Re: [PATCH v13 04/11] LSM: syscalls for current process attributes

[Casey Schaufler]

On 8/25/2023 7:59 AM, Mickaël Salaün wrote:
> On Wed, Aug 02, 2023 at 10:44:27AM -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@xxxxxxxxxxxxxxxx>
>> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
>> Reviewed-by: Serge Hallyn <serge@xxxxxxxxxx>
>> ---
>>  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            |  90 ++++++++++++++++++++
>>  kernel/sys_ni.c                     |   2 +
>>  security/Makefile                   |   1 +
>>  security/lsm_syscalls.c             |  55 ++++++++++++
>>  security/security.c                 | 125 ++++++++++++++++++++++++++++
>>  10 files changed, 372 insertions(+)
>>  create mode 100644 Documentation/userspace-api/lsm.rst
>>  create mode 100644 include/uapi/linux/lsm.h
>>  create mode 100644 security/lsm_syscalls.c
>> +/**
>> + * sys_lsm_get_self_attr - Return current task's security module attributes
>> + * @attr: which attribute to set
>> + * @ctx: the LSM contexts
>> + * @size: size of @ctx, updated on return
>> + * @flags: reserved for future use
> This documentation is out-of-sync with the LSM hook doc, especially the
> flags.

You are correct. I will repair it.

>
>> + *
>> + * 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_DEFINE4(lsm_get_self_attr, unsigned int, attr, struct lsm_ctx __user *,
>> +		ctx, size_t __user *, size, u32, flags)
>> +{
>> +	return security_getselfattr(attr, ctx, size, flags);
>> +}
>> diff --git a/security/security.c b/security/security.c
>> index 5e9cd548dd95..cde7f3a13e7c 100644
>> --- a/security/security.c
>> +++ b/security/security.c
>> @@ -3798,6 +3798,131 @@ void security_d_instantiate(struct dentry *dentry, struct inode *inode)
>>  }
>>  EXPORT_SYMBOL(security_d_instantiate);
>>  
>> +/**
>> + * security_getselfattr - Read an LSM attribute of the current process.
>> + * @attr: which attribute to return
>> + * @ctx: 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
> The final dot is missing.

Will fix.

>
>> + *
>> + * A NULL value for @ctx 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 *ctx,
>> +			 size_t __user *size, u32 flags)
>> +{
>> +	struct security_hook_list *hp;
>> +	struct lsm_ctx lctx = { .id = LSM_ID_UNDEF, };
>> +	u8 __user *base = (u8 __user *)ctx;
>> +	size_t total = 0;
>> +	size_t entrysize;
>> +	size_t left;
>> +	bool toobig = 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 & LSM_FLAG_SINGLE) == LSM_FLAG_SINGLE) {
>> +		if (!ctx)
> This doesn't fit with the documenation. I guess it should handle NULL
> ctx in both cases.

It's also not correct. I've reworked the single case logic.

>
> This cases should be tested.
>
> I'm now wondering if this LSM_FLAG_SINGLE makes sense though.

It does if you want to pick up the attributes one at a time in
separate code paths in an application.

>
>> +			return -EINVAL;
>> +		if (copy_struct_from_user(&lctx, sizeof(lctx), ctx, left))
> This check looks good but it looks inconsistent with the
> non-LSM_FLAG_SINGLE loop which doesn't check that the arrays only
> contain zeros.

The single case is the only case where the data is read from user-space.
They're inconsistent because the use of the user-space memory is read+write
in the one case and write-only in the other.

>
>> +			return -EFAULT;
>> +		if (lctx.id == LSM_ID_UNDEF)
>> +			return -EINVAL;
>> +	} else if (flags) {
> This check is not correct. It should test if there is no value other
> than LSM_FLAG_SINGLE.

I've reworked the single logic.

>
> Please add a test with something like this:
> lsm_get_self_attr(valid_attr, valid_ctx, valid_size, LSM_FLAG_SINGLE | 1 << 9)
>
>> +		return -EINVAL;
>> +	}
>> +
> Please add a comment to highlight that only LSM_FLAG_SINGLE is handled
> for this loop.

I don't follow. The loop handles both single and multiple cases.

>
>> +	hlist_for_each_entry(hp, &security_hook_heads.getselfattr, list) {
>> +		if (lctx.id != LSM_ID_UNDEF && lctx.id != hp->lsmid->id)
>> +			continue;
>> +		entrysize = left;
>> +		if (base)
>> +			ctx = (struct lsm_ctx __user *)(base + total);
> To be consistent with the previous copy_struct_from_user() call, we
> should at least check that the ctx arrays contain zeros too.

We don't care because this is the copy-out part. The user-space data
isn't being read here.

>
> We should add a test for this case.
>
>> +		rc = hp->hook.getselfattr(attr, ctx, &entrysize, flags);
>> +		if (rc == -EOPNOTSUPP) {
>> +			rc = 0;
>> +			continue;
>> +		}
>> +		if (rc == -E2BIG) {
>> +			toobig = true;
>> +			left = 0;
>> +			continue;
>> +		}
>> +		if (rc < 0)
>> +			return rc;
>> +
>> +		left -= entrysize;
>> +		total += entrysize;
>> +		count += rc;
>> +	}
>> +	if (put_user(total, size))
>> +		return -EFAULT;
>> +	if (toobig)
>> +		return -E2BIG;
>> +	if (count == 0)
>> +		return LSM_RET_DEFAULT(getselfattr);
>> +	return count;
>> +}
>> +
>> +/**
>> + * security_setselfattr - Set an LSM attribute on the current process.
>> + * @attr: which attribute to set
>> + * @ctx: 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 @ctx.
>> + *
>> + * 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 *ctx,
>> +			 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(*ctx))
>> +		return -EINVAL;
>> +
>> +	lctx = kmalloc(size, GFP_KERNEL);
> We should not allow user space to allocate arbitrary kernel buffer size.
> Limiting to PAGE_SIZE seems reasonable.

Quite reasonable.

>
>> +	if (lctx == NULL)
>> +		return -ENOMEM;
>> +
>> +	if (copy_from_user(&lctx, ctx, size))
>> +		return -EFAULT;
>> +
>> +	if (size < lctx->len || size < lctx->ctx_len + sizeof(ctx) ||
> Because we only handle one LSM at a time, size should be equal to
> lctx->len right?

Unless they've done some of the padding Paul's so keen on.

>
>> +	    lctx->len < lctx->ctx_len + sizeof(ctx))
>> +		return -EINVAL;
>> +
>> +	hlist_for_each_entry(hp, &security_hook_heads.setselfattr, list)
>> +		if ((hp->lsmid->id) == lctx->id) {
>> +			rc = hp->hook.setselfattr(attr, lctx, size, flags);
> It seems that there is no (shared) check that the whole ctx is used. It
> would be nice for the LSM hook implementations to return the processed
> size and let this code check that there is no remaining data left, or at
> least that it only contains zeros.

Again, it's the possible padding case.

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