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.

Fixes #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/+/370576
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Alex Rakoczy <alex@golang.org>

diff --git a/src/syscall/exec_unix.go b/src/syscall/exec_unix.go
index 64eb5ed9f047a80c5c2414572484fec131892421..0e41959ffe54301ff0bec90a39ddb174b02f0079 100644 (file)
--- a/src/syscall/exec_unix.go
+++ b/src/syscall/exec_unix.go
@@ -152,9 +152,6 @@ 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 {
@@ -204,14 +201,17 @@ 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()
 
@@ -243,14 +243,6 @@ 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.