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From: "flibitijibibo at flibitijibibo dot com"
 <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug libc/31198] New: realpath allocates a buffer that may not fit a
 full path
Date: Sat, 30 Dec 2023 00:36:38 +0000
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X-Bugzilla-Product: glibc
X-Bugzilla-Component: libc
X-Bugzilla-Version: 2.36
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D31198

            Bug ID: 31198
           Summary: realpath allocates a buffer that may not fit a full
                    path
           Product: glibc
           Version: 2.36
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: flibitijibibo at flibitijibibo dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

Created attachment 15273
  --> https://sourceware.org/bugzilla/attachment.cgi?id=3D15273&action=3Ded=
it
Patch to replace strdup with malloc(PATH_MAX)

Consider the following example:

  extern char *somepath;
  char *path =3D realpath(somepath, NULL);
  strcat(path, "/");

It is common to append directory separators to paths, but when realpath
allocates the buffer the size cannot be determined from the outside. While =
the
application can provide its own buffer, it is reasonable for an application=
 to
assume that a path buffer will be able to fit a full path string even if it
gets modified after the call is made. As a result, modifications to the
strdup'd return value may result in a buffer overwrite.

A good replacement for the strdup allocation in realpath would be to always
allocate a buffer of PATH_MAX size, regardless of the realpath size, so that
modifications to the return value will always fit. I've attached a patch th=
at
does this.

This would fix a crash in the Steamworks SDK, which prior to 2017 always
assumed that the buffer returned by realpath had room to append a directory
separator to the end.

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