[PATCH xserver/hw/xfree86/parser/scan.c] potential buffer overrun
Peter Hutterer
peter.hutterer at who-t.net
Thu Oct 27 16:33:24 PDT 2011
On Tue, Oct 11, 2011 at 04:11:28AM +0200, vdb at picaros.org wrote:
> >
> > On Sat, Sep 17, 2011 at 06:58:18PM +0200, vdb at picaros.org wrote:
> > > This patch fixes a potential buffer overrun in xf86addComment(). The
> > > overrun occurs if the comment string to add doesn't start with a
> > > leading '#'.
> >
> > out of interest - how do you trigger this? Presumably xf86addComment should
> > only be called for comments, which would require the leading '#'
>
> The code which calculates the new buffer size is incorrect. In the
> normal case, a leading '#', the reallocated buffer is one byte too
> long. In the other case the buffer will be one byte short and an
> overrun occurs.
>
> The code has explicit support for adding comments without a leading
> hash. It is this support which causes the bug.
>
> It seems not good to have support for a feature which causes an
> overrun when used. But indeed, no bug is seen since all calls to
> xf86addComment() include the leading '#' as read from xorg.conf.
sorry, nearly dropped this one. I've merged it in, thanks. Was about to say
that a simple if (!iscomment) len++; may be enough but this code is awful
enough and your patch cleans it up to be more sane. Much appreciated.
I've also squashed in a very basic testcase for test/xfree86.c to reproduce
the overflow.
Cheers,
Peter
>
> Just my view, Servaas Vandenberghe.
>
> > > The bug can be seen from the code assuming
> > > curlen > 0 && eol_seen == 0 && iscomment == 0 :
> > >
> > > char *
> > > xf86addComment(char *cur, char *add)
> > >
> > > <...>
> > >
> > > str = add;
> > > -> iscomment = 0;
> > > while (*str) {
> > > if (*str != ' ' && *str != '\t')
> > > break;
> > > ++str;
> > > }
> > > -> iscomment = (*str == '#');
> > >
> > > len = strlen(add);
> > > endnewline = add[len - 1] == '\n';
> > > len += 1 + iscomment + (!hasnewline) + (!endnewline) + eol_seen;
> > > ^^^^^^^^^
> > > if ((str = realloc(cur, len + curlen)) == NULL)
> > > return cur;
> > >
> > > cur = str;
> > >
> > > if (eol_seen || (curlen && !hasnewline))
> > > cur[curlen++] = '\n';
> > > -> if (!iscomment)
> > > -> cur[curlen++] = '#';
> > > strcpy(cur + curlen, add);
> > > if (!endnewline)
> > > strcat(cur, "\n");
> > >
> > > The patch below is just a bugfix and tries to stay as close as
> > > possible to the original code. See also:
> > > http://lists.x.org/archives/xorg-devel/2011-August/024775.html .
> > >
> > > Signed-off-by: Servaas Vandenberghe <vdb at picaros.org>
> > > diff --git a/hw/xfree86/parser/scan.c b/hw/xfree86/parser/scan.c
> > > index 1cff3bc..b188503 100644
> > > --- a/hw/xfree86/parser/scan.c
> > > +++ b/hw/xfree86/parser/scan.c
> > > @@ -1093,7 +1093,7 @@ char *
> > > xf86addComment(char *cur, char *add)
> > > {
> > > char *str;
> > > - int len, curlen, iscomment, hasnewline = 0, endnewline;
> > > + int len, curlen, iscomment, hasnewline = 0, insnewline, endnewline;
> > >
> > > if (add == NULL || add[0] == '\0')
> > > return cur;
> > > @@ -1108,7 +1108,6 @@ xf86addComment(char *cur, char *add)
> > > curlen = 0;
> > >
> > > str = add;
> > > - iscomment = 0;
> > > while (*str) {
> > > if (*str != ' ' && *str != '\t')
> > > break;
> > > @@ -1118,14 +1117,23 @@ xf86addComment(char *cur, char *add)
> > >
> > > len = strlen(add);
> > > endnewline = add[len - 1] == '\n';
> > > - len += 1 + iscomment + (!hasnewline) + (!endnewline) + eol_seen;
> > >
> > > - if ((str = realloc(cur, len + curlen)) == NULL)
> > > + insnewline = eol_seen || (curlen && !hasnewline);
> > > + if (insnewline)
> > > + len++;
> > > + if (!iscomment)
> > > + len++;
> > > + if (!endnewline)
> > > + len++;
> > > +
> > > + /* Allocate + 1 char for '\0' terminator. */
> > > + str = realloc(cur, curlen + len + 1);
> > > + if (!str)
> > > return cur;
> > >
> > > cur = str;
> > >
> > > - if (eol_seen || (curlen && !hasnewline))
> > > + if (insnewline)
> > > cur[curlen++] = '\n';
> > > if (!iscomment)
> > > cur[curlen++] = '#';
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