[release-branch.go1.17] syscall: fix ForkLock spurious close(0) on pipe failure

Pipe (and therefore forkLockPipe) does not make any guarantees
about the state of p after a failed Pipe(p). Avoid that assumption
and the too-clever goto, so that we don't accidentally Close a real fd
if the failed pipe leaves p[0] or p[1] set >= 0.

Updates #50057
Fixes CVE-2021-44717

Change-Id: Iff8e19a6efbba0c73cc8b13ecfae381c87600bb4
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/1291270
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/370534
Trust: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alex Rakoczy <alex@golang.org>

diff --git a/src/syscall/exec_unix.go b/src/syscall/exec_unix.go
index 54b18dccd73e266e680d9a5e3250be18ce08ead4..c9c9d1abf3d74f3284530c96675ba567d9d7aee5 100644 (file)
--- a/src/syscall/exec_unix.go
+++ b/src/syscall/exec_unix.go
@@ -153,9 +153,6 @@ func forkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error)
                sys = &zeroSysProcAttr
        }
 
-       p[0] = -1
-       p[1] = -1
-
        // Convert args to C form.
        argv0p, err := BytePtrFromString(argv0)
        if err != nil {
@@ -205,14 +202,17 @@ func forkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error)
 
        // Allocate child status pipe close on exec.
        if err = forkExecPipe(p[:]); err != nil {
-               goto error
+               ForkLock.Unlock()
+               return 0, err
        }
 
        // Kick off child.
        pid, err1 = forkAndExecInChild(argv0p, argvp, envvp, chroot, dir, attr, sys, p[1])
        if err1 != 0 {
-               err = Errno(err1)
-               goto error
+               Close(p[0])
+               Close(p[1])
+               ForkLock.Unlock()
+               return 0, Errno(err1)
        }
        ForkLock.Unlock()
 
@@ -244,14 +244,6 @@ func forkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error)
 
        // Read got EOF, so pipe closed on exec, so exec succeeded.
        return pid, nil
-
-error:
-       if p[0] >= 0 {
-               Close(p[0])
-               Close(p[1])
-       }
-       ForkLock.Unlock()
-       return 0, err
 }
 
 // Combination of fork and exec, careful to be thread safe.