[bug#53468] [RFC PATCH] gnu: linux-pam: Change path to unix_chkpwd helper.

[Andrew Tropin]

* gnu/packages/patches/change-path-to-unix_chkpwd.patch: New file
* gnu/packages/linux.scm (linux-pam): Add patch.
* gnu/system/pam.scm (pam-root-service-type): Add unix_chkpwd to setuid
binaries.
---
The quote from unix_chkpwd.c:
> * This program is designed to run setuid(root) or with sufficient
> * privilege to read all of the unix password databases. It is designed
> * to provide a mechanism for the current user (defined by this
> * process's uid) to verify their own password.

Without suid bit it will fail in various use cases: for example utilities like
xlock or swaylock compiled with pam support won't be able to unlock the
screen.  To fix it I added unix_chkpwd binary to list of Guix System's setuid
programs and added a patch, which hardcodes /run/setuid-programs/unix_chkpwd
path in pam_unix module source code of linux-pam package.  However, I'm not
sure if it's a proper solution, please share your thoughts and conserns.

 gnu/packages/linux.scm                        |  3 +-
 .../patches/change-path-to-unix_chkpwd.patch  | 54 +++++++++++++++++++
 gnu/system/pam.scm                            |  8 ++-
 3 files changed, 62 insertions(+), 3 deletions(-)
 create mode 100644 gnu/packages/patches/change-path-to-unix_chkpwd.patch

diff --git a/gnu/packages/linux.scm b/gnu/packages/linux.scm
index 7b12cb8ec1..ee0df3c625 100644
--- a/gnu/packages/linux.scm
+++ b/gnu/packages/linux.scm
@@ -1590,7 +1590,8 @@ (define-public linux-pam
        (sha256
         (base32
          "1z4jayf69qyyxln1gl6ch4qxfd66ib1g42garnrv2d8i1drl0790"))
-       (patches (search-patches "linux-pam-no-setfsuid.patch"))))
+       (patches (search-patches "change-path-to-unix_chkpwd.patch"
+                                "linux-pam-no-setfsuid.patch"))))
 
     (build-system gnu-build-system)
     (native-inputs
diff --git a/gnu/packages/patches/change-path-to-unix_chkpwd.patch 
b/gnu/packages/patches/change-path-to-unix_chkpwd.patch
new file mode 100644
index 0000000000..90a8b639f6
--- /dev/null
+++ b/gnu/packages/patches/change-path-to-unix_chkpwd.patch
@@ -0,0 +1,54 @@
+From f314ab148b488e23a2e48e7222964e46d0d03447 Mon Sep 17 00:00:00 2001
+From: Andrew Tropin <andrew@trop.in>
+Date: Wed, 12 Jan 2022 17:17:42 +0300
+Subject: [PATCH] Change path to unix_chkpwd.
+
+---
+ modules/pam_unix/pam_unix_acct.c | 4 ++--
+ modules/pam_unix/support.c       | 4 ++--
+ 2 files changed, 4 insertions(+), 4 deletions(-)
+
+diff --git a/modules/pam_unix/pam_unix_acct.c 
b/modules/pam_unix/pam_unix_acct.c
+index 8f5ed3e0..2fdec6c7 100644
+--- a/modules/pam_unix/pam_unix_acct.c
++++ b/modules/pam_unix/pam_unix_acct.c
+@@ -122,12 +122,12 @@ int _unix_run_verify_binary(pam_handle_t *pamh, unsigned 
long long ctrl,
+     }
+ 
+     /* exec binary helper */
+-    args[0] = CHKPWD_HELPER;
++    args[0] = "/run/setuid-programs/unix_chkpwd";
+     args[1] = user;
+     args[2] = "chkexpiry";
+ 
+     DIAG_PUSH_IGNORE_CAST_QUAL;
+-    execve(CHKPWD_HELPER, (char *const *) args, envp);
++    execve("/run/setuid-programs/unix_chkpwd", (char *const *) args, envp);
+     DIAG_POP_IGNORE_CAST_QUAL;
+ 
+     pam_syslog(pamh, LOG_ERR, "helper binary execve failed: %m");
+diff --git a/modules/pam_unix/support.c b/modules/pam_unix/support.c
+index 27ca7127..d02f394e 100644
+--- a/modules/pam_unix/support.c
++++ b/modules/pam_unix/support.c
+@@ -523,7 +523,7 @@ static int _unix_run_helper_binary(pam_handle_t *pamh, 
const char *passwd,
+       }
+ 
+       /* exec binary helper */
+-      args[0] = CHKPWD_HELPER;
++      args[0] = "/run/setuid-programs/unix_chkpwd";
+       args[1] = user;
+       if (off(UNIX__NONULL, ctrl)) {  /* this means we've succeeded */
+         args[2]="nullok";
+@@ -532,7 +532,7 @@ static int _unix_run_helper_binary(pam_handle_t *pamh, 
const char *passwd,
+       }
+ 
+       DIAG_PUSH_IGNORE_CAST_QUAL;
+-      execve(CHKPWD_HELPER, (char *const *) args, envp);
++      execve("/run/setuid-programs/unix_chkpwd", (char *const *) args, envp);
+       DIAG_POP_IGNORE_CAST_QUAL;
+ 
+       /* should not get here: exit with error */
+-- 
+2.34.0
+
diff --git a/gnu/system/pam.scm b/gnu/system/pam.scm
index 2574e019f1..48cd2ebf2c 100644
--- a/gnu/system/pam.scm
+++ b/gnu/system/pam.scm
@@ -375,8 +375,12 @@ (define (extend-configuration initial extensions)
 
 (define pam-root-service-type
   (service-type (name 'pam)
-                (extensions (list (service-extension etc-service-type
-                                                     /etc-entry)))
+                (extensions
+                 (list (service-extension etc-service-type /etc-entry)
+                       (service-extension
+                        setuid-program-service-type
+                        (list (file-like->setuid-program
+                               (file-append linux-pam "/sbin/unix_chkpwd"))))))
 
                 ;; Arguments include <pam-service> as well as procedures.
                 (compose concatenate)
-- 
2.34.0

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