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From: "maxkamper at outlook dot com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug stdio/25812] New: Libio vtable protection is sometimes only
 partially enforced
Date: Fri, 10 Apr 2020 17:22:27 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D25812

            Bug ID: 25812
           Summary: Libio vtable protection is sometimes only partially
                    enforced
           Product: glibc
           Version: 2.31
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: stdio
          Assignee: unassigned at sourceware dot org
          Reporter: maxkamper at outlook dot com
  Target Milestone: ---

Some versions of GLIBC bundled with popular distros still place their libio
vtables in a writable segment. E.g. Ubuntu 1904 (which uses GLIBC 2.29)
exhibits this behaviour, whilst the earlier Ubuntu 1804 (which uses GLIBC 2=
.27)
does not.

This could be a choice on behalf of the distro maintainers but it seems
unlikely.

Vtable pointers are still checked before dereferencing but I feel this is
undermined somewhat by the fact that the vtables themselves are mapped
writable.

Perhaps the issue is in the Makerules file, around line 549, which is comme=
nted
"If the linker is good enough, we can let it use its default linker script."
The issue seems to manifest depending on the preceding ifeq statement's res=
ult.

I'm not well versed enough in the workings of linkers to comment as to why =
this
might be, but I've experienced the same writable vtable issue whilst buildi=
ng
GLIBC using more recent versions of GCC.

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