[PATCH v8 3/4] sample/Landlock: Support abstract unix socket restriction
Tahera Fahimi
fahimitahera at gmail.com
Fri Aug 9 18:16:37 UTC 2024
On Fri, Aug 09, 2024 at 04:11:47PM +0200, Mickaël Salaün wrote:
> 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 <fahimitahera at gmail.com>
> >
> > ---
> > 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 ?
Right, I will add this check before calling chek_ruleset_scope function.
> > + 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.
For the second note, I think even if the user provides multiple "a"
(something like "a:a"), It would not have a different effect (for now).
Do you suggest that I change this way of handeling this environment
variable or add documents that mention this note?
>
> > + } 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.
I think this happens in check_ruleset_scope function. However, I will
add a condition (abi >=6) to this "if" statement.
> > + 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
> >
> >
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