Re: [PATCH rdma-rc] RDMA/ucma: free a queued CONNECT_REQUEST with its new context

[Leon Romanovsky]

On Mon, Nov 09, 2026 at 11:20:00AM +0800, Yuehong Wu wrote:
> ucma_cleanup_ctx_events() only frees the events that belong to the
> context being destroyed.  A CONNECT_REQUEST uevent that has not been
> delivered to user space belongs to the listening context, but it also
> keeps the newly created connection context in uevent->conn_req_ctx, and
> that context is already reachable from user space because
> ucma_finish_ctx() published its id before the event was queued.
> 
> If user space destroys the new id while the event is still queued, i.e.
> before it fetches it with get_event, the event is skipped here and stays
> in the listener's event list with a pointer to a context that
> ucma_destroy_private_ctx() then frees.  Destroying the listener later
> (which is what closing its file does) reaches the
> uevent->conn_req_ctx->id lookup in the xa_cmpxchg() below with a
> dangling pointer, and when the freed context was reallocated the
> comparison can match an unrelated live context, which is then detached
> from the id table and freed.
> 
> The match in commit 8ae291cc95e4 ("RDMA/ucma: Do not miss ctx
> destruction steps in some cases") replaced "uevent->ctx == ctx ||
> uevent->conn_req_ctx == ctx" with a plain "uevent->ctx != ctx" test and
> lost the case where the new context is the one being destroyed.
> 
> Free the event in that case and return the backlog slot it consumed: it
> was never delivered to user space, so it can no longer be acted upon.
> 
> Fixes: 8ae291cc95e4 ("RDMA/ucma: Do not miss ctx destruction steps in some cases")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Yuehong Wu <wuyuehong@xxxxxxxxxxxxxxxxxxxx>
> ---
> Validated in a QEMU/KVM guest running this tree with CONFIG_KASAN=y and
> CONFIG_RDMA_RXE=y.  A reproducer creates an rxe device, queues a
> CONNECT_REQUEST, sees it with poll() without fetching it, finds the new
> connection id by trying the small integer ids that belong to its own
> file, destroys that id and then the listener.  The unpatched kernel
> reports
> 
>   BUG: KASAN: slab-use-after-free in ucma_destroy_private_ctx+0x1c0/0x4f0
>   Read of size 4 at addr ff11000102295a00 by task c9repro/302
>   Allocated by task 12:
>    ucma_alloc_ctx+0x2b/0x170
>    ucma_event_handler+0x1ad/0x3a0
>   Freed by task 302:
>    ucma_destroy_private_ctx+0x468/0x4f0
>    ucma_destroy_id+0x180/0x1e0
> 
> With this patch the same sequence run four times produces no report, and
> a probe shows the queued event is dropped when the new context is
> destroyed.
> 
> Note: ucma_migrate_id() can move the new context to another file, which
> leaves its queued CONNECT_REQUEST in the original file's event list.
> That cross-file case predates 8ae291cc95e4 and is left for a separate
> fix.
> 
>  drivers/infiniband/core/ucma.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/infiniband/core/ucma.c b/drivers/infiniband/core/ucma.c
> --- a/drivers/infiniband/core/ucma.c
> +++ b/drivers/infiniband/core/ucma.c
> @@ -534,8 +534,23 @@ static int ucma_cleanup_ctx_events(struct ucma_context *ctx)
>  	/* Cleanup events not yet reported to the user.*/
>  	mutex_lock(&ctx->file->mut);
>  	list_for_each_entry_safe(uevent, tmp, &ctx->file->event_list, list) {
> -		if (uevent->ctx != ctx)
> +		/*
> +		 * A CONNECT_REQUEST that has not been delivered to user
> +		 * space belongs to the listening context but also holds the
> +		 * new one. 

I don't understand this sentence. conn_req_ctx is created and added to
event_list at the same function without any conditions.

Please explain with function call chain how your claim is possible.

Thanks