Re: [PATCH v38 39/39] LSM: Create lsm_module_list system call

[Mickaël Salaün]

On 27/09/2022 22:31, Casey Schaufler wrote:
> Create a system call to report the list of Linux Security Modules
> that are active on the system. The list is provided as an array
> of LSM ID numbers.

With lsm_self_attr(), this would look like a dir/file structure.

Would it be useful for user space to list all the currently used LSMs 
instead of only retrieving information about a known (list of) LSM?
What is the use case for this syscall?


> Signed-off-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
> ---
>   arch/x86/entry/syscalls/syscall_64.tbl |  1 +
>   include/linux/syscalls.h               |  1 +
>   include/uapi/asm-generic/unistd.h      |  5 ++-
>   kernel/sys_ni.c                        |  1 +
>   security/lsm_syscalls.c                | 50 ++++++++++++++++++++++++++
>   5 files changed, 57 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
> index 56d5c5202fd0..40b35e7069a7 100644
> --- a/arch/x86/entry/syscalls/syscall_64.tbl
> +++ b/arch/x86/entry/syscalls/syscall_64.tbl
> @@ -373,6 +373,7 @@
>   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
> +452	common	lsm_module_list		sys_lsm_module_list

As for the other syscall, this should also be in the same dedicated 
"wire syscalls" patch.


>   
>   #
>   # Due to a historical design error, certain syscalls are numbered differently
> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
> index 7f87ef8be546..e2e2a9e93e8c 100644
> --- a/include/linux/syscalls.h
> +++ b/include/linux/syscalls.h
> @@ -1057,6 +1057,7 @@ asmlinkage long sys_set_mempolicy_home_node(unsigned long start, unsigned long l
>   					    unsigned long home_node,
>   					    unsigned long flags);
>   asmlinkage long sys_lsm_self_attr(struct lsm_ctx *ctx, size_t *size, int flags);
> +asmlinkage long sys_lsm_module_list(unsigned int *ids, 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 aa66718e1b48..090617a9a53a 100644
> --- a/include/uapi/asm-generic/unistd.h
> +++ b/include/uapi/asm-generic/unistd.h
> @@ -889,8 +889,11 @@ __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)
>   
> +#define __NR_lsm_module_list 452
> +__SYSCALL(__NR_lsm_module_list, sys_lsm_module_list)
> +
>   #undef __NR_syscalls
> -#define __NR_syscalls 452
> +#define __NR_syscalls 453

Same here.


>   
>   /*
>    * 32 bit systems traditionally used different
> diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
> index 0fdb0341251d..bde9e74a3473 100644
> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -264,6 +264,7 @@ COND_SYSCALL(mremap);
>   
>   /* security/lsm_syscalls.c */
>   COND_SYSCALL(lsm_self_attr);
> +COND_SYSCALL(lsm_module_list);
>   
>   /* security/keys/keyctl.c */
>   COND_SYSCALL(add_key);
> diff --git a/security/lsm_syscalls.c b/security/lsm_syscalls.c
> index da0fab7065e2..41d9ef945ede 100644
> --- a/security/lsm_syscalls.c
> +++ b/security/lsm_syscalls.c
> @@ -154,3 +154,53 @@ SYSCALL_DEFINE3(lsm_self_attr,
>   	kfree(final);
>   	return rc;
>   }
> +
> +/**
> + * lsm_module_list - Return a list of the active security modules
> + * @ids: the LSM module ids
> + * @size: size of @ids, updated on return
> + * @flags: reserved for future use, must be zero
> + *
> + * Returns a list of the active LSM ids. On success this function
> + * returns the number of @ids array elements. This value may be zero
> + * if there are no LSMs active. 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_module_list,
> +	       unsigned int __user *, ids,
> +	       size_t __user *, size,
> +	       int, flags)
> +{
> +	unsigned int *interum;
> +	size_t total_size = lsm_id * sizeof(*interum);
> +	size_t usize;
> +	int rc;
> +	int i;
> +
> +	if (get_user(usize, size))
> +		return -EFAULT;
> +
> +	if (usize < total_size) {
> +		if (put_user(total_size, size) != 0)
> +			return -EFAULT;
> +		return -E2BIG;
> +	}
> +
> +	interum = kzalloc(total_size, GFP_KERNEL);
> +	if (interum == NULL)
> +		return -ENOMEM;
> +
> +	for (i = 0; i < lsm_id; i++)
> +		interum[i] = lsm_idlist[i]->id;
> +
> +	if (copy_to_user(ids, interum, total_size) != 0 ||
> +	    put_user(total_size, size) != 0)
> +		rc = -EFAULT;
> +	else
> +		rc = lsm_id;
> +
> +	kfree(interum);
> +	return rc;
> +}