Re: [PATCH bpf-next v9 0/8] MAC and Audit policy using eBPF (KRSI)

[KP Singh]

So I was able to reproduce the issue and also fix it (will separately
send a patch).

diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 9cd4455528e5..1bdd027766d4 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -55,7 +55,7 @@ LSM_HOOK(void, LSM_RET_VOID, bprm_committing_creds, struct linux_binprm *bprm)
 LSM_HOOK(void, LSM_RET_VOID, bprm_committed_creds, struct linux_binprm *bprm)
 LSM_HOOK(int, 0, fs_context_dup, struct fs_context *fc,
         struct fs_context *src_sc)
-LSM_HOOK(int, 0, fs_context_parse_param, struct fs_context *fc,
+LSM_HOOK(int, -ENOPARAM, fs_context_parse_param, struct fs_context *fc,
         struct fs_parameter *param)
 LSM_HOOK(int, 0, sb_alloc_security, struct super_block *sb)
 LSM_HOOK(void, LSM_RET_VOID, sb_free_security, struct super_block *sb)

So what was happening was that:

bpf_lsm hook for fs_context_parse_param was returning 0 which led this
bit of logic to believe the parameter was parsed by the LSM.

int vfs_parse_fs_param(struct fs_context *fc, struct fs_parameter
*param)
{

[...]
        ret = security_fs_context_parse_param(fc, param);
        if (ret != -ENOPARAM)
                /* Param belongs to the LSM or is disallowed by the
                 * LSM; so
                 * don't pass to the FS.
                 */
                return ret;

        if (fc->ops->parse_param) {
                ret = fc->ops->parse_param(fc, param);
                if (ret != -ENOPARAM)
                        return ret;
        }
[...]

This resulted in the fs_context->dev_name being NULL and the following
chain to throw an -EINVAL resulting in unsuccessful mount of the root
file-system:

- do_mount_root -> do_mount -> do_new_mount -> vfs_get_tree ->
-> fc->ops->get_tree -> legacy->get_tree -> fc->fs_type->mount ->
ext4_mount -> mount_bdev -> blkdev_get_by_path -> lookup_bdev

- KP


On 29-Apr 15:45, Mikko Ylinen wrote:
> 
> 
> On 29/04/2020 15:34, KP Singh wrote:
> > Thanks for reporting this! Can you share your Kconfig please?
> 
> This is what I originally started with
> https://raw.githubusercontent.com/clearlinux-pkgs/linux-mainline/master/config
> 
> but I also tried your _LSM_ settings found in this
> https://lore.kernel.org/bpf/20200402040357.GA217889@xxxxxxxxxx/
> 
> -- Regards, Mikko