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SubjectRe: KASAN: slab-out-of-bounds Read in hex_string
On Tue, Apr 30, 2019 at 4:13 PM Alan Stern <stern@rowland.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!

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