Work around Tcl's default PATH lookup

As per https://www.tcl.tk/man/tcl8.6/TclCmd/exec.html#M23, Tcl's `exec`
function goes out of its way to imitate the highly dangerous path lookup
of `cmd.exe`, but _of course_ only on Windows:

	If a directory name was not specified as part of the application
	name, the following directories are automatically searched in
	order when attempting to locate the application:

	    The directory from which the Tcl executable was loaded.

	    The current directory.

	    The Windows 32-bit system directory.

	    The Windows home directory.

	    The directories listed in the path.

The dangerous part is the second item, of course: `exec` _prefers_
executables in the current directory to those that are actually in the
`PATH`.

It is almost as if people wanted to Windows users vulnerable,
specifically.

To avoid that, Git GUI already has the `_which` function that does not
imitate that dangerous practice when looking up executables in the
search path.

However, Git GUI currently fails to use that function e.g. when trying to
execute `aspell` for spell checking.

That is not only dangerous but combined with Tcl's unfortunate default
behavior and with the fact that Git GUI tries to spell-check a
repository just after cloning, leads to a critical Remote Code Execution
vulnerability.

Let's override both `exec` and `open` to always use `_which` instead of
letting Tcl perform the path lookup, to prevent this attack vector.

This addresses CVE-2022-41953.

For more details, see
GHSA-v4px-mx59-w99c

Signed-off-by: Johannes Schindelin <[email protected]>

git-gui/git-gui.sh

@@ -121,6 +121,62 @@ proc _which {what args} {
 	return {}
 }
 
+proc sanitize_command_line {command_line from_index} {
+	set i $from_index
+	while {$i < [llength $command_line]} {
+		set cmd [lindex $command_line $i]
+		if {[file pathtype $cmd] ne "absolute"} {
+			set fullpath [_which $cmd]
+			if {$fullpath eq ""} {
+				throw {NOT-FOUND} "$cmd not found in PATH"
+			}
+			lset command_line $i $fullpath
+		}
+
+		# handle piped commands, e.g. `exec A | B`
+		for {incr i} {$i < [llength $command_line]} {incr i} {
+			if {[lindex $command_line $i] eq "|"} {
+				incr i
+				break
+			}
+		}
+	}
+	return $command_line
+}
+
+# Override `exec` to avoid unsafe PATH lookup
+
+rename exec real_exec
+
+proc exec {args} {
+	# skip options
+	for {set i 0} {$i < [llength $args]} {incr i} {
+		set arg [lindex $args $i]
+		if {$arg eq "--"} {
+			incr i
+			break
+		}
+		if {[string range $arg 0 0] ne "-"} {
+			break
+		}
+	}
+	set args [sanitize_command_line $args $i]
+	uplevel 1 real_exec $args
+}
+
+# Override `open` to avoid unsafe PATH lookup
+
+rename open real_open
+
+proc open {args} {
+	set arg0 [lindex $args 0]
+	if {[string range $arg0 0 0] eq "|"} {
+		set command_line [string trim [string range $arg0 1 end]]
+		lset args 0 "| [sanitize_command_line $command_line 0]"
+	}
+	uplevel 1 real_open $args
+}
+
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