rust: net: netlink: validate attribute length before casting to c_int

`put()` trusted an unchecked `as` cast from `usize` to `c_int`.
When the length exceeds `i32::MAX` that cast wraps around to a
negative value.

This ultimately resulted in a kernel panic when the reinterpreted
value via `__nla_reserve()` and `skb_put()` became enormous.

Validate payload and header both fit together in a `u16`, rejecting
any payload that wouldn't leave room for `NLA_HDRLEN`.

Fixes: 5eaa5fbb6e ("rust: netlink: add raw netlink abstraction")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Carlos Llamas <cmllamas@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Sagar Taunk <sagartaunk@proton.me>
Link: https://patch.msgid.link/20260925085449.22553-1-sagartaunk@proton.me
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

rust/kernel/net/netlink.rs

@@ -11,6 +11,7 @@
 use kernel::{
     alloc::{self, AllocError},
     error::to_result,
+    num::casts::u16_as_usize,
     prelude::*,
     transmute::AsBytes,
     types::Opaque,
@@ -86,9 +87,17 @@ fn put<T>(&mut self, attrtype: c_int, value: &T) -> Result
     where
         T: ?Sized + AsBytes,
     {
+        // `nla_len` is a 16-bit field that encodes the total attribute length
+        // (header + payload). Subtracting the header size from `u16::MAX` gives
+        // the largest payload that still fits within that field.
+        const MAX_PAYLOAD_LEN: usize = u16_as_usize(u16::MAX) - size_of::<bindings::nlattr>();
+
         let skb = self.skb.skb.as_ptr();
         let len = size_of_val(value);
         let ptr = core::ptr::from_ref(value).cast::<c_void>();
+        if len > MAX_PAYLOAD_LEN {
+            return Err(EMSGSIZE);
+        }
         // SAFETY: `skb` is valid by `NetlinkSkBuff` type invariants, and the provided value is
         // readable and initialized for its `size_of` bytes.
         to_result(unsafe { bindings::nla_put(skb, attrtype, len as c_int, ptr) })