On 4/18/2023 2:51 PM, Paul Moore wrote:

On Tue, Apr 11, 2023 at 12:02 PM Casey Schaufler [off-list ref] wrote:

quoted

Add lsm_name_to_attr(), which translates a text string to a
LSM_ATTR value if one is available.

Add lsm_fill_user_ctx(), which fills a struct lsm_ctx, including
the trailing attribute value. The .len value is padded to a multiple
of the size of the structure for alignment.

All are used in module specific components of LSM system calls.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
---
 include/linux/security.h | 13 +++++++++++
 security/lsm_syscalls.c  | 24 ++++++++++++++++++++
 security/security.c      | 48 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 85 insertions(+)

..

quoted

diff --git a/security/lsm_syscalls.c b/security/lsm_syscalls.c
index 6efbe244d304..67106f642422 100644
--- a/security/lsm_syscalls.c
+++ b/security/lsm_syscalls.c

@@ -17,6 +17,30 @@
 #include <linux/lsm_hooks.h>
 #include <uapi/linux/lsm.h>

+/**
+ * lsm_name_to_attr - map an LSM attribute name to its ID
+ * @name: name of the attribute
+ *
+ * Returns the LSM attribute value associated with @name, or 0 if
+ * there is no mapping.
+ */
+u64 lsm_name_to_attr(const char *name)
+{
+       if (!strcmp(name, "current"))
+               return LSM_ATTR_CURRENT;
+       if (!strcmp(name, "exec"))
+               return LSM_ATTR_EXEC;
+       if (!strcmp(name, "fscreate"))
+               return LSM_ATTR_FSCREATE;
+       if (!strcmp(name, "keycreate"))
+               return LSM_ATTR_KEYCREATE;
+       if (!strcmp(name, "prev"))
+               return LSM_ATTR_PREV;
+       if (!strcmp(name, "sockcreate"))
+               return LSM_ATTR_SOCKCREATE;
+       return 0;
+}

Thank you :)

It didn't hurt all that badly.

quoted

 /**
  * sys_lsm_set_self_attr - Set current task's security module attribute
  * @attr: which attribute to set

diff --git a/security/security.c b/security/security.c
index bfe9a1a426b2..453f3ff591ec 100644
--- a/security/security.c
+++ b/security/security.c

@@ -752,6 +752,54 @@ static int lsm_superblock_alloc(struct super_block *sb)
        return 0;
 }

+/**
+ * lsm_fill_user_ctx - Fill a user space lsm_ctx structure
+ * @ctx: an LSM context to be filled
+ * @context: the new context value
+ * @context_size: the size of the new context value
+ * @id: LSM id
+ * @flags: LSM defined flags
+ *
+ * Fill all of the fields in a user space lsm_ctx structure.
+ * Caller is assumed to have verified that @ctx has enough space
+ * for @context.
+ *
+ * The total length is padded to an integral number of lsm_ctx.

Considering that lsm_ctx is variable length I'm not sure that makes a
lot of sense, how about we pad the total length so that the @ctx entry
is a multiple of 64-bits?

64 is fine.

  If needed we can always change this later
as the lsm_ctx struct is inherently variable in length and userspace
will need to deal with the buffer regardless of alignment.

quoted

+ * Returns 0 on success, -EFAULT on a copyout error.
+ */
+int lsm_fill_user_ctx(struct lsm_ctx __user *ctx, void *context,
+                     size_t context_size, u64 id, u64 flags)
+{
+       struct lsm_ctx *lctx;
+       size_t locallen;
+       u8 *composite;
+       int rc = 0;
+
+       locallen = sizeof(*ctx);
+       if (context_size)
+               locallen += sizeof(*ctx) * ((context_size / sizeof(*ctx)) + 1);

It seems cleaner to use the kernel's ALIGN() macro:

Indeed. I'll do it.

  /* ensure the lsm_ctx length is a multiple of 64-bits */
  locallen = ALIGN(sizeof(*ctx) + context_size, 8);
  lctx = kzalloc(locallen, GFP_KERNEL)
  if (!lctx)
    return -ENOMEM;

quoted

+       composite = kzalloc(locallen, GFP_KERNEL);
+       if (composite == NULL)
+               return -ENOMEM;
+
+       lctx = (struct lsm_ctx *)composite;
+       lctx->id = id;
+       lctx->flags = flags;
+       lctx->ctx_len = context_size;
+       lctx->len = locallen;
+
+       memcpy(composite + sizeof(*lctx), context, context_size);

Is there a problem with doing `memcpy(lctx->ctx, context,
context_size)` in place of the memcpy above?

Nope.

  That is easier to read
and we can get rid of @composite.

Point.

quoted

+       if (copy_to_user(ctx, composite, locallen))
+               rc = -EFAULT;
+
+       kfree(composite);
+
+       return rc;
+}

I understand Mickaël asked you to do a single copy_to_user(), but I'm
not sure it is worth it if we have to add a temporary buffer
allocation like that.  How about something like below (v7 with some
tweaks/padding)?  You could be a bit more clever with the memset if
you want, I was just typing this up quickly ...

I prefer two copies to the allocation myself. I'll incorporate this.

int lsm_fill_user_ctx(...)
{
  struct lsm_ctx lctx;

  /* ensure the lctx length is a multiple of 64-bits */
  lctx.len = ALIGN(sizeof(lctx) + context_size, 8);

  lctx.id = id;
  lctx.flags = flags;
  lctx.ctx_len = context_size;

  memset(ctx, 0, lctx.len);
  if (copy_to_user(ctx, &lctx, sizeof(lctx))
    return -EFAULT;
  if (copy_to_user(&ctx[1], context, context_size)
    return -EFAULT;

  return 0;
}

--
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