Paolo Bonzini <pbonz...@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 <pbon...@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 <pbon...@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;
