OpenE2K
/
qemu-e2k
13
4
Fork 0
Code Issues 4 Pull Requests Projects 2 Releases Activity

9pfs: local: chmod: don't follow symlinks 

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 <groug@kaod.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Greg Kurz 2017-02-26 23:44:28 +01:00
parent ad0b46e6ac
commit e3187a45dd
1 changed files with 167 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

178
hw/9pfs/9p-local.c
View File

@ -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;
}