From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:37785) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cif3t-0001QD-95 for qemu-devel@nongnu.org; Tue, 28 Feb 2017 05:31:20 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cif3q-00081s-SL for qemu-devel@nongnu.org; Tue, 28 Feb 2017 05:31:17 -0500 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:54044) by eggs.gnu.org with esmtps (TLS1.0:RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1cif3q-00081V-Iv for qemu-devel@nongnu.org; Tue, 28 Feb 2017 05:31:14 -0500 Received: from pps.filterd (m0098410.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.20/8.16.0.20) with SMTP id v1SASfLE015066 for ; Tue, 28 Feb 2017 05:31:13 -0500 Received: from e06smtp14.uk.ibm.com (e06smtp14.uk.ibm.com [195.75.94.110]) by mx0a-001b2d01.pphosted.com with ESMTP id 28vpwfk93n-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Tue, 28 Feb 2017 05:31:13 -0500 Received: from localhost by e06smtp14.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 28 Feb 2017 10:31:10 -0000 From: Greg Kurz Date: Tue, 28 Feb 2017 11:30:34 +0100 In-Reply-To: <1488277840-18608-1-git-send-email-groug@kaod.org> References: <1488277840-18608-1-git-send-email-groug@kaod.org> Message-Id: <1488277840-18608-23-git-send-email-groug@kaod.org> Subject: [Qemu-devel] [PULL 22/28] 9pfs: local: chmod: don't follow symlinks List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: qemu-devel@nongnu.org Cc: Peter Maydell , "Aneesh Kumar K.V" , Greg Kurz The local_chmod() callback is vulnerable to symlink attacks because it calls: (1) chmod() which follows symbolic links for all path elements (2) local_set_xattr()->setxattr() which follows symbolic links for all path elements (3) local_set_mapped_file_attr() which calls in turn local_fopen() and mkdir(), both functions following symbolic links for all path elements but the rightmost one We would need fchmodat() to implement AT_SYMLINK_NOFOLLOW to fix (1). This isn't the case on linux unfortunately: the kernel doesn't even have a flags argument to the syscall :-\ It is impossible to fix it in userspace in a race-free manner. This patch hence converts local_chmod() to rely on open_nofollow() and fchmod(). This fixes the vulnerability but introduces a limitation: the target file must readable and/or writable for the call to openat() to succeed. It introduces a local_set_xattrat() replacement to local_set_xattr() based on fsetxattrat() to fix (2), and a local_set_mapped_file_attrat() replacement to local_set_mapped_file_attr() based on local_fopenat() and mkdirat() to fix (3). No effort is made to factor out code because both local_set_xattr() and local_set_mapped_file_attr() will be dropped when all users have been converted to use the "at" versions. This partly fixes CVE-2016-9602. Signed-off-by: Greg Kurz Reviewed-by: Stefan Hajnoczi --- hw/9pfs/9p-local.c | 178 +++++++++++++++++++++++++++++++++++++++++++++++++---- 1 file changed, 167 insertions(+), 11 deletions(-) diff --git a/hw/9pfs/9p-local.c b/hw/9pfs/9p-local.c index 2c38ea12a288..27ecbf6c5ba7 100644 --- a/hw/9pfs/9p-local.c +++ b/hw/9pfs/9p-local.c @@ -367,6 +367,155 @@ static int local_set_xattr(const char *path, FsCred *credp) return 0; } +static int local_set_mapped_file_attrat(int dirfd, const char *name, + FsCred *credp) +{ + FILE *fp; + int ret; + char buf[ATTR_MAX]; + int uid = -1, gid = -1, mode = -1, rdev = -1; + int map_dirfd; + + ret = mkdirat(dirfd, VIRTFS_META_DIR, 0700); + if (ret < 0 && errno != EEXIST) { + return -1; + } + + map_dirfd = openat_dir(dirfd, VIRTFS_META_DIR); + if (map_dirfd == -1) { + return -1; + } + + fp = local_fopenat(map_dirfd, name, "r"); + if (!fp) { + if (errno == ENOENT) { + goto update_map_file; + } else { + close_preserve_errno(map_dirfd); + return -1; + } + } + memset(buf, 0, ATTR_MAX); + while (fgets(buf, ATTR_MAX, fp)) { + if (!strncmp(buf, "virtfs.uid", 10)) { + uid = atoi(buf + 11); + } else if (!strncmp(buf, "virtfs.gid", 10)) { + gid = atoi(buf + 11); + } else if (!strncmp(buf, "virtfs.mode", 11)) { + mode = atoi(buf + 12); + } else if (!strncmp(buf, "virtfs.rdev", 11)) { + rdev = atoi(buf + 12); + } + memset(buf, 0, ATTR_MAX); + } + fclose(fp); + +update_map_file: + fp = local_fopenat(map_dirfd, name, "w"); + close_preserve_errno(map_dirfd); + if (!fp) { + return -1; + } + + if (credp->fc_uid != -1) { + uid = credp->fc_uid; + } + if (credp->fc_gid != -1) { + gid = credp->fc_gid; + } + if (credp->fc_mode != -1) { + mode = credp->fc_mode; + } + if (credp->fc_rdev != -1) { + rdev = credp->fc_rdev; + } + + if (uid != -1) { + fprintf(fp, "virtfs.uid=%d\n", uid); + } + if (gid != -1) { + fprintf(fp, "virtfs.gid=%d\n", gid); + } + if (mode != -1) { + fprintf(fp, "virtfs.mode=%d\n", mode); + } + if (rdev != -1) { + fprintf(fp, "virtfs.rdev=%d\n", rdev); + } + fclose(fp); + + return 0; +} + +static int fchmodat_nofollow(int dirfd, const char *name, mode_t mode) +{ + int fd, ret; + + /* FIXME: this should be handled with fchmodat(AT_SYMLINK_NOFOLLOW). + * Unfortunately, the linux kernel doesn't implement it yet. As an + * alternative, let's open the file and use fchmod() instead. This + * may fail depending on the permissions of the file, but it is the + * best we can do to avoid TOCTTOU. We first try to open read-only + * in case name points to a directory. If that fails, we try write-only + * in case name doesn't point to a directory. + */ + fd = openat_file(dirfd, name, O_RDONLY, 0); + if (fd == -1) { + /* In case the file is writable-only and isn't a directory. */ + if (errno == EACCES) { + fd = openat_file(dirfd, name, O_WRONLY, 0); + } + if (fd == -1 && errno == EISDIR) { + errno = EACCES; + } + } + if (fd == -1) { + return -1; + } + ret = fchmod(fd, mode); + close_preserve_errno(fd); + return ret; +} + +static int local_set_xattrat(int dirfd, const char *path, FsCred *credp) +{ + int err; + + if (credp->fc_uid != -1) { + uint32_t tmp_uid = cpu_to_le32(credp->fc_uid); + err = fsetxattrat_nofollow(dirfd, path, "user.virtfs.uid", &tmp_uid, + sizeof(uid_t), 0); + if (err) { + return err; + } + } + if (credp->fc_gid != -1) { + uint32_t tmp_gid = cpu_to_le32(credp->fc_gid); + err = fsetxattrat_nofollow(dirfd, path, "user.virtfs.gid", &tmp_gid, + sizeof(gid_t), 0); + if (err) { + return err; + } + } + if (credp->fc_mode != -1) { + uint32_t tmp_mode = cpu_to_le32(credp->fc_mode); + err = fsetxattrat_nofollow(dirfd, path, "user.virtfs.mode", &tmp_mode, + sizeof(mode_t), 0); + if (err) { + return err; + } + } + if (credp->fc_rdev != -1) { + uint64_t tmp_rdev = cpu_to_le64(credp->fc_rdev); + err = fsetxattrat_nofollow(dirfd, path, "user.virtfs.rdev", &tmp_rdev, + sizeof(dev_t), 0); + if (err) { + return err; + } + } + return 0; +} + static int local_post_create_passthrough(FsContext *fs_ctx, const char *path, FsCred *credp) { @@ -558,22 +707,29 @@ static ssize_t local_pwritev(FsContext *ctx, V9fsFidOpenState *fs, static int local_chmod(FsContext *fs_ctx, V9fsPath *fs_path, FsCred *credp) { - char *buffer; + char *dirpath = g_path_get_dirname(fs_path->data); + char *name = g_path_get_basename(fs_path->data); int ret = -1; - char *path = fs_path->data; + int dirfd; + + dirfd = local_opendir_nofollow(fs_ctx, dirpath); + if (dirfd == -1) { + goto out; + } if (fs_ctx->export_flags & V9FS_SM_MAPPED) { - buffer = rpath(fs_ctx, path); - ret = local_set_xattr(buffer, credp); - g_free(buffer); + ret = local_set_xattrat(dirfd, name, credp); } else if (fs_ctx->export_flags & V9FS_SM_MAPPED_FILE) { - return local_set_mapped_file_attr(fs_ctx, path, credp); - } else if ((fs_ctx->export_flags & V9FS_SM_PASSTHROUGH) || - (fs_ctx->export_flags & V9FS_SM_NONE)) { - buffer = rpath(fs_ctx, path); - ret = chmod(buffer, credp->fc_mode); - g_free(buffer); + ret = local_set_mapped_file_attrat(dirfd, name, credp); + } else if (fs_ctx->export_flags & V9FS_SM_PASSTHROUGH || + fs_ctx->export_flags & V9FS_SM_NONE) { + ret = fchmodat_nofollow(dirfd, name, credp->fc_mode); } + close_preserve_errno(dirfd); + +out: + g_free(dirpath); + g_free(name); return ret; } -- 2.7.4