Message-ID: <ebd82727-cc8e-d922-972b-ebd2cc47f1ee@schaufler-ca.com>
Date:   Mon, 11 Sep 2023 15:08:48 -0700
From:   Casey Schaufler <casey@...aufler-ca.com>
To:     Mickaël Salaün <mic@...ikod.net>
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, Casey Schaufler <casey@...aufler-ca.com>
Subject: Re: [PATCH v14 04/11] LSM: syscalls for current process attributes

On 9/7/2023 8:37 AM, Mickaël Salaün wrote:
> On Mon, Aug 28, 2023 at 12:57:54PM -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                 | 146 ++++++++++++++++++++++++++++
>>  10 files changed, 341 insertions(+)
>>  create mode 100644 Documentation/userspace-api/lsm.rst
>>  create mode 100644 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;
> Much easier to read with these booleans.
>
>> +	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_struct_from_user(&lctx, sizeof(lctx), uctx, left))
> Again, I'm not sure is copy_struct_from_user() should be used here
> because it checks that the user space structures ends with zeros, which
> is inconsistent with the case without LSM_FLAG_SINGLE. Anyway, this code
> should at least properly handle the copy_struct_from_user() error codes
> which includes EFAULT *and* E2BIG.

Further testing makes your point. The checking in copy_struct_from_user()
is really annoying. I'm going to fix this.

>
>> +			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) {
>> +			if (count > 0)
>> +				break;
>> +			if (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;
> Not a big deal but with LSM_FLAG_SINGLE, if the selected LSM doesn't
> implement this hook, this will uselessly loop over all LSMs.
> I'd add:
>
> if (single)
> 	/* Still try to write 0 as the total size for consistency. */
> 	break;

I'm trying to keep LSM_FLAG_SINGLE from being a complete special case.
Asking for an attribute that the specified LSM doesn't support is a
programming error, so I'm not concerned about a trivial performance
issue.

>
>> +			continue;
>> +		}
>> +		if (rc == -E2BIG) {
>> +			toobig = true;
>> +			left = 0;
>> +			total += entrysize;
> I'm not sure what is the desired behavior with LSM_FLAG_SINGLE in this
> case but I'd add:
>
> if (single)
> 	break;

I'm shifting the control logic a bit. I think it'll look cleaner.

>
>> +			continue;
>> +		}
>> +		if (rc < 0)
>> +			return rc;
> I think this should be a break instead of the return rc for consistency
> reasons.

No one has ever accused me of being consistent. I believe in using
return aggressively.

>
>> +
>> +		left -= entrysize;
>> +		total += entrysize;
>> +		count += rc;
> You could simplify a bit by replacing the first single check with this
> one:
>
> if (single)
> 	break;

As above, I think I'm making this clearer in v15.

>
>> +	}
>> +	if (put_user(total, size))
>> +		return -EFAULT;
>> +	if (toobig)
>> +		return -E2BIG;
>> +	if (count == 0)
>> +		return LSM_RET_DEFAULT(getselfattr);
>> +	return count;
>> +}
>> +
> It would be nice to add a comment before all these three syscall
> documentations to keep up-to-date the related user space documentation
> in lsm_syscalls.c

Good thinking.

>
>> +/**
>> + * security_setselfattr - Set an LSM attribute on the current process.
>> + * @attr: which attribute to set
>> + * @uctx: the user-space source for the information
>> + * @size: the size of the data
>> + * @flags: reserved for future use, must be 0
>> + *
>> + * Set an LSM attribute for the current process. The LSM, attribute
>> + * and new value are included in @uctx.
>> + *
>> + * Returns 0 on success, -EINVAL if the input is inconsistent, -EFAULT
>> + * if the user buffer is inaccessible or an LSM specific failure.
>> + */
>> +int security_setselfattr(unsigned int attr, struct lsm_ctx __user *uctx,
>> +			 size_t size, u32 flags)
>> +{
>> +	struct security_hook_list *hp;
>> +	struct lsm_ctx *lctx;
>> +	int rc = LSM_RET_DEFAULT(setselfattr);
>> +
>> +	if (flags)
>> +		return -EINVAL;
>> +	if (size < sizeof(*lctx) || size > PAGE_SIZE)
> I would be better to return -E2BIG if size > PAGE_SIZE, and update the
> (two) documentations accordingly.
>
>> +		return -EINVAL;
>> +
>> +	lctx = kmalloc(size, GFP_KERNEL);
>> +	if (lctx == NULL)
>> +		return -ENOMEM;
>> +
>> +	if (copy_from_user(lctx, uctx, size)) {
>> +		rc = -EFAULT;
>> +		goto free_out;
>> +	}
>> +
>> +	if (size < lctx->len || size < lctx->ctx_len + sizeof(*lctx) ||
>> +	    lctx->len < lctx->ctx_len + sizeof(*lctx)) {
>> +		rc = -EINVAL;
>> +		goto free_out;
>> +	}
>> +
>> +	hlist_for_each_entry(hp, &security_hook_heads.setselfattr, list)
>> +		if ((hp->lsmid->id) == lctx->id) {
>> +			rc = hp->hook.setselfattr(attr, lctx, size, flags);
> There is no need (at least for now) to directly expose lctx->id nor
> lctx->flags. It would be simpler to only pass lctx->ctx and
> lctx->ctx_len to the hooks.

No. Absolutely not. I dislike passing string/length pairs, especially when
they are both contained in a structure.

>
>> +			break;
>> +		}
>> +
>> +free_out:
>> +	kfree(lctx);
>> +	return rc;
>> +}
>> +
>>  /**
>>   * security_getprocattr() - Read an attribute for a task
>>   * @p: the task
>> -- 
>> 2.41.0
>>

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