[perl #119505] Segfault from bad backreference

[Kurt Seifried <kseifried () redhat com>]

http://perl5.git.perl.org/perl.git/commitdiff/0c2990d652e985784f095bba4bc356481a66aa06

The code that parses regex backrefs (or ambiguous backref/octal) such as
\123, did a simple atoi(), which could wrap round to negative values on
long digit strings and cause seg faults.

Include a check on the length of the digit string, and if greater than 9
digits, assume it can never be a valid backref (obviating the need for
the atoi() call).

I've also simplified the code a bit, putting most of the \g handling
code into a single block, rather than doing multiple "if (isg) {...}".

PoC:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=776046
perl -e '/\7777777777/'

not sure if this can be exploited at all, but someone creative maybe has
ideas, if so this may need a CVE.

-- 
Kurt Seifried -- Red Hat -- Product Security -- Cloud
PGP A90B F995 7350 148F 66BF 7554 160D 4553 5E26 7993

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