Re: [PATCH v7 07/11] LSM: Helpers for attribute names and filling an lsm_ctx
From: Mickaël Salaün
Date: Mon Apr 03 2023 - 07:47:22 EST
On 03/04/2023 11:54, Mickaël Salaün wrote:
On 03/04/2023 11:47, Mickaël Salaün wrote:
On 15/03/2023 23:47, Casey Schaufler wrote:
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.
All are used in module specific components of LSM system calls.
Signed-off-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
---
include/linux/security.h | 13 ++++++++++
security/lsm_syscalls.c | 51 ++++++++++++++++++++++++++++++++++++++++
security/security.c | 31 ++++++++++++++++++++++++
3 files changed, 95 insertions(+)
[...]
diff --git a/security/security.c b/security/security.c
index 2c57fe28c4f7..f7b814a3940c 100644
--- a/security/security.c
+++ b/security/security.c
@@ -753,6 +753,37 @@ 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.
+ * 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 local;
+ void __user *vc = ctx;
+
+ local.id = id;
+ local.flags = flags;
+ local.ctx_len = context_size;
+ local.len = context_size + sizeof(local);
+ vc += sizeof(local);
+ if (copy_to_user(ctx, &local, sizeof(local)))
+ return -EFAULT;
+ if (context_size > 0 && copy_to_user(vc, context, context_size))
+ return -EFAULT;
Can we do a single copy_to_user() call? That would avoid inconsistent
user space data, could speed up a bit the operation, and make the code
easier to understand. To use the stack, we need to know the maximum size
of context_size for all use cases, which seems reasonable and can be
checked at build time (on each LSM side, and potentially with specific
context type passed as enum instead of context_size) and run time (for
this generic helper).
Well, actually the context_size should be inferred from id, and the
"local" size should be defined and check at build time against all
context ID sizes.
@ctx_len should already be known by user space according to the LSM ID
and the requested attribute. @len should already be known by user space
because lsm_ctx is part of the ABI.
The only reason I can think of the rationale for @len and @ctx_len is
that struct lsm_ctx could gain more fields. If this happen, they would
then need to be inserted before @ctx. This would make this struct
lsm_ctx too flexible and complex for user space to parse correctly (e.g.
for strace, gdb).
I don't see where we could use @flags instead of relying on a new
attribute type.
I think security_getselfattr() and lsm_fill_user_ctx() could be changed
to avoid each LSM to pass their own ID to lsm_fill_user_ctx(). We could
have a lsm_get_attr_size(lsm_id, attr) helper (called by
security_getselfattr) to group these relations, based on fixed values,
exposed in the UAPI, and checked at build time with the size of the
related LSM-specific attribute type. This would also allow to factor out
the total size calculation needed before calling the getselfattr()
implementers, and then rely on a common consistent behavior. That could
also be used to not call getselfattr() implementers if they don't handle
a specific attribute, and then remove their related error handling for
this case.
For now, the getselfattr() hook (not the related syscall) doesn't need
to pass a "flags" argument to each LSM because there is no use of it.
+ return 0;
+}
+
/*
* The default value of the LSM hook is defined in linux/lsm_hook_defs.h and
* can be accessed with: