linux-kernel - Re: KASAN: slab-out-of-bounds Read in hex

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Date:   Tue, 13 Aug 2019 14:55:51 +0200
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Alan Stern <stern@...land.harvard.edu>,
        Eric Biggers <ebiggers@...nel.org>
Cc:     syzbot <syzbot+a9fefd18c7b240f19c54@...kaller.appspotmail.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>, rafael@...nel.org,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: KASAN: slab-out-of-bounds Read in hex_string

On Tue, Apr 30, 2019 at 4:13 PM Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Mon, 29 Apr 2019, Eric Biggers wrote:
>
> > On Mon, Apr 29, 2019 at 04:07:04PM -0400, Alan Stern wrote:
>
> > > Accessing beyond the end of the descriptor.
> > >
> > > #syz test: https://github.com/google/kasan.git usb-fuzzer
> > >
> > > --- a/drivers/video/fbdev/udlfb.c
> > > +++ b/drivers/video/fbdev/udlfb.c
> > > @@ -1511,6 +1511,7 @@ static int dlfb_parse_vendor_descriptor(
> > >     char *buf;
> > >     char *desc_end;
> > >     int total_len;
> > > +   int width;
> > >
> > >     buf = kzalloc(MAX_VENDOR_DESCRIPTOR_SIZE, GFP_KERNEL);
> > >     if (!buf)
> > > @@ -1529,9 +1530,10 @@ static int dlfb_parse_vendor_descriptor(
> > >     }
> > >
> > >     if (total_len > 5) {
> > > +           width = min(total_len, 11);
> > >             dev_info(&intf->dev,
> > > -                    "vendor descriptor length: %d data: %11ph\n",
> > > -                    total_len, desc);
> > > +                    "vendor descriptor length: %d data: %*ph\n",
> > > +                    total_len, width, desc);
> > >
> > >             if ((desc[0] != total_len) || /* descriptor length */
> > >                 (desc[1] != 0x5f) ||   /* vendor descriptor type */
> > >
> > >
> >
> > Why not write just:
> >
> >                 dev_info(&intf->dev,
> >                          "vendor descriptor length: %d data: %*ph\n",
> >                          total_len, min(total_len, 11), desc);
>
> I did consider doing that.  In the end I decided adding an extra
> temporary variable made the code a little more readable.  (For some
> reason, extra recursion -- a function call embedded in a function
> argument -- seems to demand more mental effort than an extra
> temporary.  Maybe my brain is just getting old...)
>
> > Also, aren't there more out-of-bounds reads in the code just after?  It only
> > checks for at least 1 byte available, but then it reads up to 7 bytes:
> >
> >               while (desc < desc_end) {
> >                       u8 length;
> >                       u16 key;
> >
> >                       key = *desc++;
> >                       key |= (u16)*desc++ << 8;
> >                       length = *desc++;
> >
> >                       switch (key) {
> >                       case 0x0200: { /* max_area */
> >                               u32 max_area = *desc++;
> >                               max_area |= (u32)*desc++ << 8;
> >                               max_area |= (u32)*desc++ << 16;
> >                               max_area |= (u32)*desc++ << 24;
> >                               dev_warn(&intf->dev,
> >                                        "DL chip limited to %d pixel modes\n",
> >                                        max_area);
> >                               dlfb->sku_pixel_limit = max_area;
> >                               break;
> >                       }
> >                       default:
> >                               break;
> >                       }
> >                       desc += length;
> >               }
>
> Quite right.  Please feel free to submit a patch fixing all these
> problems.
>
> > Also I couldn't help but notice it's also using 'char' rather than 'u8',
> > so bytes >= 0x80 are read incorrectly as they're sign extended...
>
> As I recall, the C standard doesn't specify whether char is signed or
> unsigned; it can vary with the implementation.  However you are
> certainly correct that to ensure there is no sign extension, the code
> should use unsigned char or u8.

Hi Alan and Eric,

Have any of this fixes been submitted anywhere? This bug is still open
on syzbot.

Thanks!