Re: [PATCH net 1/1] net: smc: fix splice entry lifetime imbalance in smc_rx_splice

[Jakub Kicinski <kuba@kernel.org>]

On Thu, 11 Jun 2026 01:54:11 +0800 Ren Wei wrote:
> smc_rx_splice() hands candidate pages to splice_to_pipe() without taking
> references for the lifetime of each splice entry first. That breaks the
> splice ownership contract in the VM-backed RMB path.
> 
> splice_to_pipe() drops unqueued entries through spd_release(), while
> queued entries are later dropped through the pipe buffer release
> callback. The current code only tries to take page references after the
> splice succeeds, and it derives the number of queued VM pages from a
> mutated offset value. This can underflow page refcounts and trigger a
> use-after-free. It also leaves the socket lifetime imbalanced in the
> multi-page VM case, where one sock_hold() can be followed by multiple
> sock_put() calls.
> 
> Fix this by taking the page and socket references for every candidate
> splice entry before calling splice_to_pipe(), and by releasing the
> matching private state, page reference, and socket reference from
> smc_rx_spd_release() for entries that never get queued. This makes the
> SMC splice path follow the normal splice lifetime rules and removes the
> broken post-splice VM page counting entirely.

SMC maintainers please review.