Patchwork [v2] block: Add support for Secure Shell (ssh) block device.

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Submitter Richard W.M. Jones
Date March 25, 2013, 11:49 p.m.
Message ID <1364255381-16599-2-git-send-email-rjones@redhat.com>
Download mbox | patch
Permalink /patch/231029/
State New
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Comments

Richard W.M. Jones - March 25, 2013, 11:49 p.m.
From: "Richard W.M. Jones" <rjones@redhat.com>

  qemu-system-x86_64 -drive file=ssh://hostname/some/image

QEMU will ssh into 'hostname' and open '/some/image' which is made
available as a standard block device.

You can specify a username (ssh://user@host/...) and/or a port number
(ssh://host:port/...).

Current limitations:

- Authentication must be done without passwords or passphrases, using
  ssh-agent.  Other authentication methods are not supported. (*)

- Does not check host key. (*)

- New remote files cannot be created. (*)

- Uses a single connection, instead of concurrent AIO with multiple
  SSH connections.

- Blocks during connection and authentication.

(*) = potentially easy fix

This is implemented using libssh2 on the client side.  The server just
requires a regular ssh daemon with sftp-server support.  Most ssh
daemons on Unix/Linux systems will work out of the box.
---
 block/Makefile.objs |   1 +
 block/ssh.c         | 656 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 configure           |  47 ++++
 qemu-doc.texi       |  28 +++
 4 files changed, 732 insertions(+)
 create mode 100644 block/ssh.c
Stefan Hajnoczi - March 26, 2013, 10:16 a.m.
On Mon, Mar 25, 2013 at 11:49:41PM +0000, Richard W.M. Jones wrote:
> From: "Richard W.M. Jones" <rjones@redhat.com>
> 
>   qemu-system-x86_64 -drive file=ssh://hostname/some/image
> 
> QEMU will ssh into 'hostname' and open '/some/image' which is made
> available as a standard block device.
> 
> You can specify a username (ssh://user@host/...) and/or a port number
> (ssh://host:port/...).
> 
> Current limitations:
> 
> - Authentication must be done without passwords or passphrases, using
>   ssh-agent.  Other authentication methods are not supported. (*)
> 
> - Does not check host key. (*)
> 
> - New remote files cannot be created. (*)
> 
> - Uses a single connection, instead of concurrent AIO with multiple
>   SSH connections.
> 
> - Blocks during connection and authentication.

This isn't ideal but .bdrv_open() is a blocking function anyway.  If you
open a raw image on an NFS export the open(2) call can block too.

Blocking in .bdrv_open() is fine during startup.  It's bad when
hotplugging drives since the running VM will experience downtime.

> +    /* Start SFTP. */
> +    s->sftp = libssh2_sftp_init(s->session);
> +    if (!s->sftp) {
> +        session_error_report(s, "failed to initialize sftp handle");
> +        ret = -EINVAL;
> +        goto err;
> +    }
> +
> +    s->sftp_handle = libssh2_sftp_open(s->sftp, path,
> +                                       LIBSSH2_FXF_READ|LIBSSH2_FXF_WRITE,

Probably worth handling -drive ...,readonly=on.  !(flags & BDRV_O_RDWR)

> +static coroutine_fn void set_fd_handler(BDRVSSHState *s)
> +{
> +    int r;
> +    void (*rd_handler)(void*);
> +    void (*wr_handler)(void*);

There's a typedef for this: IOHandler.

> +    Coroutine *co = qemu_coroutine_self();
> +
> +    rd_handler = wr_handler = NULL;
> +
> +    r = libssh2_session_block_directions(s->session);
> +
> +    if (r & LIBSSH2_SESSION_BLOCK_INBOUND) {
> +        rd_handler = restart_coroutine;
> +    }
> +    if (r & LIBSSH2_SESSION_BLOCK_OUTBOUND) {
> +        wr_handler = restart_coroutine;
> +    }
> +
> +    DPRINTF("s->sock=%d rd_handler=%p wr_handler=%p", s->sock,
> +            rd_handler, wr_handler);
> +
> +    qemu_aio_set_fd_handler(s->sock, rd_handler, wr_handler, return_true, co);
> +}
> +
> +/* A non-blocking call returned EAGAIN, so yield, ensuring the
> + * handlers are set up so that we'll be rescheduled when there is an
> + * interesting event on the socket.
> + */
> +static coroutine_fn void co_yield(BDRVSSHState *s)
> +{
> +    set_fd_handler(s);
> +    qemu_coroutine_yield();

Should we clear the fd handler here?  That way the caller doesn't have
to worry about fd handler state.
Richard W.M. Jones - March 26, 2013, 10:38 a.m.
Thanks -- I've included all feedback up to this point in version 3,
which I'll post in a moment.

Rich.

Patch

diff --git a/block/Makefile.objs b/block/Makefile.objs
index c067f38..6c4b5bc 100644
--- a/block/Makefile.objs
+++ b/block/Makefile.objs
@@ -13,6 +13,7 @@  block-obj-$(CONFIG_LIBISCSI) += iscsi.o
 block-obj-$(CONFIG_CURL) += curl.o
 block-obj-$(CONFIG_RBD) += rbd.o
 block-obj-$(CONFIG_GLUSTERFS) += gluster.o
+block-obj-$(CONFIG_LIBSSH2) += ssh.o
 endif
 
 common-obj-y += stream.o
diff --git a/block/ssh.c b/block/ssh.c
new file mode 100644
index 0000000..822a8e1
--- /dev/null
+++ b/block/ssh.c
@@ -0,0 +1,656 @@ 
+/*
+ * Secure Shell (ssh) backend for QEMU.
+ *
+ * Copyright (C) 2013 Red Hat Inc., Richard W.M. Jones <rjones@redhat.com>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdarg.h>
+
+#ifndef _WIN32
+#include <pwd.h>
+#endif
+
+#include <libssh2.h>
+#include <libssh2_sftp.h>
+
+#include "block/block_int.h"
+#include "qemu/sockets.h"
+#include "qemu/uri.h"
+
+#define DEBUG_SSH 0
+
+#if DEBUG_SSH
+#define DPRINTF(fmt,...)                                                \
+    do {                                                                \
+        fprintf(stderr, "ssh: %-15s " fmt "\n", __func__, ##__VA_ARGS__); \
+    } while (0)
+#else
+#define DPRINTF(fmt,...) /* nothing */
+#endif
+
+typedef struct BDRVSSHState {
+    CoMutex lock;                     /* coroutine lock */
+
+    /* ssh connection */
+    int sock;                         /* socket */
+    LIBSSH2_SESSION *session;         /* ssh session */
+    LIBSSH2_SFTP *sftp;               /* sftp session */
+    LIBSSH2_SFTP_HANDLE *sftp_handle; /* sftp remote file handle */
+
+    /* file attributes (at open) */
+    LIBSSH2_SFTP_ATTRIBUTES attrs;
+} BDRVSSHState;
+
+/* Wrappers around error_report which make sure to dump as much
+ * information from libssh2 as possible.
+ */
+static void
+session_error_report(BDRVSSHState *s, const char *fs, ...)
+{
+    va_list args;
+
+    va_start(args, fs);
+
+    if ((s)->session) {
+        char *ssh_err;
+        int ssh_err_len = sizeof ssh_err;
+        int ssh_err_code;
+
+        libssh2_session_last_error((s)->session,
+                                   &ssh_err, &ssh_err_len, 0);
+        /* This is not an errno.  See <libssh2.h>. */
+        ssh_err_code = libssh2_session_last_errno((s)->session);
+
+        error_vprintf(fs, args);
+        error_printf(": %s (libssh2 error code: %d)", ssh_err, ssh_err_code);
+    }
+    else {
+        error_vprintf(fs, args);
+    }
+
+    va_end(args);
+    error_printf("\n");
+}
+
+static void
+sftp_error_report(BDRVSSHState *s, const char *fs, ...)
+{
+    va_list args;
+
+    va_start(args, fs);
+
+    if ((s)->sftp) {
+        char *ssh_err;
+        int ssh_err_len = sizeof ssh_err;
+        int ssh_err_code;
+        unsigned long sftp_err_code;
+
+        libssh2_session_last_error((s)->session,
+                                   &ssh_err, &ssh_err_len, 0);
+        /* This is not an errno.  See <libssh2.h>. */
+        ssh_err_code = libssh2_session_last_errno((s)->session);
+        /* See <libssh2_sftp.h>. */
+        sftp_err_code = libssh2_sftp_last_error((s)->sftp);
+
+        error_vprintf(fs, args);
+        error_printf(": %s (libssh2 error code: %d, sftp error code: %lu)",
+                     ssh_err, ssh_err_code, sftp_err_code);
+    }
+    else {
+        error_vprintf(fs, args);
+    }
+
+    va_end(args);
+    error_printf("\n");
+}
+
+#ifndef _WIN32
+static const char *get_username(void)
+{
+    struct passwd *pwd;
+
+    pwd = getpwuid(geteuid ());
+    if (!pwd) {
+        error_report("failed to get current user name");
+        return NULL;
+    }
+    return pwd->pw_name;
+}
+#else
+static const char *get_username(void)
+{
+    error_report("on Windows, specify a remote user name in the URI");
+    return NULL;
+}
+#endif
+
+static int parse_uri(const char *filename, QDict *options)
+{
+    URI *uri;
+    char *host = NULL;
+
+    uri = uri_parse(filename);
+    if (!uri) {
+        return -EINVAL;
+    }
+
+    if (strcmp(uri->scheme, "ssh") != 0) {
+        error_report("URI scheme must be 'ssh'");
+        goto err;
+    }
+
+    if (!uri->server || strcmp(uri->server, "") == 0) {
+        error_report("missing hostname in URI");
+        goto err;
+    }
+
+    if (!uri->path || strcmp(uri->path, "") == 0) {
+        error_report("missing remote path in URI");
+        goto err;
+    }
+
+    if(uri->user && strcmp(uri->user, "") != 0) {
+        qdict_put(options, "user", qstring_from_str(uri->user));
+    }
+
+    /* Construct the hostname:port string for inet_connect. */
+    host = g_strdup_printf("%s:%d", uri->server, uri->port ? uri->port : 22);
+    qdict_put(options, "host", qstring_from_str(host));
+
+    qdict_put(options, "path", qstring_from_str(uri->path));
+
+    uri_free(uri);
+    free(host);
+    return 0;
+
+ err:
+    uri_free(uri);
+    free(host);
+    return -EINVAL;
+}
+
+static void ssh_parse_filename(const char *filename, QDict *options,
+                               Error **errp)
+{
+    if (qdict_haskey(options, "user") ||
+        qdict_haskey(options, "host") ||
+        qdict_haskey(options, "path")) {
+        error_setg(errp, "user, host or path cannot be used at the same time as a file option");
+        return;
+    }
+
+    if (parse_uri(filename, options) < 0) {
+        error_setg(errp, "no valid URL specified");
+    }
+}
+
+static int connect_to_ssh(BDRVSSHState *s, QDict *options)
+{
+    int r, ret;
+    Error *err = NULL;
+    const char *host, *user, *path;
+    const char *userauthlist;
+    LIBSSH2_AGENT *agent = NULL;
+    struct libssh2_agent_publickey *identity;
+    struct libssh2_agent_publickey *prev_identity = NULL;
+
+    host = qdict_get_str(options, "host");
+    path = qdict_get_str(options, "path");
+
+    if (qdict_haskey(options, "user")) {
+        user = qdict_get_str(options, "user");
+    } else {
+        user = get_username();
+        if (!user) {
+            ret = -EINVAL;
+            goto err;
+        }
+    }
+
+    /* Open the socket and connect. */
+    s->sock = inet_connect(host, &err);
+    if (err != NULL) {
+        ret = -errno;
+        qerror_report_err(err);
+        error_free(err);
+        goto err;
+    }
+
+    /* Create SSH session. */
+    s->session = libssh2_session_init();
+    if (!s->session) {
+        ret = -EINVAL;
+        session_error_report(s, "failed to initialize libssh2 session");
+        goto err;
+    }
+
+    r = libssh2_session_handshake(s->session, s->sock);
+    if (r != 0) {
+        ret = -EINVAL;
+        session_error_report(s, "failed to establish SSH session");
+        goto err;
+    }
+
+#if 0
+    /* Check the remote host's key against known_hosts. */
+    ret = check_host_key (s);
+    if (ret < 0) {
+        goto err;
+    }
+#endif
+
+    /* Authenticate. */
+    userauthlist = libssh2_userauth_list(s->session, user, strlen(user));
+    if (strstr(userauthlist, "publickey") == NULL) {
+        ret = -EPERM;
+        error_report("remote server does not support \"publickey\" authentication");
+        goto err;
+    }
+
+    /* Connect to ssh-agent and try each identity in turn. */
+    agent = libssh2_agent_init(s->session);
+    if (!agent) {
+        ret = -EINVAL;
+        session_error_report(s, "failed to initialize ssh-agent support");
+        goto err;
+    }
+    if (libssh2_agent_connect(agent)) {
+        ret = -ECONNREFUSED;
+        session_error_report(s, "failed to connect to ssh-agent");
+        goto err;
+    }
+    if (libssh2_agent_list_identities(agent)) {
+        ret = -EINVAL;
+        session_error_report(s, "failed requesting identities from ssh-agent");
+        goto err;
+    }
+
+    for(;;) {
+        r = libssh2_agent_get_identity(agent, &identity, prev_identity);
+        if (r == 1) {           /* end of list */
+            break;
+        }
+        if (r < 0) {
+            ret = -EINVAL;
+            session_error_report(s, "failed to obtain identity from ssh-agent");
+            goto err;
+        }
+        r = libssh2_agent_userauth(agent, user, identity);
+        if (r == 0) {
+            goto authenticated;
+        }
+        /* Failed to authenticate with this identity, try the next one. */
+        prev_identity = identity;
+    }
+
+    ret = -EPERM;
+    error_report("failed to authenticate using publickey authentication "
+                 "and the identities held by your ssh-agent");
+    goto err;
+
+ authenticated:
+    libssh2_agent_disconnect(agent);
+    libssh2_agent_free(agent);
+    agent = NULL;
+
+    /* Start SFTP. */
+    s->sftp = libssh2_sftp_init(s->session);
+    if (!s->sftp) {
+        session_error_report(s, "failed to initialize sftp handle");
+        ret = -EINVAL;
+        goto err;
+    }
+
+    s->sftp_handle = libssh2_sftp_open(s->sftp, path,
+                                       LIBSSH2_FXF_READ|LIBSSH2_FXF_WRITE,
+                                       0);
+    if (!s->sftp_handle) {
+        session_error_report(s, "failed to open remote file '%s'", path);
+        ret = -EINVAL;
+        goto err;
+    }
+
+    r = libssh2_sftp_fstat(s->sftp_handle, &s->attrs);
+    if (r < 0) {
+        sftp_error_report(s, "failed to read file attributes of '%s'",
+                          path);
+        ret = -EINVAL;
+        goto err;
+    }
+
+    /* Delete the options we've used; any not deleted will cause the
+     * block layer to give an error about unused options.
+     */
+    qdict_del(options, "host");
+    qdict_del(options, "user");
+    qdict_del(options, "path");
+
+    return 0;
+
+ err:
+    if (s->sftp_handle) {
+        libssh2_sftp_close(s->sftp_handle);
+    }
+    s->sftp_handle = NULL;
+    if (s->sftp) {
+        libssh2_sftp_shutdown(s->sftp);
+    }
+    s->sftp = NULL;
+    if (s->session) {
+        libssh2_session_disconnect(s->session,
+                                   "from qemu ssh client: "
+                                   "error opening connection");
+        libssh2_session_free(s->session);
+    }
+    s->session = NULL;
+
+    if (agent) {
+        libssh2_agent_disconnect(agent);
+        libssh2_agent_free(agent);
+    }
+
+    return ret;
+}
+
+static int ssh_file_open(BlockDriverState *bs, const char *filename,
+                         QDict *options, int bdrv_flags)
+{
+    BDRVSSHState *s = bs->opaque;
+    int ret;
+
+    s->sock = -1;
+    qemu_co_mutex_init(&s->lock);
+
+    /* Start up SSH. */
+    ret = connect_to_ssh(s, options);
+    if (ret < 0) {
+        goto err;
+    }
+
+    /* Go non-blocking. */
+    libssh2_session_set_blocking(s->session, 0);
+
+    return 0;
+
+ err:
+    if (s->sock >= 0) {
+        close(s->sock);
+    }
+    s->sock = -1;
+
+    return ret;
+}
+
+static void ssh_close(BlockDriverState *bs)
+{
+    BDRVSSHState *s = bs->opaque;
+
+    if (s->sftp_handle) {
+        libssh2_sftp_close(s->sftp_handle);
+    }
+    if (s->sftp) {
+        libssh2_sftp_shutdown(s->sftp);
+    }
+    if (s->session) {
+        libssh2_session_disconnect(s->session,
+                                   "from qemu ssh client: "
+                                   "user closed the connection");
+        libssh2_session_free(s->session);
+    }
+    if (s->sock >= 0) {
+        close(s->sock);
+    }
+}
+
+static void restart_coroutine(void *opaque)
+{
+    Coroutine *co = opaque;
+
+    qemu_coroutine_enter(co, NULL);
+}
+
+/* Always true because when we have called set_fd_handler there is
+ * always a request being processed.
+ */
+static int return_true(void *opaque)
+{
+    return 1;
+}
+
+static coroutine_fn void set_fd_handler(BDRVSSHState *s)
+{
+    int r;
+    void (*rd_handler)(void*);
+    void (*wr_handler)(void*);
+    Coroutine *co = qemu_coroutine_self();
+
+    rd_handler = wr_handler = NULL;
+
+    r = libssh2_session_block_directions(s->session);
+
+    if (r & LIBSSH2_SESSION_BLOCK_INBOUND) {
+        rd_handler = restart_coroutine;
+    }
+    if (r & LIBSSH2_SESSION_BLOCK_OUTBOUND) {
+        wr_handler = restart_coroutine;
+    }
+
+    DPRINTF("s->sock=%d rd_handler=%p wr_handler=%p", s->sock,
+            rd_handler, wr_handler);
+
+    qemu_aio_set_fd_handler(s->sock, rd_handler, wr_handler, return_true, co);
+}
+
+/* A non-blocking call returned EAGAIN, so yield, ensuring the
+ * handlers are set up so that we'll be rescheduled when there is an
+ * interesting event on the socket.
+ */
+static coroutine_fn void co_yield(BDRVSSHState *s)
+{
+    set_fd_handler(s);
+    qemu_coroutine_yield();
+}
+
+static coroutine_fn void clear_fd_handler(BDRVSSHState *s)
+{
+    qemu_aio_set_fd_handler(s->sock, NULL, NULL, NULL, NULL);
+}
+
+static coroutine_fn int ssh_read(BDRVSSHState *s,
+                                 int64_t offset, size_t size,
+                                 QEMUIOVector *qiov)
+{
+    ssize_t r;
+    size_t got;
+    char *buf, *end_of_vec;
+    struct iovec *i;
+
+    DPRINTF("offset=%" PRIi64 " size=%zu", offset, size);
+
+    /* According to the docs, this just updates a field in the
+     * sftp_handle structure, so there is no network traffic and it
+     * cannot fail.
+     */
+    libssh2_sftp_seek64(s->sftp_handle, offset);
+
+    /* This keeps track of the current iovec element ('i'), where we
+     * will write to next ('buf'), and the end of the current iovec
+     * ('end_of_vec').
+     */
+    i = &qiov->iov[0];
+    buf = i->iov_base;
+    end_of_vec = i->iov_base + i->iov_len;
+
+    /* libssh2 has a hard-coded limit of 2000 bytes per request,
+     * although it will also do readahead behind our backs.  Therefore
+     * we may have to do repeated reads here until we have read 'size'
+     * bytes.
+     */
+    for (got = 0; got < size; ) {
+    again:
+        DPRINTF("sftp_read buf=%p size=%zu", buf, end_of_vec - buf);
+        r = libssh2_sftp_read(s->sftp_handle, buf, end_of_vec - buf);
+
+        if (r == LIBSSH2_ERROR_EAGAIN || r == LIBSSH2_ERROR_TIMEOUT) {
+            co_yield(s);
+            goto again;
+        }
+        if (r < 0) {
+            clear_fd_handler(s);
+            sftp_error_report(s, "read failed");
+            return -EIO;
+        }
+        if (r == 0) {
+            clear_fd_handler(s);
+            error_report("read failed (EOF)");
+            return -EIO;
+        }
+
+        got += r;
+        buf += r;
+        if (buf >= end_of_vec) {
+            i++;
+            buf = i->iov_base;
+            end_of_vec = i->iov_base + i->iov_len;
+        }
+    }
+
+    clear_fd_handler(s);
+    return 0;
+}
+
+static coroutine_fn int ssh_co_readv(BlockDriverState *bs,
+                                     int64_t sector_num,
+                                     int nb_sectors, QEMUIOVector *qiov)
+{
+    BDRVSSHState *s = bs->opaque;
+    int ret;
+
+    qemu_co_mutex_lock(&s->lock);
+    ret = ssh_read(s, sector_num * BDRV_SECTOR_SIZE,
+                   nb_sectors * BDRV_SECTOR_SIZE, qiov);
+    qemu_co_mutex_unlock(&s->lock);
+
+    return ret;
+}
+
+static int ssh_write(BDRVSSHState *s,
+                     int64_t offset, size_t size,
+                     QEMUIOVector *qiov)
+{
+    ssize_t r;
+    size_t written;
+    char *buf, *end_of_vec;
+    struct iovec *i;
+
+    DPRINTF("offset=%" PRIi64 " size=%zu", offset, size);
+
+    /* According to the docs, this just updates a field in the
+     * sftp_handle structure, so there is no network traffic and it
+     * cannot fail.
+     */
+    libssh2_sftp_seek64(s->sftp_handle, offset);
+
+    /* This keeps track of the current iovec element ('i'), where we
+     * will read from next ('buf'), and the end of the current iovec
+     * ('end_of_vec').
+     */
+    i = &qiov->iov[0];
+    buf = i->iov_base;
+    end_of_vec = i->iov_base + i->iov_len;
+
+    for (written = 0; written < size; ) {
+    again:
+        DPRINTF("sftp_write buf=%p size=%zu", buf, end_of_vec - buf);
+        r = libssh2_sftp_write(s->sftp_handle, buf, end_of_vec - buf);
+
+        if (r == LIBSSH2_ERROR_EAGAIN || r == LIBSSH2_ERROR_TIMEOUT) {
+            co_yield(s);
+            goto again;
+        }
+        if (r < 0) {
+            clear_fd_handler(s);
+            sftp_error_report(s, "write failed");
+            return -EIO;
+        }
+
+        written += r;
+        buf += r;
+        if (buf >= end_of_vec) {
+            i++;
+            buf = i->iov_base;
+            end_of_vec = i->iov_base + i->iov_len;
+        }
+    }
+
+    clear_fd_handler(s);
+    return 0;
+}
+
+static coroutine_fn int ssh_co_writev(BlockDriverState *bs,
+                                      int64_t sector_num,
+                                      int nb_sectors, QEMUIOVector *qiov)
+{
+    BDRVSSHState *s = bs->opaque;
+    int ret;
+
+    qemu_co_mutex_lock(&s->lock);
+    ret = ssh_write(s, sector_num * BDRV_SECTOR_SIZE,
+                    nb_sectors * BDRV_SECTOR_SIZE, qiov);
+    qemu_co_mutex_unlock(&s->lock);
+
+    return ret;
+}
+
+static int64_t ssh_getlength(BlockDriverState *bs)
+{
+    BDRVSSHState *s = bs->opaque;
+    return (int64_t) s->attrs.filesize;
+}
+
+static BlockDriver bdrv_ssh = {
+    .format_name                  = "ssh",
+    .protocol_name                = "ssh",
+    .instance_size                = sizeof(BDRVSSHState),
+    .bdrv_parse_filename          = ssh_parse_filename,
+    .bdrv_file_open               = ssh_file_open,
+    .bdrv_close                   = ssh_close,
+    .bdrv_co_readv                = ssh_co_readv,
+    .bdrv_co_writev               = ssh_co_writev,
+    .bdrv_getlength               = ssh_getlength,
+};
+
+static void bdrv_ssh_init(void)
+{
+    int r;
+
+    r = libssh2_init(0);
+    if (r != 0) {
+        fprintf(stderr, "libssh2 initialization failed, %d\n", r);
+        exit(EXIT_FAILURE);
+    }
+
+    bdrv_register(&bdrv_ssh);
+}
+
+block_init(bdrv_ssh_init);
diff --git a/configure b/configure
index f2af714..d666066 100755
--- a/configure
+++ b/configure
@@ -229,6 +229,7 @@  virtio_blk_data_plane=""
 gtk=""
 gtkabi="2.0"
 tpm="no"
+libssh2=""
 
 # parse CC options first
 for opt do
@@ -908,6 +909,10 @@  for opt do
   ;;
   --enable-tpm) tpm="yes"
   ;;
+  --disable-libssh2) libssh2="no"
+  ;;
+  --enable-libssh2) libssh2="yes"
+  ;;
   *) echo "ERROR: unknown option $opt"; show_help="yes"
   ;;
   esac
@@ -1166,6 +1171,7 @@  echo "  --disable-glusterfs      disable GlusterFS backend"
 echo "  --enable-gcov            enable test coverage analysis with gcov"
 echo "  --gcov=GCOV              use specified gcov [$gcov_tool]"
 echo "  --enable-tpm             enable TPM support"
+echo "  --enable-libssh2         enable ssh block device support"
 echo ""
 echo "NOTE: The object files are built at the place where configure is launched"
 exit 1
@@ -2330,6 +2336,42 @@  EOF
 fi
 
 ##########################################
+# libssh2 probe
+if test "$libssh2" != "no" ; then
+  cat > $TMPC <<EOF
+#include <stdio.h>
+#include <libssh2.h>
+#include <libssh2_sftp.h>
+int main(void) {
+    LIBSSH2_SESSION *session;
+    session = libssh2_session_init ();
+    (void) libssh2_sftp_init (session);
+    return 0;
+}
+EOF
+
+  if $pkg_config libssh2 --modversion >/dev/null 2>&1; then
+    libssh2_cflags=`$pkg_config libssh2 --cflags`
+    libssh2_libs=`$pkg_config libssh2 --libs`
+  else
+    libssh2_cflags=
+    libssh2_libs="-lssh2"
+  fi
+
+  if compile_prog "$libssh2_cflags" "$libssh2_libs" ; then
+    libssh2=yes
+    libs_tools="$libssh2_libs $libs_tools"
+    libs_softmmu="$libssh2_libs $libs_softmmu"
+    QEMU_CFLAGS="$QEMU_CFLAGS $libssh2_cflags"
+  else
+    if test "$libssh2" = "yes" ; then
+      feature_not_found "libssh2"
+    fi
+    libssh2=no
+  fi
+fi
+
+##########################################
 # linux-aio probe
 
 if test "$linux_aio" != "no" ; then
@@ -3441,6 +3483,7 @@  echo "virtio-blk-data-plane $virtio_blk_data_plane"
 echo "gcov              $gcov_tool"
 echo "gcov enabled      $gcov"
 echo "TPM support       $tpm"
+echo "libssh2 support   $libssh2"
 
 if test "$sdl_too_old" = "yes"; then
 echo "-> Your SDL version is too old - please upgrade to have SDL support"
@@ -3806,6 +3849,10 @@  if test "$glusterfs" = "yes" ; then
   echo "CONFIG_GLUSTERFS=y" >> $config_host_mak
 fi
 
+if test "$libssh2" = "yes" ; then
+  echo "CONFIG_LIBSSH2=y" >> $config_host_mak
+fi
+
 if test "$virtio_blk_data_plane" = "yes" ; then
   echo "CONFIG_VIRTIO_BLK_DATA_PLANE=y" >> $config_host_mak
 fi
diff --git a/qemu-doc.texi b/qemu-doc.texi
index af84bef..004b2ff 100644
--- a/qemu-doc.texi
+++ b/qemu-doc.texi
@@ -423,6 +423,7 @@  snapshots.
 * disk_images_sheepdog::      Sheepdog disk images
 * disk_images_iscsi::         iSCSI LUNs
 * disk_images_gluster::       GlusterFS disk images
+* disk_images_ssh::           Secure Shell (ssh) disk images
 @end menu
 
 @node disk_images_quickstart
@@ -1038,6 +1039,33 @@  qemu-system-x86_64 -drive file=gluster+unix:///testvol/dir/a.img?socket=/tmp/glu
 qemu-system-x86_64 -drive file=gluster+rdma://1.2.3.4:24007/testvol/a.img
 @end example
 
+@node disk_images_ssh
+@subsection Secure Shell (ssh) disk images
+
+You can access disk images located on a remote ssh server
+by using the ssh protocol:
+@example
+qemu-system-x86_64 -drive file=ssh://[@var{user}@@]@var{server}[:@var{port}]/@var{path}
+@end example
+
+@var{ssh} is the protocol.
+
+@var{user} is the remote user.  If not specified, then the local
+username is tried.
+
+@var{server} specifies the remote ssh server.  Any ssh server can be
+used, but it must implement the sftp-server protocol.  Most Unix/Linux
+systems should work without requiring any extra configuration.
+
+@var{port} is the port number on which sshd is listening.  By default
+the standard ssh port (22) is used.
+
+@var{path} is the path to the disk image.
+
+Authentication must currently be done without requiring a password or
+passphrase.  Set up ssh-agent with authorized keys, or Kerberos or
+similar.
+
 @node pcsys_network
 @section Network emulation