Subject: Re: [PATCH v8 3/4] sample/Landlock: Support abstract unix socket restriction

From: Mickaël Salaün <[email-masked]>
Date: Fri, 9 Aug 2024 16:11:47 +0200

On Thu, Aug 01, 2024 at 10:02:35PM -0600, Tahera Fahimi wrote:
> A sandboxer can receive the character "a" as input from the environment
> variable LL_SCOPE to restrict the abstract unix sockets from connecting
> to a process outside its scoped domain.
> 
> Example
> =======
> Create an abstract unix socket to listen with socat(1):
> socat abstract-listen:mysocket -
> Create a sandboxed shell and pass the character "a" to LL_SCOPED:
> LL_FS_RO=/ LL_FS_RW=. LL_SCOPED="a" ./sandboxer /bin/bash
> If the sandboxed process tries to connect to the listening socket
> with command "socat - abstract-connect:mysocket", the connection
> will fail.
> 
> Signed-off-by: Tahera Fahimi <[email-masked]>
> 
> ---
> v8:
> - Adding check_ruleset_scope function to parse the scope environment
>   variable and update the landlock attribute based on the restriction
>   provided by the user.
> - Adding Mickaël Salaün reviews on version 7.
> 
> v7:
> - Adding IPC scoping to the sandbox demo by defining a new "LL_SCOPED"
>   environment variable. "LL_SCOPED" gets value "a" to restrict abstract
>   unix sockets.
> - Change LANDLOCK_ABI_LAST to 6.
> ---
>  samples/landlock/sandboxer.c | 56 +++++++++++++++++++++++++++++++++---
>  1 file changed, 52 insertions(+), 4 deletions(-)
> 
> diff --git a/samples/landlock/sandboxer.c b/samples/landlock/sandboxer.c
> index e8223c3e781a..98132fd823ad 100644
> --- a/samples/landlock/sandboxer.c
> +++ b/samples/landlock/sandboxer.c
> @@ -14,6 +14,7 @@
>  #include <fcntl.h>
>  #include <linux/landlock.h>
>  #include <linux/prctl.h>
> +#include <linux/socket.h>
>  #include <stddef.h>
>  #include <stdio.h>
>  #include <stdlib.h>
> @@ -22,6 +23,7 @@
>  #include <sys/stat.h>
>  #include <sys/syscall.h>
>  #include <unistd.h>
> +#include <stdbool.h>
>  
>  #ifndef landlock_create_ruleset
>  static inline int
> @@ -55,6 +57,7 @@ static inline int landlock_restrict_self(const int ruleset_fd,
>  #define ENV_FS_RW_NAME "LL_FS_RW"
>  #define ENV_TCP_BIND_NAME "LL_TCP_BIND"
>  #define ENV_TCP_CONNECT_NAME "LL_TCP_CONNECT"
> +#define ENV_SCOPED_NAME "LL_SCOPED"
>  #define ENV_DELIMITER ":"
>  
>  static int parse_path(char *env_path, const char ***const path_list)
> @@ -184,6 +187,38 @@ static int populate_ruleset_net(const char *const env_var, const int ruleset_fd,
>  	return ret;
>  }
>  
> +static bool check_ruleset_scope(const char *const env_var,
> +				struct landlock_ruleset_attr *ruleset_attr)
> +{
> +	bool ret = true;
> +	char *env_type_scope, *env_type_scope_next, *ipc_scoping_name;
> +
> +	ruleset_attr->scoped &= ~LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET;

Why always removing the suported scope?
What happen if ABI < 6 ?

> +	env_type_scope = getenv(env_var);
> +	/* scoping is not supported by the user */
> +	if (!env_type_scope)
> +		return true;
> +	env_type_scope = strdup(env_type_scope);
> +	unsetenv(env_var);
> +
> +	env_type_scope_next = env_type_scope;
> +	while ((ipc_scoping_name =
> +			strsep(&env_type_scope_next, ENV_DELIMITER))) {
> +		if (strcmp("a", ipc_scoping_name) == 0) {
> +			ruleset_attr->scoped |=
> +				LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET;

There are two issues here:
1. this would not work for ABI < 6
2. "a" can be repeated several times, which should probably not be
   allowed because we don't want to support this
   unspecified/undocumented behavior.

> +		} else {
> +			fprintf(stderr, "Unsupported scoping \"%s\"\n",
> +				ipc_scoping_name);
> +			ret = false;
> +			goto out_free_name;
> +		}
> +	}
> +out_free_name:
> +	free(env_type_scope);
> +	return ret;
> +}
> +
>  /* clang-format off */
>  
>  #define ACCESS_FS_ROUGHLY_READ ( \
> @@ -208,7 +243,7 @@ static int populate_ruleset_net(const char *const env_var, const int ruleset_fd,
>  
>  /* clang-format on */
>  
> -#define LANDLOCK_ABI_LAST 5
> +#define LANDLOCK_ABI_LAST 6
>  
>  int main(const int argc, char *const argv[], char *const *const envp)
>  {
> @@ -223,14 +258,15 @@ int main(const int argc, char *const argv[], char *const *const envp)
>  		.handled_access_fs = access_fs_rw,
>  		.handled_access_net = LANDLOCK_ACCESS_NET_BIND_TCP |
>  				      LANDLOCK_ACCESS_NET_CONNECT_TCP,
> +		.scoped = LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET,
>  	};
>  
>  	if (argc < 2) {
>  		fprintf(stderr,
> -			"usage: %s=\"...\" %s=\"...\" %s=\"...\" %s=\"...\"%s "
> +			"usage: %s=\"...\" %s=\"...\" %s=\"...\" %s=\"...\" %s=\"...\" %s "
>  			"<cmd> [args]...\n\n",
>  			ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
> -			ENV_TCP_CONNECT_NAME, argv[0]);
> +			ENV_TCP_CONNECT_NAME, ENV_SCOPED_NAME, argv[0]);
>  		fprintf(stderr,
>  			"Execute a command in a restricted environment.\n\n");
>  		fprintf(stderr,
> @@ -251,15 +287,18 @@ int main(const int argc, char *const argv[], char *const *const envp)
>  		fprintf(stderr,
>  			"* %s: list of ports allowed to connect (client).\n",
>  			ENV_TCP_CONNECT_NAME);
> +		fprintf(stderr, "* %s: list of restrictions on IPCs.\n",
> +			ENV_SCOPED_NAME);
>  		fprintf(stderr,
>  			"\nexample:\n"
>  			"%s=\"${PATH}:/lib:/usr:/proc:/etc:/dev/urandom\" "
>  			"%s=\"/dev/null:/dev/full:/dev/zero:/dev/pts:/tmp\" "
>  			"%s=\"9418\" "
>  			"%s=\"80:443\" "
> +			"%s=\"a\" "
>  			"%s bash -i\n\n",
>  			ENV_FS_RO_NAME, ENV_FS_RW_NAME, ENV_TCP_BIND_NAME,
> -			ENV_TCP_CONNECT_NAME, argv[0]);
> +			ENV_TCP_CONNECT_NAME, ENV_SCOPED_NAME, argv[0]);
>  		fprintf(stderr,
>  			"This sandboxer can use Landlock features "
>  			"up to ABI version %d.\n",
> @@ -327,6 +366,10 @@ int main(const int argc, char *const argv[], char *const *const envp)
>  		/* Removes LANDLOCK_ACCESS_FS_IOCTL_DEV for ABI < 5 */
>  		ruleset_attr.handled_access_fs &= ~LANDLOCK_ACCESS_FS_IOCTL_DEV;
>  
> +		__attribute__((fallthrough));
> +	case 5:
> +		/* Removes LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET for ABI < 6 */
> +		ruleset_attr.scoped &= ~LANDLOCK_SCOPED_ABSTRACT_UNIX_SOCKET;
>  		fprintf(stderr,
>  			"Hint: You should update the running kernel "
>  			"to leverage Landlock features "
> @@ -358,6 +401,11 @@ int main(const int argc, char *const argv[], char *const *const envp)
>  			~LANDLOCK_ACCESS_NET_CONNECT_TCP;
>  	}
>  
> +	if (!check_ruleset_scope(ENV_SCOPED_NAME, &ruleset_attr)) {

You should use the same pattern as for TCP access rigths: if the
environment variable is not set then remove the ruleset's scopes.

> +		perror("Unsupported IPC scoping requested");
> +		return 1;
> +	}
> +
>  	ruleset_fd =
>  		landlock_create_ruleset(&ruleset_attr, sizeof(ruleset_attr), 0);
>  	if (ruleset_fd < 0) {
> -- 
> 2.34.1
> 
>