[PATCH RFC] LSM: lsm_self_attr system call to get security module attributes
Serge E. Hallyn
serge at hallyn.com
Sat Sep 10 02:51:24 UTC 2022
On Fri, Sep 09, 2022 at 06:01:33PM -0700, Casey Schaufler wrote:
> This is probably a tin-man proposal for the first in a series of
> system calls dealing with Linux security module data. It is based
> on suggestions by Paul Moore, however the flaws in design and
> implementation are all mine.
>
> Create a system call lsm_self_attr() to provide the security
> module maintained attributes of the current process. Historically
> these attributes have been exposed to user space via entries in
> procfs under /proc/self/attr.
>
> Attributes are provided as a collection of lsm_ctx structures
> which are placed into a user supplied buffer. Each structure
> identifies the security module providing the attribute, which
> of the possible attributes is provided, the size of the
> attribute, and finally the attribute value as a nul terminated
> string.
>
> An LSM ID table is introduced to map IDs to security modules.
Probably best to cc: linux-api right off the bat, right?
> Signed-off-by: Casey Schaufler <casey at schaufler-ca.com>
>
> --
> arch/x86/entry/syscalls/syscall_64.tbl | 1 +
> include/linux/syscalls.h | 1 +
> include/uapi/asm-generic/unistd.h | 5 +-
> include/uapi/linux/lsm.h | 67 +++++++++++++
> kernel/sys_ni.c | 3 +
> security/Makefile | 2 +-
> security/lsm_syscalls.c | 166 +++++++++++++++++++++++++++++++++
> 7 files changed, 243 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index c84d12608cd2..56d5c5202fd0 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -372,6 +372,7 @@
> 448 common process_mrelease sys_process_mrelease
> 449 common futex_waitv sys_futex_waitv
> 450 common set_mempolicy_home_node sys_set_mempolicy_home_node
> +451 common lsm_self_attr sys_lsm_self_attr
>
> #
> # Due to a historical design error, certain syscalls are numbered differently
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index a34b0f9a9972..7f87ef8be546 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -1056,6 +1056,7 @@ asmlinkage long sys_memfd_secret(unsigned int flags);
> asmlinkage long sys_set_mempolicy_home_node(unsigned long start, unsigned long len,
> unsigned long home_node,
> unsigned long flags);
> +asmlinkage long sys_lsm_self_attr(struct lsm_ctx *ctx, size_t *size, int flags);
>
> /*
> * Architecture-specific system calls
> diff --git a/include/uapi/asm-generic/unistd.h b/include/uapi/asm-generic/unistd.h
> index 45fa180cc56a..aa66718e1b48 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -886,8 +886,11 @@ __SYSCALL(__NR_futex_waitv, sys_futex_waitv)
> #define __NR_set_mempolicy_home_node 450
> __SYSCALL(__NR_set_mempolicy_home_node, sys_set_mempolicy_home_node)
>
> +#define __NR_lsm_self_attr 451
> +__SYSCALL(__NR_lsm_self_attr, sys_lsm_self_attr)
> +
> #undef __NR_syscalls
> -#define __NR_syscalls 451
> +#define __NR_syscalls 452
>
> /*
> * 32 bit systems traditionally used different
> diff --git a/include/uapi/linux/lsm.h b/include/uapi/linux/lsm.h
> new file mode 100644
> index 000000000000..ec7bb1a7b943
> --- /dev/null
> +++ b/include/uapi/linux/lsm.h
> @@ -0,0 +1,67 @@
> +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> +/*
> + * Linus Security Modules (LSM) - User space API
> + *
> + * Copyright (C) 2022 Casey Schaufler <casey at schaufler-ca.com>
> + * Copyright (C) Intel Corporation
> + */
> +
> +#ifndef _UAPI_LINUX_LSM_H
> +#define _UAPI_LINUX_LSM_H
> +
> +#include <linux/types.h>
> +#include <linux/unistd.h>
> +
> +/**
> + * struct lsm_ctx - LSM context
> + * @id: the LSM id number, see LSM_ID_XXX
> + * @flags: context specifier and LSM specific flags
> + * @ctx_len: the size of @ctx
> + * @ctx: the LSM context, a nul terminated string
> + *
> + * @ctx in a nul terminated string.
> + * (strlen(@ctx) < @ctx_len) is always true.
> + * (strlen(@ctx) == @ctx_len + 1) is not guaranteed.
> + */
> +struct lsm_ctx {
> + unsigned int id;
> + unsigned int flags;
> + __kernel_size_t ctx_len;
> + unsigned char ctx[];
> +};
> +
> +/*
> + * ID values to identify security modules.
> + * A system may use more than one security module.
> + *
> + * LSM_ID_XXX values 32 and below are reserved for future use
> + */
> +#define LSM_ID_SELINUX 33
> +#define LSM_ID_SMACK 34
> +#define LSM_ID_TOMOYO 35
> +#define LSM_ID_IMA 36
> +#define LSM_ID_APPARMOR 37
> +#define LSM_ID_YAMA 38
> +#define LSM_ID_LOADPIN 39
> +#define LSM_ID_SAFESETID 40
> +#define LSM_ID_LOCKDOWN 41
> +#define LSM_ID_BPF 42
> +#define LSM_ID_LANDLOCK 43
> +
> +/*
> + * Flag values.
> + *
> + * LSM_ATTR_XXX values identify the /proc/.../attr entry that the
> + * context represents. Not all security modules provide all of these
> + * values. Some security modules provide none of them.
> + */
> +/* clang-format off */
> +#define LSM_ATTR_CURRENT (1UL << 0)
> +#define LSM_ATTR_EXEC (1UL << 1)
> +#define LSM_ATTR_FSCREATE (1UL << 2)
> +#define LSM_ATTR_KEYCREATE (1UL << 3)
> +#define LSM_ATTR_PREV (1UL << 4)
> +#define LSM_ATTR_SOCKCREATE (1UL << 5)
> +/* clang-format on */
> +
> +#endif /* _UAPI_LINUX_LSM_H */
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index a492f159624f..c579ffc50454 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -262,6 +262,9 @@ COND_SYSCALL_COMPAT(recvmsg);
> /* mm/nommu.c, also with MMU */
> COND_SYSCALL(mremap);
>
> +/* security/lsm_syscalls.c */
> +COND_SYSCALL(lsm_attr_self);
> +
> /* security/keys/keyctl.c */
> COND_SYSCALL(add_key);
> COND_SYSCALL(request_key);
> diff --git a/security/Makefile b/security/Makefile
> index 18121f8f85cd..409c47a25fcf 100644
> --- a/security/Makefile
> +++ b/security/Makefile
> @@ -6,7 +6,7 @@
> obj-$(CONFIG_KEYS) += keys/
>
> # always enable default capabilities
> -obj-y += commoncap.o
> +obj-y += commoncap.o lsm_syscalls.o
> obj-$(CONFIG_MMU) += min_addr.o
>
> # Object file lists
> diff --git a/security/lsm_syscalls.c b/security/lsm_syscalls.c
> new file mode 100644
> index 000000000000..fba8aeea1a10
> --- /dev/null
> +++ b/security/lsm_syscalls.c
> @@ -0,0 +1,166 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * System calls implementing the Linux Security Module API.
> + *
> + * Copyright (C) 2022 Casey Schaufler <casey at schaufler-ca.com>
> + * Copyright (C) Intel Corporation
> + */
> +
> +#include <asm/current.h>
> +#include <linux/compiler_types.h>
> +#include <linux/err.h>
> +#include <linux/errno.h>
> +#include <linux/security.h>
> +#include <linux/stddef.h>
> +#include <linux/syscalls.h>
> +#include <linux/types.h>
> +#include <uapi/linux/lsm.h>
> +
> +struct id_map {
> + char *name;
> + int id;
> +};
> +
> +static const struct id_map lsm_attr_names[] = {
> + { "current", LSM_ATTR_CURRENT, },
> + { "exec", LSM_ATTR_EXEC, },
> + { "fscreate", LSM_ATTR_FSCREATE, },
> + { "keycreate", LSM_ATTR_KEYCREATE, },
> + { "prev", LSM_ATTR_PREV, },
> + { "sockcreate", LSM_ATTR_SOCKCREATE, },
> +};
> +
> +static const struct id_map lsm_names[] = {
> + { "selinux", LSM_ID_SELINUX, },
> + { "smack", LSM_ID_SMACK, },
> + { "tomoyo", LSM_ID_TOMOYO, },
> + { "ima", LSM_ID_IMA, },
> + { "apparmor", LSM_ID_APPARMOR, },
> + { "yama", LSM_ID_YAMA, },
> + { "loadpin", LSM_ID_LOADPIN, },
> + { "safesetid", LSM_ID_SAFESETID, },
> + { "lockdown", LSM_ID_LOCKDOWN, },
> + { "bpf", LSM_ID_BPF, },
> + { "landlock", LSM_ID_LANDLOCK, },
> +};
> +
> +/**
> + * lsm_self_attr - Return current task's security module attributes
> + * @ctx: the LSM contexts
> + * @size: size of @ctx, updated on return
> + * @flags: reserved for future use, must be zero
> + *
> + * Returns the calling task's LSM contexts. On success this
> + * function returns the number of @ctx array elements. This value
> + * may be zero if there are no LSM contexts assigned. If @size is
> + * insufficient to contain the return data -E2BIG is returned and
> + * @size is set to the minimum required size. In all other cases
> + * a negative value indicating the error is returned.
> + */
> +SYSCALL_DEFINE3(lsm_self_attr,
> + struct lsm_ctx __user *, ctx,
> + size_t __user *, size,
> + int, flags)
> +{
> + struct lsm_ctx *final = NULL;
> + struct lsm_ctx *interum;
> + struct lsm_ctx *ip;
> + void *curr;
> + char **interum_ctx;
> + char *cp;
> + size_t total_size = 0;
> + int count = 0;
> + int attr;
> + int lsm;
> + int len;
> + int rc = 0;
> + int i;
> +
> + interum = kzalloc(ARRAY_SIZE(lsm_attr_names) * ARRAY_SIZE(lsm_names) *
> + sizeof(*interum), GFP_KERNEL);
> + if (interum == NULL)
> + return -ENOMEM;
> + ip = interum;
> +
> + interum_ctx = kzalloc(ARRAY_SIZE(lsm_attr_names) *
> + ARRAY_SIZE(lsm_names) * sizeof(*interum_ctx),
> + GFP_KERNEL);
> + if (interum_ctx == NULL) {
> + kfree(interum);
> + return -ENOMEM;
> + }
> +
> + for (attr = 0; attr < ARRAY_SIZE(lsm_attr_names); attr++) {
> + for (lsm = 0; lsm < ARRAY_SIZE(lsm_names); lsm++) {
> + len = security_getprocattr(current,
> + lsm_names[lsm].name,
> + lsm_attr_names[attr].name,
> + &cp);
> + if (len <= 0)
> + continue;
> +
> + ip->id = lsm_names[lsm].id;
> + ip->flags = lsm_attr_names[attr].id;
> + ip->ctx_len = len;
> + interum_ctx[count] = cp;
> + /*
> + * Security modules have been inconsistent about
> + * including the \0 terminator in the size. Add
> + * space for it from an abundance of caution.
> + * At least one security module adds a \n at the
> + * end of a context to make it look nicer. Change
> + * that to a \0 so that user space does't have to
> + * work around it. Because of this meddling it is
> + * safe to assume that lsm_ctx.name is terminated
> + * and that strlen(lsm_ctx.name) < lsm.ctx_len.
> + */
> + total_size += sizeof(*interum) + len + 1;
> + cp = strnchr(cp, len, '\n');
> + if (cp != NULL)
> + *cp = '\0';
> + ip++;
> + count++;
> + }
> + }
> +
> + if (count == 0)
> + goto free_out;
> +
> + final = kzalloc(total_size, GFP_KERNEL);
> + if (final == NULL) {
> + rc = -ENOMEM;
> + goto free_out;
> + }
> +
> + curr = final;
> + ip = interum;
> + for (i = 0; i < count; i++) {
> + memcpy(curr, ip, sizeof(*interum));
> + curr += sizeof(*interum);
> + memcpy(curr, interum_ctx[i], ip->ctx_len);
> + curr += interum[i].ctx_len;
> + ip++;
> + }
> +
> + if (get_user(len, size)) {
> + rc = -EFAULT;
> + goto free_out;
> + }
> + if (total_size > len) {
> + rc = -ERANGE;
> + goto free_out;
> + }
> + if (copy_to_user(ctx, final, total_size) != 0 ||
> + put_user(total_size, size) != 0)
> + rc = -EFAULT;
> + else
> + rc = count;
> +
> +free_out:
> + for (i = 0; i < count; i++)
> + kfree(interum_ctx[i]);
> + kfree(interum_ctx);
> + kfree(interum);
> + kfree(final);
> + return rc;
> +}
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