On 3/7/2023 3:56 AM, Mickaël Salaün wrote:

Let's say an LSM need to pass a file descriptor instead of a text
value. Would that be possible or would it need to use another interface?

You could use this interface. LSM_ATTR_MAGICFD would have a
ctx_len = sizeof(fd) and the value in ctx. The underlying
plumbing is another matter entirely. It's likely you'd need
to provide more information in the ctx than the fd, but I
couldn't say what it would be, and I won't speculate.

I would not advocate such a use, as I am not now nor have
ever been a fan of passed file descriptors.

On 22/02/2023 21:08, Casey Schaufler wrote:

quoted

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
identifys 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.