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

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The local_chown() callback is vulnerable to symlink attacks because it
calls:

(1) lchown() which follows symbolic links for all path elements but the
    rightmost one
(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

This patch converts local_chown() to rely on open_nofollow() and
fchownat() to fix (1), as well as local_set_xattrat() and
local_set_mapped_file_attrat() to fix (2) and (3) respectively.

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 23:44:37 +01:00
parent e3187a45dd
commit d369f20763
1 changed files with 17 additions and 9 deletions
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26
hw/9pfs/9p-local.c
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@ -1160,23 +1160,31 @@ static int local_truncate(FsContext *ctx, V9fsPath *fs_path, off_t size)
static int local_chown(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 ((credp->fc_uid == -1 && credp->fc_gid == -1) ||
(fs_ctx->export_flags & V9FS_SM_PASSTHROUGH) ||
(fs_ctx->export_flags & V9FS_SM_NONE)) {
buffer = rpath(fs_ctx, path);
ret = lchown(buffer, credp->fc_uid, credp->fc_gid);
g_free(buffer);
ret = fchownat(dirfd, name, credp->fc_uid, credp->fc_gid,
AT_SYMLINK_NOFOLLOW);
} else 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);
ret = local_set_mapped_file_attrat(dirfd, name, credp);
}
close_preserve_errno(dirfd);
out:
g_free(name);
g_free(dirpath);
return ret;
}