URL:
<http://savannah.gnu.org/bugs/?45582>
Summary: Race condition creating the socket directories
Project: GNU Screen
Submitted by: None
Submitted on: Mon 20 Jul 2015 08:27:17 AM UTC
Category: Program Logic
Severity: 3 - Normal
Priority: 5 - Normal
Status: None
Privacy: Public
Assigned to: None
Open/Closed: Open
Discussion Lock: Any
Release: 4.2.0
Fixed Release: None
Planned Release: None
Work Required: None
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Details:
There is a race condition in screen creating the socket directories.
Both the main and the user specific directory (child to the main) suffer from
the same type of race condition.
Taken from "screen.c" (version 4.1.0 of Ubuntu, but the same for 4.2),
creating the child obviously got patched:
-----------
sprintf(SockPath, "%s/S-%s", SockDir, LoginName);
if (access(SockPath, F_OK))
{
if (mkdir(SockPath, 0700) == -1 && errno != EEXIST)
Panic(errno, "Cannot make directory '%s'", SockPath);
(void) chown(SockPath, real_uid, real_gid);
}
}
-----------
However a few lines up, creating the parent directory is still suffering from
the race condition:
-----------
if (stat(SockDir, &st))
{
n = (eff_uid == 0 && (real_uid || eff_gid == real_gid)) ? 0755 :
(eff_gid != real_gid) ? 0775 :
#ifdef S_ISVTX
0777|S_ISVTX;
#else
0777;
#endif
// START: !!
if (mkdir(SockDir, n) == -1)
Panic(errno, "Cannot make directory '%s'", SockDir);
// END: !!
}
-----------
If the screen tool is started at the same time, then between the "stat" call
and the "mkdir" call it may happen that that another instance of screen
already created the directory. Causing the second "mkdir" to fail with
"EEXIST". Which is "ok", since the directory simple has to be there.
_______________________________________________________
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