Re: [PATCH v2] of: fdt: fix possible overflow during parsing of fdt

[AbdelRahman Yossef <abdelrahmanyossef12@gmail.com>]

I will update the patch and send it as v4.

Is it enough to just add the changes to Changelog or change the commit message?

On Wed, Nov 13, 2024 at 2:17 PM Sascha Hauer <s.hauer@pengutronix.de> wrote:
>
> On Tue, Nov 12, 2024 at 08:56:58PM +0100, Ahmad Fatoum wrote:
> > Hello Abdelrahman,
> >
> > Thanks for your patch.
> >
> > On 12.11.24 20:10, Abdelrahman Youssef wrote:
> > > While fuzzing, the name marked by FDT_BEGIN_NODE sometimes extends beyond
> > > the struct block area, Causing a heap-overflow.
> > >
> > > Since `maxlen` is an unsigned integer representing the length of name,
> > > It can be negative, So it overflows to large numbers, Causing strnlen()
> > > to overflow.
> > >
> > > So we can just change the type of maxlen to signed and check if it's negative.
> > >
> > > Signed-off-by: Abdelrahman Youssef <abdelrahmanyossef12@gmail.com>
> > > ---
> >
> > Changelog would've been nice. This also should have been v3 not v2.
> >
> > >  drivers/of/fdt.c | 7 ++++++-
> > >  1 file changed, 6 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> > > index 2c3ea31394..d8d8a4922c 100644
> > > --- a/drivers/of/fdt.c
> > > +++ b/drivers/of/fdt.c
> > > @@ -176,7 +176,7 @@ static struct device_node *__of_unflatten_dtb(const void *infdt, int size,
> > >     void *dt_strings;
> > >     struct fdt_header f;
> > >     int ret;
> > > -   unsigned int maxlen;
> > > +   int maxlen;
> > >     const struct fdt_header *fdt = infdt;
> > >
> > >     ret = fdt_parse_header(infdt, size, &f);
> > > @@ -210,6 +210,11 @@ static struct device_node *__of_unflatten_dtb(const void *infdt, int size,
> > >                     maxlen = (unsigned long)fdt + f.off_dt_struct +
> > >                             f.size_dt_struct - (unsigned long)name;
> > >
> > > +                   if (maxlen < 0) {
> > > +                           ret = -ESPIPE;
> > > +                           goto err;
> > > +                   }
> > > +
> > >                     len = strnlen(name, maxlen + 1);
> >
> > Hmm is this + 1 correct? I am wondering if we should be dropping
> > the + 1 here and make it maxlen <= 0 above.
>
> I think maxlen <= 0 is correct indepent of what follows next, because
> maxlen is the length of a string and a valid string has a minimal length
> of one byte ('\0').
>
> Next we shouldn't look at bytes exceeding maxlen, so indeed
> strnlen(name, maxlen) should be correct. When changing this we have
> to adjust the following if (len > maxlen) check to >=.
>
> Sascha
>
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