[Bug stdio/27124] New: libio vtable hardening bypass via obstack files

["willcoster at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=27124

            Bug ID: 27124
           Summary: libio vtable hardening bypass via obstack files
           Product: glibc
           Version: 2.32
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: stdio
          Assignee: unassigned at sourceware dot org
          Reporter: willcoster at gmail dot com
  Target Milestone: ---

It is possible to bypass existing libio vtable hardening by using the
_IO_obstack_jumps vtable and a forged obstack.

The obstack struct contains unmangled pointers to a custom allocator [1] which
is used when growing an obstack [2, 3, 4].

Both of the methods listed in the _IO_obstack_jumps vtable attempt to grow an
obstack when certain conditions are met [5, 6].

This is similar to Bug 23236 and would allow an attacker with the ability to
corrupt FILE objects to gain code execution. I have tested and confirmed that
this works on glibc 2.32.

I am filing this in the public tracker in accordance with the guidance that
hardening bypasses are not considered security issues in and of themselves [7].

[1]
https://sourceware.org/git/?p=glibc.git;a=blob;f=malloc/obstack.h;h=4b1ba80047b378fa63739e5dbbbf748ca026e197;hb=refs/heads/release/2.32/master#l153
[2]
https://sourceware.org/git/?p=glibc.git;a=blob;f=malloc/obstack.h;h=4b1ba80047b378fa63739e5dbbbf748ca026e197;hb=refs/heads/release/2.32/master#l296
[3]
https://sourceware.org/git/?p=glibc.git;a=blob;f=malloc/obstack.c;h=0d652be9f5d4c0e88e55bf84bbc1053c20d7ce83;hb=refs/heads/release/2.32/master#l261
[4]
https://sourceware.org/git/?p=glibc.git;a=blob;f=malloc/obstack.c;h=0d652be9f5d4c0e88e55bf84bbc1053c20d7ce83;hb=refs/heads/release/2.32/master#l121
[5]
https://sourceware.org/git/?p=glibc.git;a=blob;f=libio/obprintf.c;h=e440cd74c3caf3ee5bbfec9aa1e3ff880f0b3fb7;hb=refs/heads/release/2.32/master#l48
[6]
https://sourceware.org/git/?p=glibc.git;a=blob;f=libio/obprintf.c;h=e440cd74c3caf3ee5bbfec9aa1e3ff880f0b3fb7;hb=refs/heads/release/2.32/master#l76
[7]
https://sourceware.org/glibc/wiki/Security%20Exceptions#Post-exploitation_countermeasures

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