| From a3fdbbc7f271bff7d53d0501b29d910ece0b3789 Mon Sep 17 00:00:00 2001 |
| From: Stefan Hajnoczi <stefanha@redhat.com> |
| Date: Thu, 4 Feb 2021 15:02:08 +0000 |
| Subject: [PATCH] virtiofsd: prevent opening of special files (CVE-2020-35517) |
| |
| A well-behaved FUSE client does not attempt to open special files with |
| FUSE_OPEN because they are handled on the client side (e.g. device nodes |
| are handled by client-side device drivers). |
| |
| The check to prevent virtiofsd from opening special files is missing in |
| a few cases, most notably FUSE_OPEN. A malicious client can cause |
| virtiofsd to open a device node, potentially allowing the guest to |
| escape. This can be exploited by a modified guest device driver. It is |
| not exploitable from guest userspace since the guest kernel will handle |
| special files inside the guest instead of sending FUSE requests. |
| |
| This patch fixes this issue by introducing the lo_inode_open() function |
| to check the file type before opening it. This is a short-term solution |
| because it does not prevent a compromised virtiofsd process from opening |
| device nodes on the host. |
| |
| Restructure lo_create() to try O_CREAT | O_EXCL first. Note that O_CREAT |
| | O_EXCL does not follow symlinks, so O_NOFOLLOW masking is not |
| necessary here. If the file exists and the user did not specify O_EXCL, |
| open it via lo_do_open(). |
| |
| Reported-by: Alex Xu <alex@alxu.ca> |
| Fixes: CVE-2020-35517 |
| Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com> |
| Reviewed-by: Vivek Goyal <vgoyal@redhat.com> |
| Reviewed-by: Greg Kurz <groug@kaod.org> |
| Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> |
| Message-Id: <20210204150208.367837-4-stefanha@redhat.com> |
| Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> |
| |
| Upstream-Status: Backport |
| [https://github.com/qemu/qemu/commit/a3fdbbc7f271bff7d53d0501b29d910ece0b3789] |
| |
| CVE: CVE-2020-35517 |
| |
| Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com> |
| Signed-off-by: Khairul Rohaizzat Jamaluddin <khairul.rohaizzat.jamaluddin@intel.com> |
| --- |
| tools/virtiofsd/passthrough_ll.c | 144 ++++++++++++++++++++----------- |
| 1 file changed, 92 insertions(+), 52 deletions(-) |
| |
| diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c |
| index aa35fc6ba5a5..147b59338a18 100644 |
| --- a/tools/virtiofsd/passthrough_ll.c |
| +++ b/tools/virtiofsd/passthrough_ll.c |
| @@ -555,6 +555,38 @@ static int lo_fd(fuse_req_t req, fuse_ino_t ino) |
| return fd; |
| } |
| |
| +/* |
| + * Open a file descriptor for an inode. Returns -EBADF if the inode is not a |
| + * regular file or a directory. |
| + * |
| + * Use this helper function instead of raw openat(2) to prevent security issues |
| + * when a malicious client opens special files such as block device nodes. |
| + * Symlink inodes are also rejected since symlinks must already have been |
| + * traversed on the client side. |
| + */ |
| +static int lo_inode_open(struct lo_data *lo, struct lo_inode *inode, |
| + int open_flags) |
| +{ |
| + g_autofree char *fd_str = g_strdup_printf("%d", inode->fd); |
| + int fd; |
| + |
| + if (!S_ISREG(inode->filetype) && !S_ISDIR(inode->filetype)) { |
| + return -EBADF; |
| + } |
| + |
| + /* |
| + * The file is a symlink so O_NOFOLLOW must be ignored. We checked earlier |
| + * that the inode is not a special file but if an external process races |
| + * with us then symlinks are traversed here. It is not possible to escape |
| + * the shared directory since it is mounted as "/" though. |
| + */ |
| + fd = openat(lo->proc_self_fd, fd_str, open_flags & ~O_NOFOLLOW); |
| + if (fd < 0) { |
| + return -errno; |
| + } |
| + return fd; |
| +} |
| + |
| static void lo_init(void *userdata, struct fuse_conn_info *conn) |
| { |
| struct lo_data *lo = (struct lo_data *)userdata; |
| @@ -684,9 +716,9 @@ static void lo_setattr(fuse_req_t req, fuse_ino_t ino, struct stat *attr, |
| if (fi) { |
| truncfd = fd; |
| } else { |
| - sprintf(procname, "%i", ifd); |
| - truncfd = openat(lo->proc_self_fd, procname, O_RDWR); |
| + truncfd = lo_inode_open(lo, inode, O_RDWR); |
| if (truncfd < 0) { |
| + errno = -truncfd; |
| goto out_err; |
| } |
| } |
| @@ -848,7 +880,7 @@ static int lo_do_lookup(fuse_req_t req, fuse_ino_t parent, const char *name, |
| struct lo_inode *dir = lo_inode(req, parent); |
| |
| if (inodep) { |
| - *inodep = NULL; |
| + *inodep = NULL; /* in case there is an error */ |
| } |
| |
| /* |
| @@ -1664,19 +1696,26 @@ static void update_open_flags(int writeback, int allow_direct_io, |
| } |
| } |
| |
| +/* |
| + * Open a regular file, set up an fd mapping, and fill out the struct |
| + * fuse_file_info for it. If existing_fd is not negative, use that fd instead |
| + * opening a new one. Takes ownership of existing_fd. |
| + * |
| + * Returns 0 on success or a positive errno. |
| + */ |
| static int lo_do_open(struct lo_data *lo, struct lo_inode *inode, |
| - struct fuse_file_info *fi) |
| + int existing_fd, struct fuse_file_info *fi) |
| { |
| - char buf[64]; |
| ssize_t fh; |
| - int fd; |
| + int fd = existing_fd; |
| |
| update_open_flags(lo->writeback, lo->allow_direct_io, fi); |
| |
| - sprintf(buf, "%i", inode->fd); |
| - fd = openat(lo->proc_self_fd, buf, fi->flags & ~O_NOFOLLOW); |
| - if (fd == -1) { |
| - return errno; |
| + if (fd < 0) { |
| + fd = lo_inode_open(lo, inode, fi->flags); |
| + if (fd < 0) { |
| + return -fd; |
| + } |
| } |
| |
| pthread_mutex_lock(&lo->mutex); |
| @@ -1699,9 +1738,10 @@ static int lo_do_open(struct lo_data *lo, struct lo_inode *inode, |
| static void lo_create(fuse_req_t req, fuse_ino_t parent, const char *name, |
| mode_t mode, struct fuse_file_info *fi) |
| { |
| - int fd; |
| + int fd = -1; |
| struct lo_data *lo = lo_data(req); |
| struct lo_inode *parent_inode; |
| + struct lo_inode *inode = NULL; |
| struct fuse_entry_param e; |
| int err; |
| struct lo_cred old = {}; |
| @@ -1727,36 +1767,38 @@ static void lo_create(fuse_req_t req, fuse_ino_t parent, const char *name, |
| |
| update_open_flags(lo->writeback, lo->allow_direct_io, fi); |
| |
| - fd = openat(parent_inode->fd, name, (fi->flags | O_CREAT) & ~O_NOFOLLOW, |
| - mode); |
| + /* Try to create a new file but don't open existing files */ |
| + fd = openat(parent_inode->fd, name, fi->flags | O_CREAT | O_EXCL, mode); |
| err = fd == -1 ? errno : 0; |
| - lo_restore_cred(&old); |
| |
| - if (!err) { |
| - ssize_t fh; |
| + lo_restore_cred(&old); |
| |
| - pthread_mutex_lock(&lo->mutex); |
| - fh = lo_add_fd_mapping(lo, fd); |
| - pthread_mutex_unlock(&lo->mutex); |
| - if (fh == -1) { |
| - close(fd); |
| - err = ENOMEM; |
| - goto out; |
| - } |
| + /* Ignore the error if file exists and O_EXCL was not given */ |
| + if (err && (err != EEXIST || (fi->flags & O_EXCL))) { |
| + goto out; |
| + } |
| |
| - fi->fh = fh; |
| - err = lo_do_lookup(req, parent, name, &e, NULL); |
| + err = lo_do_lookup(req, parent, name, &e, &inode); |
| + if (err) { |
| + goto out; |
| } |
| - if (lo->cache == CACHE_NONE) { |
| - fi->direct_io = 1; |
| - } else if (lo->cache == CACHE_ALWAYS) { |
| - fi->keep_cache = 1; |
| + |
| + err = lo_do_open(lo, inode, fd, fi); |
| + fd = -1; /* lo_do_open() takes ownership of fd */ |
| + if (err) { |
| + /* Undo lo_do_lookup() nlookup ref */ |
| + unref_inode_lolocked(lo, inode, 1); |
| } |
| |
| out: |
| + lo_inode_put(lo, &inode); |
| lo_inode_put(lo, &parent_inode); |
| |
| if (err) { |
| + if (fd >= 0) { |
| + close(fd); |
| + } |
| + |
| fuse_reply_err(req, err); |
| } else { |
| fuse_reply_create(req, &e, fi); |
| @@ -1770,7 +1812,6 @@ static struct lo_inode_plock *lookup_create_plock_ctx(struct lo_data *lo, |
| pid_t pid, int *err) |
| { |
| struct lo_inode_plock *plock; |
| - char procname[64]; |
| int fd; |
| |
| plock = |
| @@ -1787,12 +1828,10 @@ static struct lo_inode_plock *lookup_create_plock_ctx(struct lo_data *lo, |
| } |
| |
| /* Open another instance of file which can be used for ofd locks. */ |
| - sprintf(procname, "%i", inode->fd); |
| - |
| /* TODO: What if file is not writable? */ |
| - fd = openat(lo->proc_self_fd, procname, O_RDWR); |
| - if (fd == -1) { |
| - *err = errno; |
| + fd = lo_inode_open(lo, inode, O_RDWR); |
| + if (fd < 0) { |
| + *err = -fd; |
| free(plock); |
| return NULL; |
| } |
| @@ -1949,7 +1988,7 @@ static void lo_open(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi) |
| return; |
| } |
| |
| - err = lo_do_open(lo, inode, fi); |
| + err = lo_do_open(lo, inode, -1, fi); |
| lo_inode_put(lo, &inode); |
| if (err) { |
| fuse_reply_err(req, err); |
| @@ -2014,39 +2053,40 @@ static void lo_flush(fuse_req_t req, fuse_ino_t ino, struct fuse_file_info *fi) |
| static void lo_fsync(fuse_req_t req, fuse_ino_t ino, int datasync, |
| struct fuse_file_info *fi) |
| { |
| + struct lo_inode *inode = lo_inode(req, ino); |
| + struct lo_data *lo = lo_data(req); |
| int res; |
| int fd; |
| - char *buf; |
| |
| fuse_log(FUSE_LOG_DEBUG, "lo_fsync(ino=%" PRIu64 ", fi=0x%p)\n", ino, |
| (void *)fi); |
| |
| - if (!fi) { |
| - struct lo_data *lo = lo_data(req); |
| - |
| - res = asprintf(&buf, "%i", lo_fd(req, ino)); |
| - if (res == -1) { |
| - return (void)fuse_reply_err(req, errno); |
| - } |
| + if (!inode) { |
| + fuse_reply_err(req, EBADF); |
| + return; |
| + } |
| |
| - fd = openat(lo->proc_self_fd, buf, O_RDWR); |
| - free(buf); |
| - if (fd == -1) { |
| - return (void)fuse_reply_err(req, errno); |
| + if (!fi) { |
| + fd = lo_inode_open(lo, inode, O_RDWR); |
| + if (fd < 0) { |
| + res = -fd; |
| + goto out; |
| } |
| } else { |
| fd = lo_fi_fd(req, fi); |
| } |
| |
| if (datasync) { |
| - res = fdatasync(fd); |
| + res = fdatasync(fd) == -1 ? errno : 0; |
| } else { |
| - res = fsync(fd); |
| + res = fsync(fd) == -1 ? errno : 0; |
| } |
| if (!fi) { |
| close(fd); |
| } |
| - fuse_reply_err(req, res == -1 ? errno : 0); |
| +out: |
| + lo_inode_put(lo, &inode); |
| + fuse_reply_err(req, res); |
| } |
| |
| static void lo_read(fuse_req_t req, fuse_ino_t ino, size_t size, off_t offset, |