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SubjectRe: Bug report - slab-out-of-bounds in vcs_scr_readw
@Nicolas Pitre  - I agree with you, "vcs_size" may return a negative
error code, so the patch is correct but as @jslaby@suse.com said it
won't fix the issue.
In my debugging session, "vcs_size" returns a positive integer ( 8000
decimal ) and the bug still triggers.

Maybe it's related to the following logic in "vcs_size"? ( not sure
about that.. )

221 if (use_attributes(inode)) {
222 if (use_unicode(inode))
223 return -EOPNOTSUPP;
224 size = 2*size + HEADER_SIZE;
225 } else if (use_unicode(inode))
226 size *= 4;
227 return size;

Why in the case of "use_unicode(inode)" size is multiplied by 4 and
not 2? ( as we can see in line 224 )





On Mon, Nov 4, 2019 at 10:54 PM Jiri Slaby <jslaby@suse.com> wrote:
>
> On 04. 11. 19, 19:33, Nicolas Pitre wrote:
> > On Mon, 4 Nov 2019, Or Cohen wrote:
> >
> >> @gregkh@linuxfoundation.org @nico@fluxnic.net - Thanks for the quick response.
> >> @gregkh@linuxfoundation.org - Regarding your question, I don't think
> >> the 1 byte buffer is related to the problem. ( it's just was there in
> >> the initial reproducer the fuzzer created, and I forgot to remove it
> >> while reducing code from the reproducer ).
> >
> > I think I know what the problem is. I have no time to test it though.
> >
> > Please try this (untested) patch. Also please try running the same test
> > code but with vcsa6 in addition to vcsu6 to be sure.
> >
> > ---------- >8
> > Subject: [PATCH] vcs: add missing validation on vcs_size() returned value
> >
> > One usage instance didn't account for the fact that vcs_size() may
> > return a negative error code.
> >
> > Signed-off-by: Nicolas Pitre <npitre@baylibre.com>
> >
> > diff --git a/drivers/tty/vt/vc_screen.c b/drivers/tty/vt/vc_screen.c
> > index 1f042346e7..fa07d79027 100644
> > --- a/drivers/tty/vt/vc_screen.c
> > +++ b/drivers/tty/vt/vc_screen.c
> > @@ -474,6 +474,10 @@ vcs_write(struct file *file, const char __user *buf, size_t count, loff_t *ppos)
> > goto unlock_out;
> >
> > size = vcs_size(inode);
> > + if (size < 0) {
> > + ret = size;
> > + goto unlock_out;
> > + }
> > ret = -EINVAL;
> > if (pos < 0 || pos > size)
> > goto unlock_out;
>
> pos must be >= 0, so "pos > size" would catch this case as a side
> effect, or am I missing something? That being said, the patch is
> correct, but won't fix the issue IMO.
>
> thanks,
> --
> js
> suse labs

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