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[Bug dynamic-link/25204] LD_PREFER_MAP_32BIT_EXEC not ignored in setuid binaries (CVE-2019-19126)


https://sourceware.org/bugzilla/show_bug.cgi?id=25204

--- Comment #3 from Florian Weimer <fweimer at redhat dot com> ---
Introduced in glibc 2.23:

commit b9eb92ab05204df772eb4929eccd018637c9f3e9
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Wed Oct 21 14:44:23 2015 -0700

    Add Prefer_MAP_32BIT_EXEC to map executable pages with MAP_32BIT

    According to Silvermont software optimization guide, for 64-bit
    applications, branch prediction performance can be negatively impacted
    when the target of a branch is more than 4GB away from the branch.  Add
    the Prefer_MAP_32BIT_EXEC bit so that mmap will try to map executable
    pages with MAP_32BIT first.  NB: MAP_32BIT will map to lower 2GB, not
    lower 4GB, address.  Prefer_MAP_32BIT_EXEC reduces bits available for
    address space layout randomization (ASLR), which is always disabled for
    SUID programs and can only be enabled by setting environment variable,
    LD_PREFER_MAP_32BIT_EXEC.

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