Re: [PATCH net v2] vxlan: keep the last remote linked during FDB flush

[Ido Schimmel <idosch@nvidia.com>]

On Tue, Aug 04, 2026 at 06:09:58AM +0000, David Lee wrote:
> From: Kyle Zeng <kylebot@openai.com>
> 
> A non-nexthop FDB entry is expected to have at least one remote while it
> remains reachable through the FDB hash table. A filtered bulk flush
> violates this invariant when every remote matches: It unlinks the last
> remote in vxlan_fdb_dst_destroy() and only afterwards tells vxlan_flush()
> to destroy the parent FDB entry.
> 
> An RCU reader can find the parent during this interval.
> first_remote_rcu() then applies list_entry_rcu() to the empty list head,
> producing an invalid remote pointer that the receive learning path can
> read from and write to.
> 
> When a matching remote is the sole remaining remote, leave it linked and
> ask the caller to destroy the entire FDB entry. vxlan_fdb_destroy() keeps
> the remote attached while sending the deletion notification and removing
> the parent from the lookup structures.
> 
> Fixes: c499fccb71cb ("vxlan: vxlan_core: Support FDB flushing by destination VNI")
> Cc: stable@vger.kernel.org
> Assisted-by: Codex:gpt-5.6-sol Codex:gpt-5.5-cyber
> Signed-off-by: Kyle Zeng <kylebot@openai.com>
> Co-developed-by: David Lee <david.lee@trailofbits.com>
> Signed-off-by: David Lee <david.lee@trailofbits.com>
> ---
> Changes in v2:
> - Add the net tree prefix to the subject.
> - Restore Kyle Zeng as the patch author and correct the sign-off chain.
> - Move the research credit below the commit-message separator.
> - Add the recipients reported by netdev CI.
> 
> v1: https://lore.kernel.org/all/20260731141311.570187-1-david.lee@trailofbits.com/
> 
> Bug found and triaged by OpenAI Security Research and
> validated by Trail of Bits.
> 
> Trail of Bits has a reproducer for this bug that triggers a
> KASAN slab-out-of-bounds read in vxlan_snoop() and can share if needed.
> 
>  drivers/net/vxlan/vxlan_core.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c
> index d834a4865..a00df127d 100644
> --- a/drivers/net/vxlan/vxlan_core.c
> +++ b/drivers/net/vxlan/vxlan_core.c
> @@ -3058,6 +3058,11 @@ vxlan_fdb_flush_match_remotes(struct vxlan_fdb *f, struct vxlan_dev *vxlan,
>  		if (!vxlan_fdb_flush_remote_matches(desc, rd))
>  			continue;
>  
> +		if (list_is_singular(&f->remotes)) {
> +			*p_destroy_fdb = true;
> +			return;
> +		}
> +
>  		vxlan_fdb_dst_destroy(vxlan, f, rd, true);
>  		remotes_flushed = true;
>  	}

Can you squash the below into v3?

'*p_destroy_fdb' at the end is always false and it's initialized as such
by the caller, so we can remove this line and 'remotes_flushed'.

diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c
index 117fae9c05a4..0f686bcc5a20 100644
--- a/drivers/net/vxlan/vxlan_core.c
+++ b/drivers/net/vxlan/vxlan_core.c
@@ -3058,7 +3058,6 @@ vxlan_fdb_flush_match_remotes(struct vxlan_fdb *f, struct vxlan_dev *vxlan,
 			      const struct vxlan_fdb_flush_desc *desc,
 			      bool *p_destroy_fdb)
 {
-	bool remotes_flushed = false;
 	struct vxlan_rdst *rd, *tmp;
 
 	list_for_each_entry_safe(rd, tmp, &f->remotes, list) {
@@ -3071,10 +3070,7 @@ vxlan_fdb_flush_match_remotes(struct vxlan_fdb *f, struct vxlan_dev *vxlan,
 		}
 
 		vxlan_fdb_dst_destroy(vxlan, f, rd, true);
-		remotes_flushed = true;
 	}
-
-	*p_destroy_fdb = remotes_flushed && list_empty(&f->remotes);
 }
 
 /* Purge the forwarding table */