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Message-ID: <20231003.kooghohS2Aiz@digikod.net>
Date: Tue, 3 Oct 2023 16:09:23 +0200
From: Mickaël Salaün <mic@...ikod.net>
To: Casey Schaufler <casey@...aufler-ca.com>
Cc: paul@...l-moore.com, linux-security-module@...r.kernel.org,
jmorris@...ei.org, serge@...lyn.com, keescook@...omium.org,
john.johansen@...onical.com, penguin-kernel@...ove.sakura.ne.jp,
stephen.smalley.work@...il.com, linux-kernel@...r.kernel.org,
linux-api@...r.kernel.org
Subject: Re: [PATCH v15 04/11] LSM: syscalls for current process attributes
On Tue, Sep 12, 2023 at 01:56:49PM -0700, Casey Schaufler wrote:
> Create a system call lsm_get_self_attr() to provide the security
> module maintained attributes of the current process.
> Create a system call lsm_set_self_attr() to set a security
> module maintained attribute of the current process.
> Historically these attributes have been exposed to user space via
> entries in procfs under /proc/self/attr.
>
> The attribute value is provided in a lsm_ctx structure. The structure
> identifies the size of the attribute, and the attribute value. The format
> of the attribute value is defined by the security module. A flags field
> is included for LSM specific information. It is currently unused and must
> be 0. The total size of the data, including the lsm_ctx structure and any
> padding, is maintained as well.
>
> struct lsm_ctx {
> __u64 id;
> __u64 flags;
> __u64 len;
> __u64 ctx_len;
> __u8 ctx[];
> };
>
> Two new LSM hooks are used to interface with the LSMs.
> security_getselfattr() collects the lsm_ctx values from the
> LSMs that support the hook, accounting for space requirements.
> security_setselfattr() identifies which LSM the attribute is
> intended for and passes it along.
>
> Signed-off-by: Casey Schaufler <casey@...aufler-ca.com>
> Reviewed-by: Kees Cook <keescook@...omium.org>
> Reviewed-by: Serge Hallyn <serge@...lyn.com>
> Reviewed-by: John Johansen <john.johansen@...onical.com>
> ---
> Documentation/userspace-api/lsm.rst | 70 +++++++++++++
> include/linux/lsm_hook_defs.h | 4 +
> include/linux/lsm_hooks.h | 1 +
> include/linux/security.h | 19 ++++
> include/linux/syscalls.h | 5 +
> include/uapi/linux/lsm.h | 36 +++++++
> kernel/sys_ni.c | 2 +
> security/Makefile | 1 +
> security/lsm_syscalls.c | 57 +++++++++++
> security/security.c | 152 ++++++++++++++++++++++++++++
> 10 files changed, 347 insertions(+)
> create mode 100644 Documentation/userspace-api/lsm.rst
> create mode 100644 security/lsm_syscalls.c
>
> diff --git a/security/security.c b/security/security.c
> index a3489c04b783..0d179750d964 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -3837,6 +3837,158 @@ void security_d_instantiate(struct dentry *dentry, struct inode *inode)
> }
> EXPORT_SYMBOL(security_d_instantiate);
>
> +/*
> + * Please keep this in sync with it's counterpart in security/lsm_syscalls.c
> + */
> +
> +/**
> + * security_getselfattr - Read an LSM attribute of the current process.
> + * @attr: which attribute to return
> + * @uctx: the user-space destination for the information, or NULL
> + * @size: pointer to the size of space available to receive the data
> + * @flags: special handling options. LSM_FLAG_SINGLE indicates that only
> + * attributes associated with the LSM identified in the passed @ctx be
> + * reported.
> + *
> + * A NULL value for @uctx can be used to get both the number of attributes
> + * and the size of the data.
> + *
> + * Returns the number of attributes found on success, negative value
> + * on error. @size is reset to the total size of the data.
> + * If @size is insufficient to contain the data -E2BIG is returned.
> + */
> +int security_getselfattr(unsigned int attr, struct lsm_ctx __user *uctx,
> + size_t __user *size, u32 flags)
> +{
> + struct security_hook_list *hp;
> + struct lsm_ctx lctx = { .id = LSM_ID_UNDEF, };
> + u8 __user *base = (u8 __user *)uctx;
> + size_t total = 0;
> + size_t entrysize;
> + size_t left;
> + bool toobig = false;
> + bool single = false;
> + int count = 0;
> + int rc;
> +
> + if (attr == LSM_ATTR_UNDEF)
> + return -EINVAL;
> + if (size == NULL)
> + return -EINVAL;
> + if (get_user(left, size))
> + return -EFAULT;
> +
> + if (flags) {
> + /*
> + * Only flag supported is LSM_FLAG_SINGLE
> + */
> + if (flags != LSM_FLAG_SINGLE)
> + return -EINVAL;
> + if (uctx && copy_from_user(&lctx, uctx, sizeof(lctx)))
I'm not sure if we should return -EINVAL or -EFAULT when uctx == NULL.
Because uctx is optional (when LSM_FLAG_SINGLE is not set), I guess
-EINVAL is OK.
> + return -EFAULT;
> + /*
> + * If the LSM ID isn't specified it is an error.
> + */
> + if (lctx.id == LSM_ID_UNDEF)
> + return -EINVAL;
> + single = true;
> + }
> +
> + /*
> + * In the usual case gather all the data from the LSMs.
> + * In the single case only get the data from the LSM specified.
> + */
> + hlist_for_each_entry(hp, &security_hook_heads.getselfattr, list) {
> + if (single && lctx.id != hp->lsmid->id)
> + continue;
> + entrysize = left;
> + if (base)
> + uctx = (struct lsm_ctx __user *)(base + total);
> + rc = hp->hook.getselfattr(attr, uctx, &entrysize, flags);
> + if (rc == -EOPNOTSUPP) {
> + rc = 0;
> + continue;
> + }
> + if (rc == -E2BIG) {
> + toobig = true;
> + left = 0;
> + } else if (rc < 0)
> + return rc;
> + else
> + left -= entrysize;
> +
> + total += entrysize;
> + count += rc;
There is a bug if rc == -E2BIG
> + if (single)
> + break;
> + }
> + if (put_user(total, size))
> + return -EFAULT;
> + if (toobig)
> + return -E2BIG;
> + if (count == 0)
> + return LSM_RET_DEFAULT(getselfattr);
> + return count;
> +}
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