[PATCH net v6] rust: net: netlink: validate attribute length before casting to `c_int`

[PATCH net v6] rust: net: netlink: validate attribute length before casting to `c_int`

[Sagar Taunk]

`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: 5eaa5fbb6e6c ("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>
---
changes since v5: 
Rebased on net/main

 rust/kernel/net/netlink.rs | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/rust/kernel/net/netlink.rs b/rust/kernel/net/netlink.rs
index 22ef3dde36fa..2f8733a694a9 100644
--- a/rust/kernel/net/netlink.rs
+++ b/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) })
-- 
2.55.0

Re: [PATCH net v6] rust: net: netlink: validate attribute length before casting to `c_int`

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to netdev/net.git (main)
by Paolo Abeni <pabeni@redhat.com>:

On Fri, 25 Sep 2026 08:55:04 +0000 you wrote:
> `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.
> 
> [...]

Here is the summary with links:
  - [net,v6] rust: net: netlink: validate attribute length before casting to `c_int`
    https://git.kernel.org/netdev/net/c/0923198be4ae

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html