Re: [Qemu-devel] [PATCH] virtfs-proxy-helper: check return code of setfsgid/setfsuid

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

Paolo Bonzini <pbonzini@redhat.com> writes:

> Il 11/10/2012 09:25, M. Mohan Kumar ha scritto:
>> Also as per the man page:
>>        When glibc determines that the argument is not a valid user ID,
>>        it will return -1 and set errno  to  EINVAL
>>        without attempting the system call.
>> 
>> If it mean a nonexistent id by 'not a valid user ID' it may be a
>> problem in virtfs case.
>
> I think only -1 would be an invalid user ID, or perhaps a user ID >
> 65535 if the kernel only supports 16-bit user IDs.
>
> Rather than dealing with the kernel, can we just use
> setresuid/setresgid like in the following (untested) patch?
>
> Paolo
>
> ps: so far in my short life I had managed to stay away from privilege
> dropping, so please review with extra care.
>
> ------------------- 8< -----------------------
> From: Paolo Bonzini <pbonini@redhat.com>
> Date: Thu, 11 Oct 2012 14:20:23 +0200
> Subject: [PATCH] virtfs-proxy-helper: use setresuid and setresgid
>
> The setfsuid and setfsgid system calls are obscure and they complicate
> the error checking (that glibc's warn_unused_result "feature" forces
> us to do).  Switch to the standard setresuid and setresgid functions.
>
> ---
> diff --git a/fsdev/virtfs-proxy-helper.c b/fsdev/virtfs-proxy-helper.c
> index f9a8270..07b3b5b 100644
> --- a/fsdev/virtfs-proxy-helper.c
> +++ b/fsdev/virtfs-proxy-helper.c
> @@ -272,31 +272,76 @@ static int send_status(int sockfd, struct iovec *iovec, int status)
>  /*
>   * from man 7 capabilities, section
>   * Effect of User ID Changes on Capabilities:
> - * 4. If the file system user ID is changed from 0 to nonzero (see setfsuid(2))
> - * then the following capabilities are cleared from the effective set:
> - * CAP_CHOWN, CAP_DAC_OVERRIDE, CAP_DAC_READ_SEARCH,  CAP_FOWNER, CAP_FSETID,
> - * CAP_LINUX_IMMUTABLE  (since  Linux 2.2.30), CAP_MAC_OVERRIDE, and CAP_MKNOD
> - * (since Linux 2.2.30). If the file system UID is changed from nonzero to 0,
> - * then any of these capabilities that are enabled in the permitted set
> - * are enabled in the effective set.
> + * If the effective user ID is changed from nonzero to 0, then the permitted
> + * set is copied to the effective set.  If the effective user ID is changed
> + * from 0 to nonzero, then all capabilities are are cleared from the effective
> + * set.
> + *
> + * The setfsuid/setfsgid man pages warn that changing the effective user ID may
> + * expose the program to unwanted signals, but this is not true anymore: for an
> + * unprivileged (without CAP_KILL) program to send a signal, the real or
> + * effective user ID of the sending process must equal the real or saved user
> + * ID of the target process.  Even when dropping privileges, it is enough to
> + * keep the saved UID to a "privileged" value and virtfs-proxy-helper won't
> + * be exposed to signals.  So just use setresuid/setresgid.
>   */
> -static int setfsugid(int uid, int gid)
> +static int setugid(int uid, int gid, int *suid, int *sgid)
>  {
> +    int retval;
> +
>      /*
> -     * We still need DAC_OVERRIDE because  we don't change
> +     * We still need DAC_OVERRIDE because we don't change
>       * supplementary group ids, and hence may be subjected DAC rules
>       */
>      cap_value_t cap_list[] = {
>          CAP_DAC_OVERRIDE,
>      };
>
> -    setfsgid(gid);
> -    setfsuid(uid);
> +    /*
> +     * If suid/sgid are NULL, the saved uid/gid is set to the
> +     * new effective uid/gid.  If they are not, the saved uid/gid
> +     * is set to the current effective user id and stored into
> +     * *suid and *sgid.
> +     */
> +    if (!suid) {
> +        suid = &uid;
> +    } else {
> +        *suid = geteuid();
> +    }
> +    if (!sgid) {
> +        sgid = &gid;
> +    } else {
> +        *sgid = getegid();
> +    }
> +

I found this to be confusing. How about avoiding all those pointers, something
like below ? If you are ok can I add the signed-off-by for this ? I can
test this and get a pull request out with the build fix.

commit 24cc9f0d07c2a505bfafbdcb72006f2eda1288a4
Author: Paolo Bonzini <pbonini@redhat.com>
Date:   Thu Oct 11 14:20:23 2012 +0200

    virtfs-proxy-helper: use setresuid and setresgid
    
    The setfsuid and setfsgid system calls are obscure and they complicate
    the error checking (that glibc's warn_unused_result "feature" forces
    us to do).  Switch to the standard setresuid and setresgid functions.

diff --git a/fsdev/virtfs-proxy-helper.c b/fsdev/virtfs-proxy-helper.c
index f9a8270..49ab0eb 100644
--- a/fsdev/virtfs-proxy-helper.c
+++ b/fsdev/virtfs-proxy-helper.c
@@ -272,31 +272,59 @@ static int send_status(int sockfd, struct iovec *iovec, int status)
 /*
  * from man 7 capabilities, section
  * Effect of User ID Changes on Capabilities:
- * 4. If the file system user ID is changed from 0 to nonzero (see setfsuid(2))
- * then the following capabilities are cleared from the effective set:
- * CAP_CHOWN, CAP_DAC_OVERRIDE, CAP_DAC_READ_SEARCH,  CAP_FOWNER, CAP_FSETID,
- * CAP_LINUX_IMMUTABLE  (since  Linux 2.2.30), CAP_MAC_OVERRIDE, and CAP_MKNOD
- * (since Linux 2.2.30). If the file system UID is changed from nonzero to 0,
- * then any of these capabilities that are enabled in the permitted set
- * are enabled in the effective set.
+ * If the effective user ID is changed from nonzero to 0, then the permitted
+ * set is copied to the effective set.  If the effective user ID is changed
+ * from 0 to nonzero, then all capabilities are are cleared from the effective
+ * set.
+ *
+ * The setfsuid/setfsgid man pages warn that changing the effective user ID may
+ * expose the program to unwanted signals, but this is not true anymore: for an
+ * unprivileged (without CAP_KILL) program to send a signal, the real or
+ * effective user ID of the sending process must equal the real or saved user
+ * ID of the target process.  Even when dropping privileges, it is enough to
+ * keep the saved UID to a "privileged" value and virtfs-proxy-helper won't
+ * be exposed to signals.  So just use setresuid/setresgid.
  */
-static int setfsugid(int uid, int gid)
+static int setugid(int uid, int gid, int suid, int sgid)
 {
+    int retval;
+
     /*
-     * We still need DAC_OVERRIDE because  we don't change
+     * We still need DAC_OVERRIDE because we don't change
      * supplementary group ids, and hence may be subjected DAC rules
      */
     cap_value_t cap_list[] = {
         CAP_DAC_OVERRIDE,
     };
 
-    setfsgid(gid);
-    setfsuid(uid);
+    if (setresuid(-1, uid, suid) == -1) {
+        retval = -errno;
+        goto err_out;
+    }
+    if (setresgid(-1, gid, sgid) == -1) {
+        retval = -errno;
+        goto err_suid;
+    }
 
     if (uid != 0 || gid != 0) {
-        return do_cap_set(cap_list, ARRAY_SIZE(cap_list), 0);
+        if (do_cap_set(cap_list, ARRAY_SIZE(cap_list), 0) < 0) {
+            retval = -errno;
+            goto err_sgid;
+        }
     }
+
     return 0;
+
+err_sgid:
+    if (setresgid(-1, sgid, sgid) == -1) {
+        abort();
+    }
+err_suid:
+    if (setresuid(-1, suid, suid) == -1) {
+        abort();
+    }
+err_out:
+    return retval;
 }
 
 /*
@@ -586,9 +614,8 @@ static int do_create_others(int type, struct iovec *iovec)
         return retval;
     }
     offset += retval;
-    retval = setfsugid(uid, gid);
+    retval = setugid(uid, gid, cur_uid, cur_gid);
     if (retval < 0) {
-        retval = -errno;
         goto err_out;
     }
     switch (type) {
@@ -621,7 +648,7 @@ static int do_create_others(int type, struct iovec *iovec)
 err_out:
     v9fs_string_free(&path);
     v9fs_string_free(&oldpath);
-    setfsugid(cur_uid, cur_gid);
+    setugid(cur_uid, cur_gid, cur_uid, cur_gid);
     return retval;
 }
 
@@ -641,24 +668,20 @@ static int do_create(struct iovec *iovec)
     if (ret < 0) {
         goto unmarshal_err_out;
     }
+
     cur_uid = geteuid();
     cur_gid = getegid();
-    ret = setfsugid(uid, gid);
+    ret = setugid(uid, gid, cur_uid, cur_gid);
     if (ret < 0) {
-        /*
-         * On failure reset back to the
-         * old uid/gid
-         */
-        ret = -errno;
-        goto err_out;
+        goto unmarshal_err_out;
     }
     ret = open(path.data, flags, mode);
     if (ret < 0) {
         ret = -errno;
     }
 
-err_out:
-    setfsugid(cur_uid, cur_gid);
+    setugid(cur_uid, cur_gid, cur_uid, cur_gid);
+
 unmarshal_err_out:
     v9fs_string_free(&path);
     return ret;