[08/12] os-posix: Provide new -runas <uid>:<gid> facility

Commit Message

Ian Jackson March 8, 2018, 7:03 p.m. UTC

This allows the caller to specify a uid and gid to use, even if there
is no corresponding password entry.  This will be useful in certain
Xen configurations.

We don't support just -runas <uid> because: (i) deprivileging without
calling setgroups would be ineffective (ii) given only a uid we don't
know what gid we ought to use (since uids may eppear in multiple
passwd file entries with different gids).

Signed-off-by: Ian Jackson <[e-mail]>
Reviewed-by: Anthony PERARD <[e-mail]>
CC: Paolo Bonzini <[e-mail]>
CC: Markus Armbruster <[e-mail]>
CC: Daniel P. Berrange <[e-mail]>
---
v6.1: Fix constness of qemu_strtoul end pointer parameter.
v6: Use qemu_strtoul for the first strtoul.
    Use error_report rather than fprintf to print usage error message.
    Fix an error message which still referred to . rather than :
v5: Use : rather than . to separate uid from gid
v4: Changed to reuse option -runas
v3: Error messages fixed.  Thanks to Peter Maydell and Ross Lagerwall.
v2: Coding style fixes.

Signed-off-by: Ian Jackson <[e-mail]>
---
 os-posix.c      | 62 +++++++++++++++++++++++++++++++++++++++++++++++----------
 qemu-options.hx |  3 ++-
 2 files changed, 53 insertions(+), 12 deletions(-)

Comments

Markus Armbruster April 13, 2018, 3:51 p.m. UTC | #1

Ian Jackson <[e-mail]> writes:

> This allows the caller to specify a uid and gid to use, even if there
> is no corresponding password entry.  This will be useful in certain
> Xen configurations.
>
> We don't support just -runas <uid> because: (i) deprivileging without
> calling setgroups would be ineffective (ii) given only a uid we don't
> know what gid we ought to use (since uids may eppear in multiple
> passwd file entries with different gids).
>
> Signed-off-by: Ian Jackson <[e-mail]>
> Reviewed-by: Anthony PERARD <[e-mail]>
> CC: Paolo Bonzini <[e-mail]>
> CC: Markus Armbruster <[e-mail]>
> CC: Daniel P. Berrange <[e-mail]>
> ---
> v6.1: Fix constness of qemu_strtoul end pointer parameter.
> v6: Use qemu_strtoul for the first strtoul.
>     Use error_report rather than fprintf to print usage error message.
>     Fix an error message which still referred to . rather than :
> v5: Use : rather than . to separate uid from gid
> v4: Changed to reuse option -runas
> v3: Error messages fixed.  Thanks to Peter Maydell and Ross Lagerwall.
> v2: Coding style fixes.
>
> Signed-off-by: Ian Jackson <[e-mail]>
> ---
>  os-posix.c      | 62 +++++++++++++++++++++++++++++++++++++++++++++++----------
>  qemu-options.hx |  3 ++-
>  2 files changed, 53 insertions(+), 12 deletions(-)
>
> diff --git a/os-posix.c b/os-posix.c
> index b9c2343..214f8fb 100644
> --- a/os-posix.c
> +++ b/os-posix.c
> @@ -42,6 +42,8 @@
>  #endif
>  
>  static struct passwd *user_pwd;
> +static uid_t user_uid = (uid_t)-1;
> +static gid_t user_gid = (gid_t)-1;

As we'll see below, @user_pwd->pw_uid, @user_pwd_pw_gid take precedence
over @user_uid, @user_gid.  Awkward.

>  static const char *chroot_dir;
>  static int daemonize;
>  static int daemon_pipe;
> @@ -127,6 +129,34 @@ void os_set_proc_name(const char *s)
>  #endif
>  }
>  
> +
> +static bool os_parse_runas_uid_gid(const char *optarg)
> +{
> +    unsigned long lv;
> +    const char *ep;
> +    uid_t got_uid;
> +    gid_t got_gid;
> +    int rc;
> +
> +    errno = 0;
> +    rc = qemu_strtoul(optarg, &ep, 0, &lv);
> +    got_uid = lv; /* overflow here is ID in C99 */
> +    if (rc || *ep != ':' || got_uid != lv || got_uid == (uid_t)-1) {
> +        return false;
> +    }
> +
> +    lv = 0;

Either zero lv before both qemu_strtoul() or neither one.

> +    rc = qemu_strtoul(ep + 1, 0, 0, &lv);
> +    got_gid = lv; /* overflow here is ID in C99 */
> +    if (rc || got_gid != lv || got_gid == (gid_t)-1) {
> +        return false;
> +    }
> +
> +    user_uid = got_uid;
> +    user_gid = got_gid;
> +    return true;
> +}
> +
>  /*
>   * Parse OS specific command line options.
>   * return 0 if option handled, -1 otherwise
> @@ -144,8 +174,8 @@ void os_parse_cmd_args(int index, const char *optarg)
>  #endif
>      case QEMU_OPTION_runas:
>          user_pwd = getpwnam(optarg);
> -        if (!user_pwd) {
> -            fprintf(stderr, "User \"%s\" doesn't exist\n", optarg);
> +        if (!user_pwd && !os_parse_runas_uid_gid(optarg)) {
> +            error_report("User doesn't exist (and is not <uid>:<gid>)");

The error message no longer includes the offending value.  Intentional?

Note for later: @user_uid and @user_gid get set only when @user_pwd
remains null.

>              exit(1);
>          }
>          break;
> @@ -165,18 +195,28 @@ void os_parse_cmd_args(int index, const char *optarg)
>  
>  static void change_process_uid(void)
>  {
> -    if (user_pwd) {
> -        if (setgid(user_pwd->pw_gid) < 0) {
> -            fprintf(stderr, "Failed to setgid(%d)\n", user_pwd->pw_gid);
> +    if (user_pwd || user_uid != (uid_t)-1) {
> +        gid_t intended_gid = user_pwd ? user_pwd->pw_gid : user_gid;
> +        uid_t intended_uid = user_pwd ? user_pwd->pw_uid : user_uid;
> +        if (setgid(intended_gid) < 0) {
> +            fprintf(stderr, "Failed to setgid(%d)\n", intended_gid);

error_report(), please.  More of the same below.

>              exit(1);
>          }
> -        if (initgroups(user_pwd->pw_name, user_pwd->pw_gid) < 0) {
> -            fprintf(stderr, "Failed to initgroups(\"%s\", %d)\n",
> -                    user_pwd->pw_name, user_pwd->pw_gid);
> -            exit(1);
> +        if (user_pwd) {
> +            if (initgroups(user_pwd->pw_name, user_pwd->pw_gid) < 0) {
> +                fprintf(stderr, "Failed to initgroups(\"%s\", %d)\n",
> +                        user_pwd->pw_name, user_pwd->pw_gid);
> +                exit(1);
> +            }
> +        } else {
> +            if (setgroups(1, &user_gid) < 0) {
> +                fprintf(stderr, "Failed to setgroups(1, [%d])",
> +                        user_gid);
> +                exit(1);
> +            }
>          }
> -        if (setuid(user_pwd->pw_uid) < 0) {
> -            fprintf(stderr, "Failed to setuid(%d)\n", user_pwd->pw_uid);
> +        if (setuid(intended_uid) < 0) {
> +            fprintf(stderr, "Failed to setuid(%d)\n", intended_uid);
>              exit(1);
>          }
>          if (setuid(0) != -1) {

This function is the only user of @user_pwd, @user_uid, @user_gid.

Have you considered replacing global @user_pwd by @user_uid, @user_gid
and @user_name?  --runas with numeric uid and gid would leave @user_name
null.

> diff --git a/qemu-options.hx b/qemu-options.hx
> index 6585058..211f2a6 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -3763,7 +3763,8 @@ ETEXI
>  
>  #ifndef _WIN32
>  DEF("runas", HAS_ARG, QEMU_OPTION_runas, \
> -    "-runas user     change to user id user just before starting the VM\n",
> +    "-runas user     change to user id user just before starting the VM\n" \
> +    "                user can be numeric uid:gid instead\n",
>      QEMU_ARCH_ALL)
>  #endif
>  STEXI

Ian Jackson April 16, 2018, 2:06 p.m. UTC | #2

Thanks for the review.  Taking your comments out of order slightly:

Markus Armbruster writes ("Re: [Qemu-devel] [PATCH 08/12] os-posix: Provide new -runas <uid>:<gid> facility"):
> [change_process_uid] is the only user of @user_pwd, @user_uid, @user_gid.
> 
> Have you considered replacing global @user_pwd by @user_uid, @user_gid
> and @user_name?  --runas with numeric uid and gid would leave @user_name
> null.

That would defer the getpwnam from argument parsing to os_setup_post.
I think that's undesriable.

> Ian Jackson <[e-mail]> writes:
> >  static struct passwd *user_pwd;
> > +static uid_t user_uid = (uid_t)-1;
> > +static gid_t user_gid = (gid_t)-1;
> 
> As we'll see below, @user_pwd->pw_uid, @user_pwd_pw_gid take precedence
> over @user_uid, @user_gid.  Awkward.

My patch has the right behaviour: each -runas completely overrides the
previous one.  -runas that sets user_{uid,gid} always clears user_pwd
on the way.  So user_pwd can only be set if the most recent -runas was
a name, and then we should honour the name.

This is rather obscure.  I think you are right that this is confusing.
It ought to be clearer.

I will
  - add a comment next to these three variables saying they must
    all be set at the same time
  - explicitly (redundantly) clear user_pwd in os_parse_runas_uid_gid
  - explicitly set user_{uid,gid} to -1 when -runas gets a
    success from getpwnam
  - assert in change_process_uid that the combination is legal

> > +    errno = 0;
> > +    rc = qemu_strtoul(optarg, &ep, 0, &lv);
...
> > +    lv = 0;
> 
> Either zero lv before both qemu_strtoul() or neither one.

This is a hangover from the previous version which used raw strtoul.
The assignments to both lv and errno are redundant.

> > -        if (!user_pwd) {
> > -            fprintf(stderr, "User \"%s\" doesn't exist\n", optarg);
> > +        if (!user_pwd && !os_parse_runas_uid_gid(optarg)) {
> > +            error_report("User doesn't exist (and is not <uid>:<gid>)");
> 
> The error message no longer includes the offending value.  Intentional?

I was in two minds.  I will put it back.

> > +    if (user_pwd || user_uid != (uid_t)-1) {
> > +        gid_t intended_gid = user_pwd ? user_pwd->pw_gid : user_gid;
> > +        uid_t intended_uid = user_pwd ? user_pwd->pw_uid : user_uid;
> > +        if (setgid(intended_gid) < 0) {
> > +            fprintf(stderr, "Failed to setgid(%d)\n", intended_gid);
> 
> error_report(), please.  More of the same below.

I was following the existing code in this function.  I'll add a
pre-patch to fix this up here, and maybe a post-patch to fix the rest
of this file.

Thanks,
Ian.

Markus Armbruster April 16, 2018, 4:17 p.m. UTC | #3

Ian Jackson <[e-mail]> writes:

> Thanks for the review.  Taking your comments out of order slightly:
>
> Markus Armbruster writes ("Re: [Qemu-devel] [PATCH 08/12] os-posix: Provide new -runas <uid>:<gid> facility"):
>> [change_process_uid] is the only user of @user_pwd, @user_uid, @user_gid.
>> 
>> Have you considered replacing global @user_pwd by @user_uid, @user_gid
>> and @user_name?  --runas with numeric uid and gid would leave @user_name
>> null.
>
> That would defer the getpwnam from argument parsing to os_setup_post.
> I think that's undesriable.

No argument.  But why can't os_parse_cmd_args() call getpwnam() as it
does now, then store user_pwd->pw_uid, ->pw_gid and ->pw_name instead of
user_pwd?  Store a null name when it parses the argument as UID:GID.

>> Ian Jackson <[e-mail]> writes:
>> >  static struct passwd *user_pwd;
>> > +static uid_t user_uid = (uid_t)-1;
>> > +static gid_t user_gid = (gid_t)-1;
>> 
>> As we'll see below, @user_pwd->pw_uid, @user_pwd_pw_gid take precedence
>> over @user_uid, @user_gid.  Awkward.
>
> My patch has the right behaviour: each -runas completely overrides the
> previous one.  -runas that sets user_{uid,gid} always clears user_pwd
> on the way.  So user_pwd can only be set if the most recent -runas was
> a name, and then we should honour the name.
>
> This is rather obscure.  I think you are right that this is confusing.
> It ought to be clearer.
>
> I will
>   - add a comment next to these three variables saying they must
>     all be set at the same time
>   - explicitly (redundantly) clear user_pwd in os_parse_runas_uid_gid
>   - explicitly set user_{uid,gid} to -1 when -runas gets a
>     success from getpwnam
>   - assert in change_process_uid that the combination is legal

Yes, that's better.  But perhaps you like my idea above.

[...]

Ian Jackson April 16, 2018, 5 p.m. UTC | #4

Markus Armbruster writes ("Re: [Qemu-devel] [PATCH 08/12] os-posix: Provide new -runas <uid>:<gid> facility"):
> Ian Jackson <[e-mail]> writes:
> > That would defer the getpwnam from argument parsing to os_setup_post.
> > I think that's undesriable.
> 
> No argument.  But why can't os_parse_cmd_args() call getpwnam() as it
> does now, then store user_pwd->pw_uid, ->pw_gid and ->pw_name instead of
> user_pwd?  Store a null name when it parses the argument as UID:GID.

Oh, I see.  It seems less obvious to me than what I have done, but I
can do it like that if you like.

Ian.

Markus Armbruster April 16, 2018, 6:06 p.m. UTC | #5

Ian Jackson <[e-mail]> writes:

> Markus Armbruster writes ("Re: [Qemu-devel] [PATCH 08/12] os-posix: Provide new -runas <uid>:<gid> facility"):
>> Ian Jackson <[e-mail]> writes:
>> > That would defer the getpwnam from argument parsing to os_setup_post.
>> > I think that's undesriable.
>> 
>> No argument.  But why can't os_parse_cmd_args() call getpwnam() as it
>> does now, then store user_pwd->pw_uid, ->pw_gid and ->pw_name instead of
>> user_pwd?  Store a null name when it parses the argument as UID:GID.
>
> Oh, I see.  It seems less obvious to me than what I have done, but I
> can do it like that if you like.

I just wanted to toss out the idea.  Please use your judgenment and do
it the way you like better.

Patch

diff --git a/os-posix.c b/os-posix.c
index b9c2343..214f8fb 100644
--- a/os-posix.c
+++ b/os-posix.c
@@ -42,6 +42,8 @@ 
 #endif
 
 static struct passwd *user_pwd;
+static uid_t user_uid = (uid_t)-1;
+static gid_t user_gid = (gid_t)-1;
 static const char *chroot_dir;
 static int daemonize;
 static int daemon_pipe;
@@ -127,6 +129,34 @@  void os_set_proc_name(const char *s)
 #endif
 }
 
+
+static bool os_parse_runas_uid_gid(const char *optarg)
+{
+    unsigned long lv;
+    const char *ep;
+    uid_t got_uid;
+    gid_t got_gid;
+    int rc;
+
+    errno = 0;
+    rc = qemu_strtoul(optarg, &ep, 0, &lv);
+    got_uid = lv; /* overflow here is ID in C99 */
+    if (rc || *ep != ':' || got_uid != lv || got_uid == (uid_t)-1) {
+        return false;
+    }
+
+    lv = 0;
+    rc = qemu_strtoul(ep + 1, 0, 0, &lv);
+    got_gid = lv; /* overflow here is ID in C99 */
+    if (rc || got_gid != lv || got_gid == (gid_t)-1) {
+        return false;
+    }
+
+    user_uid = got_uid;
+    user_gid = got_gid;
+    return true;
+}
+
 /*
  * Parse OS specific command line options.
  * return 0 if option handled, -1 otherwise
@@ -144,8 +174,8 @@  void os_parse_cmd_args(int index, const char *optarg)
 #endif
     case QEMU_OPTION_runas:
         user_pwd = getpwnam(optarg);
-        if (!user_pwd) {
-            fprintf(stderr, "User \"%s\" doesn't exist\n", optarg);
+        if (!user_pwd && !os_parse_runas_uid_gid(optarg)) {
+            error_report("User doesn't exist (and is not <uid>:<gid>)");
             exit(1);
         }
         break;
@@ -165,18 +195,28 @@  void os_parse_cmd_args(int index, const char *optarg)
 
 static void change_process_uid(void)
 {
-    if (user_pwd) {
-        if (setgid(user_pwd->pw_gid) < 0) {
-            fprintf(stderr, "Failed to setgid(%d)\n", user_pwd->pw_gid);
+    if (user_pwd || user_uid != (uid_t)-1) {
+        gid_t intended_gid = user_pwd ? user_pwd->pw_gid : user_gid;
+        uid_t intended_uid = user_pwd ? user_pwd->pw_uid : user_uid;
+        if (setgid(intended_gid) < 0) {
+            fprintf(stderr, "Failed to setgid(%d)\n", intended_gid);
             exit(1);
         }
-        if (initgroups(user_pwd->pw_name, user_pwd->pw_gid) < 0) {
-            fprintf(stderr, "Failed to initgroups(\"%s\", %d)\n",
-                    user_pwd->pw_name, user_pwd->pw_gid);
-            exit(1);
+        if (user_pwd) {
+            if (initgroups(user_pwd->pw_name, user_pwd->pw_gid) < 0) {
+                fprintf(stderr, "Failed to initgroups(\"%s\", %d)\n",
+                        user_pwd->pw_name, user_pwd->pw_gid);
+                exit(1);
+            }
+        } else {
+            if (setgroups(1, &user_gid) < 0) {
+                fprintf(stderr, "Failed to setgroups(1, [%d])",
+                        user_gid);
+                exit(1);
+            }
         }
-        if (setuid(user_pwd->pw_uid) < 0) {
-            fprintf(stderr, "Failed to setuid(%d)\n", user_pwd->pw_uid);
+        if (setuid(intended_uid) < 0) {
+            fprintf(stderr, "Failed to setuid(%d)\n", intended_uid);
             exit(1);
         }
         if (setuid(0) != -1) {
diff --git a/qemu-options.hx b/qemu-options.hx
index 6585058..211f2a6 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -3763,7 +3763,8 @@  ETEXI
 
 #ifndef _WIN32
 DEF("runas", HAS_ARG, QEMU_OPTION_runas, \
-    "-runas user     change to user id user just before starting the VM\n",
+    "-runas user     change to user id user just before starting the VM\n" \
+    "                user can be numeric uid:gid instead\n",
     QEMU_ARCH_ALL)
 #endif
 STEXI