a/drivers/staging/android/ashmem.c
> >> +++ b/drivers/staging/android/ashmem.c
> >> @@ -548,7 +548,8 @@ static int set_name(struct ashmem_area *asma, void __user *name)
> >>       if (unlikely(asma->file))
> >>               ret = -EINVAL;
> >>       else
> >> -             strcpy(asma->name + ASHMEM_NAME_PREFIX_LEN, local_name);
> >> +             strlcpy(asma->name + ASHMEM_NAME_PREFIX_LEN, local_name,
> >> +                     sizeof(asma->name + ASHMEM_NAME_PREFIX_LEN));
> >
> > This appears to be the old patch.
> >
> > Actually, looking closer, I don't think this is right at all.  There seems
> > to be a goal of leaving some space at the beginning of asma->name.  So the
> > amount of space available for local_name would be computed using a
> > subtraction, not an addition.
> >
> Julia, By mistake I sent the wrong patch.
> And I am not getting you want to mention which space.

The following is the whole asma->name array.  The | in the middle is the
offset ASHMEM_NAME_PREFIX_LEN.  The xxx characters are in the
ASHMEM_NAME_PREFIX_LEN region.  The space for local_name is only the part
indicates with the ys.  This space is smaller than the size of asma->name.
In your calculation, you are allowing it to be larger.  You will
potentially overflow the buffer by 2 * ASHMEM_NAME_PREFIX_LEN.

     --------------------------
    |xxxxxxx|yyyyyyyyyyyyyyyyyy|
     --------------------------

julia