Re: [PATCH v2] drm/i915/gt: Fix request use-after-free in heartbeat()

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] heartbeat() dereferences the local pointer `rq` without holding a strong reference, making it vulnerable to a Use-After-Free if intel_engine_park_heartbeat() concurrently clears the shared pointer.
--

commit aa7ae2412e3834bb064c9442f0f96ffd0058a8ae
Author: Shuangpeng Bai <shuangpeng.kernel@gmail.com>

drm/i915/gt: Fix request use-after-free in heartbeat()

This commit addresses a request use-after-free in heartbeat() by using cmpxchg 
to manage the detached systole reference and replacing direct struct accesses 
with a local pointer.

Link: https://patch.msgid.link/20260808184302.1509585-1-shuangpeng.kernel@gmail.com

> diff --git a/drivers/gpu/drm/i915/gt/intel_engine_heartbeat.c b/drivers/gpu/drm/i915/gt/intel_engine_heartbeat.c
> index 6424ecce8bcbe..fc6c53c93cf57 100644
> --- a/drivers/gpu/drm/i915/gt/intel_engine_heartbeat.c
> +++ b/drivers/gpu/drm/i915/gt/intel_engine_heartbeat.c
[ ... ]
> @@ -152,9 +160,11 @@ static void heartbeat(struct work_struct *wrk)
>  	if (rq) {
>  		if (i915_request_completed(rq))
>  			i915_request_put(rq);
> -		else
> -			engine->heartbeat.systole = rq;
> +		/* Keep a newer pulse that raced with the detached systole. */
> +		else if (cmpxchg(&engine->heartbeat.systole, NULL, rq))
> +			i915_request_put(rq);
>  	}
> +	rq = READ_ONCE(engine->heartbeat.systole);

[Severity: High]
This is a pre-existing issue, but does heartbeat() safely dereference rq
without holding a strong reference?

If userspace toggles sysfs heartbeat intervals, intel_engine_park_heartbeat()
can execute concurrently, clearing the shared pointer and dropping the
reference:

intel_engine_park_heartbeat() {
    ...
    rq = xchg(&engine->heartbeat.systole, NULL);
    if (rq)
        i915_request_put(rq);
    ...
}

Since rq is loaded locklessly here without a reference bump, could this
result in a Use-After-Free later in heartbeat() when accessing
rq->emitted_jiffies?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260830225708.3959691-1-andi.shyti@linux.intel.com?part=1