LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Nicolas Pitre <nico@fluxnic.net>
To: Gen Zhang <blackgod016574@gmail.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] vt: Fix a missing-check bug in drivers/tty/vt/vt.c
Date: Tue, 21 May 2019 00:30:38 -0400 (EDT)	[thread overview]
Message-ID: <nycvar.YSQ.7.76.1905210022050.1558@knanqh.ubzr> (raw)
In-Reply-To: <20190521040019.GD5263@zhanggen-UX430UQ>

On Tue, 21 May 2019, Gen Zhang wrote:

> On Mon, May 20, 2019 at 11:26:20PM -0400, Nicolas Pitre wrote:
> > On Tue, 21 May 2019, Gen Zhang wrote:
> > 
> > > On Mon, May 20, 2019 at 10:55:40PM -0400, Nicolas Pitre wrote:
> > > > On Tue, 21 May 2019, Gen Zhang wrote:
> > > > 
> > > > > In function con_init(), the pointer variable vc_cons[currcons].d, vc and
> > > > > vc->vc_screenbuf is allocated a memory space via kzalloc(). And they are
> > > > > used in the following codes.
> > > > > However, when there is a memory allocation error, kzalloc() can fail.
> > > > > Thus null pointer (vc_cons[currcons].d, vc and vc->vc_screenbuf)
> > > > > dereference may happen. And it will cause the kernel to crash. Therefore,
> > > > > we should check return value and handle the error.
> > > > > Further,the loop condition MIN_NR_CONSOLES is defined as 1 in
> > > > > include/uapi/linux/vt.h. So there is no need to unwind the loop.
> > > > 
> > > > But what if someone changes that define? It won't be obvious that some 
> > > > code did rely on it to be defined to 1.
> > > I re-examine the source code. MIN_NR_CONSOLES is only defined once and
> > > no other changes to it.
> > 
> > Yes, that is true today.  But if someone changes that in the future, how 
> > will that person know that you relied on it to be 1 for not needing to 
> > unwind the loop?
> > 
> > 
> > Nicolas
> Hi Nicolas,
> Thanks for your explaination! And I got your point. And is this way 
> proper?

Not quite.

> err_vc_screenbuf:
>         kfree(vc);
> 	for (currcons = 0; currcons < MIN_NR_CONSOLES; currcons++)
> 		vc_cons[currcons].d = NULL;
> 	return -ENOMEM;
> err_vc:
> 	console_unlock();
> 	return -ENOMEM;

Now imagine that MIN_NR_CONSOLES is defined to 10 instead of 1.

What happens with allocated memory if the err_vc condition is met on the 
5th loop?

If err_vc_screenbuf condition is encountered on the 5th loop (curcons = 
4), what is the value of vc_cons[4].d? Isn't it the same as vc that you 
just freed?


Nicolas

  reply	other threads:[~2019-05-21  4:30 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-21  2:29 Gen Zhang
2019-05-21  2:55 ` Nicolas Pitre
2019-05-21  3:09   ` Gen Zhang
2019-05-21  3:26     ` Nicolas Pitre
2019-05-21  4:00       ` Gen Zhang
2019-05-21  4:30         ` Nicolas Pitre [this message]
2019-05-21  7:39           ` Gen Zhang
2019-05-22  2:43             ` Nicolas Pitre
2019-05-22  8:10               ` Gen Zhang
2019-05-22 12:19               ` [PATCH v3] " Gen Zhang
2019-05-22 14:18                 ` Nicolas Pitre
2019-05-24  2:27                   ` [PATCH v3] vt: Fix a missing-check bug in con_init() Gen Zhang
2019-05-21  3:21   ` [PATCH v3] vt: Fix a missing-check bug in drivers/tty/vt/vt.c Gen Zhang
2019-05-21  6:46     ` Oleksandr Natalenko

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=nycvar.YSQ.7.76.1905210022050.1558@knanqh.ubzr \
    --to=nico@fluxnic.net \
    --cc=blackgod016574@gmail.com \
    --cc=linux-kernel@vger.kernel.org \
    --subject='Re: [PATCH v2] vt: Fix a missing-check bug in drivers/tty/vt/vt.c' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).