[bug#44700] services: setuid: More configurable setuid support.

[Christopher Lemmer Webber]

This patch allows for configuring the specific user, group, and whether
to set the setuid and setgid bits.

See also:
  https://lists.gnu.org/archive/html/guix-devel/2020-11/msg00369.html

But I thought I'd open this here so we could track changes since this is
technically independent of the postfix stuff.  Anyway, patch attached.
One change since the last email above is that I added support for
string-based username/groups.

This also needs documentation, I suppose, so that should be done.
But it would be good to know if this patch looks like it's on the "right
path" or not.

>From eadac673fb22132c555a4e1cee57a6308ecfdad4 Mon Sep 17 00:00:00 2001
From: Christopher Lemmer Webber <cwebber@dustycloud.org>
Date: Sun, 15 Nov 2020 16:58:52 -0500
Subject: [PATCH] services: setuid: More configurable setuid support.

New record <setuid-program> with fields for setting the specific user and
group, as well as specifically selecting the setuid and setgid bits, for a
program within the setuid-program-service.

* gnu/services.scm (<setuid-program>): New record type.
  (setuid-program, make-setuid-program, setuid-program?)
  (setuid-program-program, stuid-program-setuid?, setuid-program-setgid?)
  (setuid-program-user, setuid-program-group): New variables, export them.
  (setuid-program-entry): New variable, a procedure used for the
  service-extension of activation-service-type as set up by
  setuid-program-service-type.  Unpacks the <setuid-program> record,
  handing off within the gexp to activate-setuid-programs.
  (setuid-program-service-type): Make use of setuid-program-entry.
* gnu/build/activation.scm (activate-setuid-programs): Update to expect a
  ftagged list for each program entry, pre-unpacked from the <setuid-program>
  record before being handed to this procedure.
---
 gnu/build/activation.scm | 46 +++++++++++++++++++++----------------
 gnu/services.scm         | 49 +++++++++++++++++++++++++++++++++++++---
 2 files changed, 73 insertions(+), 22 deletions(-)

diff --git a/gnu/build/activation.scm b/gnu/build/activation.scm
index 4b67926e88..fd17ce0434 100644
--- a/gnu/build/activation.scm
+++ b/gnu/build/activation.scm
@@ -229,13 +229,6 @@ they already exist."
 (define (activate-setuid-programs programs)
   "Turn PROGRAMS, a list of file names, into setuid programs stored under
 %SETUID-DIRECTORY."
-  (define (make-setuid-program prog)
-    (let ((target (string-append %setuid-directory
-                                 "/" (basename prog))))
-      (copy-file prog target)
-      (chown target 0 0)
-      (chmod target #o6555)))
-
   (format #t "setting up setuid programs in '~a'...~%"
           %setuid-directory)
   (if (file-exists? %setuid-directory)
@@ -247,18 +240,33 @@ they already exist."
                          string<?))
       (mkdir-p %setuid-directory))
 
-  (for-each (lambda (program)
-              (catch 'system-error
-                (lambda ()
-                  (make-setuid-program program))
-                (lambda args
-                  ;; If we fail to create a setuid program, better keep going
-                  ;; so that we don't leave %SETUID-DIRECTORY empty or
-                  ;; half-populated.  This can happen if PROGRAMS contains
-                  ;; incorrect file names: <https://bugs.gnu.org/38800>.
-                  (format (current-error-port)
-                          "warning: failed to make '~a' setuid-root: ~a~%"
-                          program (strerror (system-error-errno args))))))
+  (for-each (match-lambda
+              [('setuid-program src-path setuid? setgid? user group)
+               (let ((uid (match user
+                            [(? string?) (passwd:uid (getpwnam user))]
+                            [(? integer?) user]))
+                     (gid (match group
+                            [(? string?) (group:gid (getgrnam user))]
+                            [(? integer?) group])))
+                 (catch 'system-error
+                   (lambda ()
+                     (let ((target (string-append %setuid-directory
+                                                  "/" (basename src-path)))
+                           (mode (+ #o0555                   ; base permissions
+                                    (if setuid? #o4000 0)    ; setuid bit
+                                    (if setgid? #o2000 0)))) ; setgid bit
+                       (copy-file src-path target)
+                       (chown target uid gid)
+                       (chmod target mode)))
+                   (lambda args
+                     ;; If we fail to create a setuid program, better keep 
going
+                     ;; so that we don't leave %SETUID-DIRECTORY empty or
+                     ;; half-populated.  This can happen if PROGRAMS contains
+                     ;; incorrect file names: <https://bugs.gnu.org/38800>.
+                     (format (current-error-port)
+                             "warning: failed to make '~a' setuid-root: ~a~%"
+                             (setuid-program-program program)
+                             (strerror (system-error-errno args))))))])
             programs))
 
 (define (activate-special-files special-files)
diff --git a/gnu/services.scm b/gnu/services.scm
index 4b30399adc..a5b4734152 100644
--- a/gnu/services.scm
+++ b/gnu/services.scm
@@ -87,6 +87,14 @@
             ambiguous-target-service-error-service
             ambiguous-target-service-error-target-type
 
+            setuid-program
+            setuid-program?
+            setuid-program-program
+            setuid-program-setuid?
+            setuid-program-setgid?
+            setuid-program-user
+            setuid-program-group
+
             system-service-type
             provenance-service-type
             sexp->system-provenance
@@ -773,13 +781,48 @@ directory."
 FILES must be a list of name/file-like object pairs."
   (service etc-service-type files))
 
+(define-record-type* <setuid-program> setuid-program make-setuid-program
+  setuid-program?
+  ;; Path to program to link with setuid permissions
+  (program       setuid-program-program)          ;string
+  ;; Whether to set user setuid bit
+  (setuid?       setuid-program-setuid?           ;boolean
+                 (default #t))
+  ;; Whether to set user setgid bit
+  (setgid?       setuid-program-setgid?           ;boolean
+                 (default #t))
+  ;; The user this should be set to (defaults to root)
+  (user          setuid-program-user              ;integer or string
+                 (default 0))
+  ;; Group we want to set this to (defaults to root)
+  (group         setuid-program-group             ;integer or string
+                 (default 0)))
+
+(define (setuid-program-entry programs)
+  #~(activate-setuid-programs
+     ;; convert into a tagged list structure as expected by
+     ;; activate-setuid-programs
+     (list #$@(map (match-lambda
+                     [(? setuid-program? sp)
+                      #~(list 'setuid-program
+                              #$(setuid-program-program sp)
+                              #$(setuid-program-setuid? sp)
+                              #$(setuid-program-setgid? sp)
+                              #$(setuid-program-user sp)
+                              #$(setuid-program-group sp))]
+                     ;; legacy, non-<setuid-program> structure
+                     [program
+                      ;; TODO: Spit out a warning here?
+                      #~(list 'setuid-program
+                              #$program
+                              #t #t 0 0)])
+                   programs))))
+
 (define setuid-program-service-type
   (service-type (name 'setuid-program)
                 (extensions
                  (list (service-extension activation-service-type
-                                          (lambda (programs)
-                                            #~(activate-setuid-programs
-                                               (list #$@programs))))))
+                                          setuid-program-entry)))
                 (compose concatenate)
                 (extend append)
                 (description
-- 
2.29.1