[release-branch.go1.16] 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/+/370514
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>

1 files changed, 6 insertions, 14 deletions

diff --git a/src/syscall/exec_unix.go b/src/syscall/exec_unix.go
index 1f49c78ef9..290dd383ec 100644
--- 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.