ignore for _apt inaccessible TMPDIR in pkgAcqChangelog

Using libpam-tmpdir caused us to create our download tmp directory in
root's private tmp before changing to _apt, which wouldn't have access
to it.

By extending our GetTempDir method with an optional wrapper changing the
effective user, we can test if a given user can access the directory and
ignore TMPDIR if not instead of ignoring TMPDIR completely.

Closes: 797270

1 files changed, 28 insertions, 3 deletions

diff --git a/apt-pkg/contrib/fileutl.cc b/apt-pkg/contrib/fileutl.cc
index d7c9424cf..1d20c9c35 100644
--- a/apt-pkg/contrib/fileutl.cc
+++ b/apt-pkg/contrib/fileutl.cc
@@ -2123,13 +2123,38 @@ std::string GetTempDir()						/*{{{*/
 
    struct stat st;
    if (!tmpdir || strlen(tmpdir) == 0 || // tmpdir is set
-	 stat(tmpdir, &st) != 0 || (st.st_mode & S_IFDIR) == 0 || // exists and is directory
-	 access(tmpdir, R_OK | W_OK | X_OK) != 0 // current user has rwx access to directory
-      )
+	 stat(tmpdir, &st) != 0 || (st.st_mode & S_IFDIR) == 0) // exists and is directory
+      tmpdir = "/tmp";
+   else if (geteuid() != 0 && // root can do everything anyway
+	 faccessat(-1, tmpdir, R_OK | W_OK | X_OK, AT_EACCESS | AT_SYMLINK_NOFOLLOW) != 0) // current user has rwx access to directory
       tmpdir = "/tmp";
 
    return string(tmpdir);
 }
+std::string GetTempDir(std::string const &User)
+{
+   // no need/possibility to drop privs
+   if(getuid() != 0 || User.empty() || User == "root")
+      return GetTempDir();
+
+   struct passwd const * const pw = getpwnam(User.c_str());
+   if (pw == NULL)
+      return GetTempDir();
+
+   if (setegid(pw->pw_gid) != 0)
+      _error->Errno("setegid", "setegid %u failed", pw->pw_gid);
+   if (seteuid(pw->pw_uid) != 0)
+      _error->Errno("seteuid", "seteuid %u failed", pw->pw_uid);
+
+   std::string const tmp = GetTempDir();
+
+   if (seteuid(0) != 0)
+      _error->Errno("seteuid", "seteuid %u failed", 0);
+   if (setegid(0) != 0)
+      _error->Errno("setegid", "setegid %u failed", 0);
+
+   return tmp;
+}
 									/*}}}*/
 FileFd* GetTempFile(std::string const &Prefix, bool ImmediateUnlink, FileFd * const TmpFd)	/*{{{*/
 {