[PATCH] text-utils/ul: Fix buffer overflow

[PATCH] text-utils/ul: Fix buffer overflow

[Tobias Stoeckmann]

The text-utility ul can run into a buffer overflow on very long lines.
See this proof of concept how to reproduce the issue:

$ dd if=/dev/zero bs=1M count=10 | tr '\000' '\041' > poc.txt
$ echo -ne '\xe\x5f\x8\x5f\x61\x2\xf\x5f\x8\x5f' | dd of=poc.txt conv=notrunc
$ ul -i poc.txt > /dev/null # output would take ages
Segmentation fault
$ _

The problem manifests by using alloca with "maxcol", which can be as
large as INT_MAX, based on the input line.

A very long line (> 8 MB) with modes must be supplied to ul, as seen in
my proof of concept byte sequence above.

It is rather easy to fix this issue: allocate space on the heap instead.
maxcol could overflow here, but in that case no system will have enough
space to handle the request, properly ending ul through an err() call.


Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
---
 text-utils/ul.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/text-utils/ul.c b/text-utils/ul.c
index 6721974..3fd0b6a 100644
--- a/text-utils/ul.c
+++ b/text-utils/ul.c
@@ -402,11 +402,7 @@ static void flushln(void)
 static void overstrike(void)
 {
 	register int i;
-#ifdef __GNUC__
-	register wchar_t *lbuf = __builtin_alloca((maxcol + 1) * sizeof(wchar_t));
-#else
-	wchar_t lbuf[BUFSIZ];
-#endif
+	register wchar_t *lbuf = xmalloc((maxcol + 1) * sizeof(wchar_t));
 	register wchar_t *cp = lbuf;
 	int hadbold=0;
 
@@ -439,16 +435,13 @@ static void overstrike(void)
 		for (cp = lbuf; *cp; cp++)
 			putwchar(*cp == '_' ? ' ' : *cp);
 	}
+	free(lbuf);
 }
 
 static void iattr(void)
 {
 	register int i;
-#ifdef __GNUC__
-	register wchar_t *lbuf = __builtin_alloca((maxcol+1)*sizeof(wchar_t));
-#else
-	wchar_t lbuf[BUFSIZ];
-#endif
+	register wchar_t *lbuf = xmalloc((maxcol + 1) * sizeof(wchar_t));
 	register wchar_t *cp = lbuf;
 
 	for (i = 0; i < maxcol; i++)
@@ -465,6 +458,7 @@ static void iattr(void)
 		*cp = 0;
 	fputws(lbuf, stdout);
 	putwchar('\n');
+	free(lbuf);
 }
 
 static void initbuf(void)
-- 
2.10.0

Re: [PATCH] text-utils/ul: Fix buffer overflow

[Shaun Tancheff]

On Thu, Sep 8, 2016 at 2:19 PM, Tobias Stoeckmann <tobias@stoeckmann.org> w=
rote:
> The text-utility ul can run into a buffer overflow on very long lines.
> See this proof of concept how to reproduce the issue:
>
> $ dd if=3D/dev/zero bs=3D1M count=3D10 | tr '\000' '\041' > poc.txt
> $ echo -ne '\xe\x5f\x8\x5f\x61\x2\xf\x5f\x8\x5f' | dd of=3Dpoc.txt conv=
=3Dnotrunc
> $ ul -i poc.txt > /dev/null # output would take ages
> Segmentation fault
> $ _
>
> The problem manifests by using alloca with "maxcol", which can be as
> large as INT_MAX, based on the input line.
>
> A very long line (> 8 MB) with modes must be supplied to ul, as seen in
> my proof of concept byte sequence above.
>
> It is rather easy to fix this issue: allocate space on the heap instead.
> maxcol could overflow here, but in that case no system will have enough
> space to handle the request, properly ending ul through an err() call.
>
>
> Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
> ---
>  text-utils/ul.c | 14 ++++----------
>  1 file changed, 4 insertions(+), 10 deletions(-)

Just a nit .. may as well change 'i' from an int to a long here as well.
That should cover the theoretical overflow for 64 bit machines.

> diff --git a/text-utils/ul.c b/text-utils/ul.c
> index 6721974..3fd0b6a 100644
> --- a/text-utils/ul.c
> +++ b/text-utils/ul.c
> @@ -402,11 +402,7 @@ static void flushln(void)
>  static void overstrike(void)
>  {
>         register int i;
> -#ifdef __GNUC__
> -       register wchar_t *lbuf =3D __builtin_alloca((maxcol + 1) * sizeof=
(wchar_t));
> -#else
> -       wchar_t lbuf[BUFSIZ];
> -#endif
> +       register wchar_t *lbuf =3D xmalloc((maxcol + 1) * sizeof(wchar_t)=
);
>         register wchar_t *cp =3D lbuf;
>         int hadbold=3D0;
>
> @@ -439,16 +435,13 @@ static void overstrike(void)
>                 for (cp =3D lbuf; *cp; cp++)
>                         putwchar(*cp =3D=3D '_' ? ' ' : *cp);
>         }
> +       free(lbuf);
>  }
>
>  static void iattr(void)
>  {
>         register int i;
> -#ifdef __GNUC__
> -       register wchar_t *lbuf =3D __builtin_alloca((maxcol+1)*sizeof(wch=
ar_t));
> -#else
> -       wchar_t lbuf[BUFSIZ];
> -#endif
> +       register wchar_t *lbuf =3D xmalloc((maxcol + 1) * sizeof(wchar_t)=
);
>         register wchar_t *cp =3D lbuf;
>
>         for (i =3D 0; i < maxcol; i++)
> @@ -465,6 +458,7 @@ static void iattr(void)
>                 *cp =3D 0;
>         fputws(lbuf, stdout);
>         putwchar('\n');
> +       free(lbuf);
>  }
>
>  static void initbuf(void)
> --
> 2.10.0
>
> --
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e=3D



--=20
Shaun Tancheff

Re: [PATCH] text-utils/ul: Fix buffer overflow

[Karel Zak]

On Sun, Sep 11, 2016 at 02:47:22PM -0500, Shaun Tancheff wrote:
> On Thu, Sep 8, 2016 at 2:19 PM, Tobias Stoeckmann <tobias@stoeckmann.org> wrote:
> > The text-utility ul can run into a buffer overflow on very long lines.
> > See this proof of concept how to reproduce the issue:
> >
> > $ dd if=/dev/zero bs=1M count=10 | tr '\000' '\041' > poc.txt
> > $ echo -ne '\xe\x5f\x8\x5f\x61\x2\xf\x5f\x8\x5f' | dd of=poc.txt conv=notrunc
> > $ ul -i poc.txt > /dev/null # output would take ages
> > Segmentation fault
> > $ _
> >
> > The problem manifests by using alloca with "maxcol", which can be as
> > large as INT_MAX, based on the input line.
> >
> > A very long line (> 8 MB) with modes must be supplied to ul, as seen in
> > my proof of concept byte sequence above.
> >
> > It is rather easy to fix this issue: allocate space on the heap instead.
> > maxcol could overflow here, but in that case no system will have enough
> > space to handle the request, properly ending ul through an err() call.
> >
> >
> > Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
> > ---
> >  text-utils/ul.c | 14 ++++----------
> >  1 file changed, 4 insertions(+), 10 deletions(-)

Applied, thanks.

> 
> Just a nit .. may as well change 'i' from an int to a long here as well.
> That should cover the theoretical overflow for 64 bit machines.

This is generic problem of the 'ul' code, on many many places it uses 'int'
rather than 'size_t'. It would be better to fix it completely rather
than just one 'i' ;-)

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com