nss-systemd: ensure returned strings point into provided buffer

Jamie Bainbridge found an issue where glib's g_get_user_database_entry()
may crash after doing:

```
error = getpwnam_r (logname, &pwd, buffer, bufsize, &pw);
// ...
pw->pw_name[0] = g_ascii_toupper (pw->pw_name[0]);
```

in order to uppercase the first letter of the user's real name. This is
a glib bug, because there is a different codepath that gets the pwd from
vanilla getpwnam instead of getpwnam_r as shown here. When the pwd
struct is returned by getpwnam, its fields point to static data owned by
glibc/NSS, and so it must not be modified by the caller. After much
debugging, Jamie Bainbridge has fixed this in https://gitlab.gnome.org/GNOME/glib/-/merge_requests/2244
by making a copy of the data before modifying it, and that resolves all
problems for glib. Yay!

However, glib is crashing even when getpwnam_r is used instead of
getpwnam! According to getpwnam_r(3), the strings in the pwd struct are
supposed to be pointers into the buffer passed by the caller, so glib
should be able to safely edit it directly in this case, so long as it
doesn't try to increase the size of any of the strings.

Problem is various functions throughout nss-systemd.c return synthesized
records declared at the top of the file. These records are returned
directly and so contain pointers to static strings owned by
libsystemd-nss. systemd must instead copy all the strings into the
provided buffer.

This crash is reproducible if nss-systemd is listed first on the passwd
line in /etc/nsswitch.conf, and the application looks up one of the
synthesized user accounts "root" or "nobody", and finally the
application attempts to edit one of the strings in the returned struct.
All our synthesized records for the other struct types have the same
problem, so this commit fixes them all at once.

Fixes #20679

1 files changed, 168 insertions, 36 deletions

diff --git a/src/nss-systemd/nss-systemd.c b/src/nss-systemd/nss-systemd.c
index 1b0866109a..1840a0d508 100644
--- a/src/nss-systemd/nss-systemd.c
+++ b/src/nss-systemd/nss-systemd.c
@@ -2,6 +2,7 @@
 
 #include <nss.h>
 #include <pthread.h>
+#include <string.h>
 
 #include "env-util.h"
 #include "errno-util.h"
@@ -139,6 +140,155 @@ NSS_GRENT_PROTOTYPES(systemd);
 NSS_SGENT_PROTOTYPES(systemd);
 NSS_INITGROUPS_PROTOTYPE(systemd);
 
+/* Since our NSS functions implement reentrant glibc APIs, we have to guarantee
+ * all the string pointers we return point into the buffer provided by the
+ * caller, not into our own static memory. */
+
+static enum nss_status copy_synthesized_passwd(
+                struct passwd *dest,
+                const struct passwd *src,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        size_t required;
+
+        assert(dest);
+        assert(src);
+        assert(src->pw_name);
+        assert(src->pw_passwd);
+        assert(src->pw_gecos);
+        assert(src->pw_dir);
+        assert(src->pw_shell);
+
+        required = strlen(src->pw_name) + 1;
+        required += strlen(src->pw_passwd) + 1;
+        required += strlen(src->pw_gecos) + 1;
+        required += strlen(src->pw_dir) + 1;
+        required += strlen(src->pw_shell) + 1;
+
+        if (buflen < required) {
+                *errnop = ERANGE;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        assert(buffer);
+
+        *dest = *src;
+
+        /* String fields point into the user-provided buffer */
+        dest->pw_name = buffer;
+        dest->pw_passwd = stpcpy(dest->pw_name, src->pw_name) + 1;
+        dest->pw_gecos = stpcpy(dest->pw_passwd, src->pw_passwd) + 1;
+        dest->pw_dir = stpcpy(dest->pw_gecos, src->pw_gecos) + 1;
+        dest->pw_shell = stpcpy(dest->pw_dir, src->pw_dir) + 1;
+        strcpy(dest->pw_shell, src->pw_shell);
+
+        return NSS_STATUS_SUCCESS;
+}
+
+static enum nss_status copy_synthesized_spwd(
+                struct spwd *dest,
+                const struct spwd *src,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        size_t required;
+
+        assert(dest);
+        assert(src);
+        assert(src->sp_namp);
+        assert(src->sp_pwdp);
+
+        required = strlen(src->sp_namp) + 1;
+        required += strlen(src->sp_pwdp) + 1;
+
+        if (buflen < required) {
+                *errnop = ERANGE;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        assert(buffer);
+
+        *dest = *src;
+
+        /* String fields point into the user-provided buffer */
+        dest->sp_namp = buffer;
+        dest->sp_pwdp = stpcpy(dest->sp_namp, src->sp_namp) + 1;
+        strcpy(dest->sp_pwdp, src->sp_pwdp);
+
+        return NSS_STATUS_SUCCESS;
+}
+
+static enum nss_status copy_synthesized_group(
+                struct group *dest,
+                const struct group *src,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        size_t required;
+
+        assert(dest);
+        assert(src);
+        assert(src->gr_name);
+        assert(src->gr_passwd);
+        assert(src->gr_mem);
+        assert(!*src->gr_mem); /* Our synthesized records' gr_mem is always just NULL... */
+
+        required = strlen(src->gr_name) + 1;
+        required += strlen(src->gr_passwd) + 1;
+        required += 1; /* ...but that NULL still needs to be stored into the buffer! */
+
+        if (buflen < required) {
+                *errnop = ERANGE;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        assert(buffer);
+
+        *dest = *src;
+
+        /* String fields point into the user-provided buffer */
+        dest->gr_name = buffer;
+        dest->gr_passwd = stpcpy(dest->gr_name, src->gr_name) + 1;
+        dest->gr_mem = (char **) strcpy(dest->gr_passwd, src->gr_passwd) + 1;
+        *dest->gr_mem = NULL;
+
+        return NSS_STATUS_SUCCESS;
+}
+
+static enum nss_status copy_synthesized_sgrp(
+                struct sgrp *dest,
+                const struct sgrp *src,
+                char *buffer, size_t buflen,
+                int *errnop) {
+
+        size_t required;
+
+        assert(dest);
+        assert(src);
+        assert(src->sg_namp);
+        assert(src->sg_passwd);
+
+        required = strlen(src->sg_namp) + 1;
+        required += strlen(src->sg_passwd) + 1;
+
+        if (buflen < required) {
+                *errnop = ERANGE;
+                return NSS_STATUS_TRYAGAIN;
+        }
+
+        assert(buffer);
+
+        *dest = *src;
+
+        /* String fields point into the user-provided buffer */
+        dest->sg_namp = buffer;
+        dest->sg_passwd = stpcpy(dest->sg_namp, src->sg_namp) + 1;
+        strcpy(dest->sg_passwd, src->sg_passwd);
+
+        return NSS_STATUS_SUCCESS;
+}
+
 enum nss_status _nss_systemd_getpwnam_r(
                 const char *name,
                 struct passwd *pwd,
@@ -164,17 +314,14 @@ enum nss_status _nss_systemd_getpwnam_r(
         /* Synthesize entries for the root and nobody users, in case they are missing in /etc/passwd */
         if (getenv_bool_secure("SYSTEMD_NSS_BYPASS_SYNTHETIC") <= 0) {
 
-                if (streq(name, root_passwd.pw_name)) {
-                        *pwd = root_passwd;
-                        return NSS_STATUS_SUCCESS;
-                }
+                if (streq(name, root_passwd.pw_name))
+                        return copy_synthesized_passwd(pwd, &root_passwd, buffer, buflen, errnop);
 
                 if (streq(name, nobody_passwd.pw_name)) {
                         if (!synthesize_nobody())
                                 return NSS_STATUS_NOTFOUND;
 
-                        *pwd = nobody_passwd;
-                        return NSS_STATUS_SUCCESS;
+                        return copy_synthesized_passwd(pwd, &nobody_passwd, buffer, buflen, errnop);
                 }
 
         } else if (STR_IN_SET(name, root_passwd.pw_name, nobody_passwd.pw_name))
@@ -211,17 +358,14 @@ enum nss_status _nss_systemd_getpwuid_r(
         /* Synthesize data for the root user and for nobody in case they are missing from /etc/passwd */
         if (getenv_bool_secure("SYSTEMD_NSS_BYPASS_SYNTHETIC") <= 0) {
 
-                if (uid == root_passwd.pw_uid) {
-                        *pwd = root_passwd;
-                        return NSS_STATUS_SUCCESS;
-                }
+                if (uid == root_passwd.pw_uid)
+                        return copy_synthesized_passwd(pwd, &root_passwd, buffer, buflen, errnop);
 
                 if (uid == nobody_passwd.pw_uid) {
                         if (!synthesize_nobody())
                                 return NSS_STATUS_NOTFOUND;
 
-                        *pwd = nobody_passwd;
-                        return NSS_STATUS_SUCCESS;
+                        return copy_synthesized_passwd(pwd, &nobody_passwd, buffer, buflen, errnop);
                 }
 
         } else if (uid == root_passwd.pw_uid || uid == nobody_passwd.pw_uid)
@@ -259,17 +403,14 @@ enum nss_status _nss_systemd_getspnam_r(
         /* Synthesize entries for the root and nobody users, in case they are missing in /etc/passwd */
         if (getenv_bool_secure("SYSTEMD_NSS_BYPASS_SYNTHETIC") <= 0) {
 
-                if (streq(name, root_spwd.sp_namp)) {
-                        *spwd = root_spwd;
-                        return NSS_STATUS_SUCCESS;
-                }
+                if (streq(name, root_spwd.sp_namp))
+                        return copy_synthesized_spwd(spwd, &root_spwd, buffer, buflen, errnop);
 
                 if (streq(name, nobody_spwd.sp_namp)) {
                         if (!synthesize_nobody())
                                 return NSS_STATUS_NOTFOUND;
 
-                        *spwd = nobody_spwd;
-                        return NSS_STATUS_SUCCESS;
+                        return copy_synthesized_spwd(spwd, &nobody_spwd, buffer, buflen, errnop);
                 }
 
         } else if (STR_IN_SET(name, root_spwd.sp_namp, nobody_spwd.sp_namp))
@@ -309,17 +450,14 @@ enum nss_status _nss_systemd_getgrnam_r(
         /* Synthesize records for root and nobody, in case they are missing from /etc/group */
         if (getenv_bool_secure("SYSTEMD_NSS_BYPASS_SYNTHETIC") <= 0) {
 
-                if (streq(name, root_group.gr_name)) {
-                        *gr = root_group;
-                        return NSS_STATUS_SUCCESS;
-                }
+                if (streq(name, root_group.gr_name))
+                        return copy_synthesized_group(gr, &root_group, buffer, buflen, errnop);
 
                 if (streq(name, nobody_group.gr_name)) {
                         if (!synthesize_nobody())
                                 return NSS_STATUS_NOTFOUND;
 
-                        *gr = nobody_group;
-                        return NSS_STATUS_SUCCESS;
+                        return copy_synthesized_group(gr, &nobody_group, buffer, buflen, errnop);
                 }
 
         } else if (STR_IN_SET(name, root_group.gr_name, nobody_group.gr_name))
@@ -356,17 +494,14 @@ enum nss_status _nss_systemd_getgrgid_r(
         /* Synthesize records for root and nobody, in case they are missing from /etc/group */
         if (getenv_bool_secure("SYSTEMD_NSS_BYPASS_SYNTHETIC") <= 0) {
 
-                if (gid == root_group.gr_gid) {
-                        *gr = root_group;
-                        return NSS_STATUS_SUCCESS;
-                }
+                if (gid == root_group.gr_gid)
+                        return copy_synthesized_group(gr, &root_group, buffer, buflen, errnop);
 
                 if (gid == nobody_group.gr_gid) {
                         if (!synthesize_nobody())
                                 return NSS_STATUS_NOTFOUND;
 
-                        *gr = nobody_group;
-                        return NSS_STATUS_SUCCESS;
+                        return copy_synthesized_group(gr, &nobody_group, buffer, buflen, errnop);
                 }
 
         } else if (gid == root_group.gr_gid || gid == nobody_group.gr_gid)
@@ -404,17 +539,14 @@ enum nss_status _nss_systemd_getsgnam_r(
         /* Synthesize records for root and nobody, in case they are missing from /etc/group */
         if (getenv_bool_secure("SYSTEMD_NSS_BYPASS_SYNTHETIC") <= 0) {
 
-                if (streq(name, root_sgrp.sg_namp)) {
-                        *sgrp = root_sgrp;
-                        return NSS_STATUS_SUCCESS;
-                }
+                if (streq(name, root_sgrp.sg_namp))
+                        return copy_synthesized_sgrp(sgrp, &root_sgrp, buffer, buflen, errnop);
 
                 if (streq(name, nobody_sgrp.sg_namp)) {
                         if (!synthesize_nobody())
                                 return NSS_STATUS_NOTFOUND;
 
-                        *sgrp = nobody_sgrp;
-                        return NSS_STATUS_SUCCESS;
+                        return copy_synthesized_sgrp(sgrp, &nobody_sgrp, buffer, buflen, errnop);
                 }
 
         } else if (STR_IN_SET(name, root_sgrp.sg_namp, nobody_sgrp.sg_namp))