linux-kernel - Re: [PATCH] drm/nouveau/gsp: fix mismatched alloc/free for kvmalloc()

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Message-Id: <DC18XPLOI092.3BTLNFSDWJU8G@kernel.org>
Date: Wed, 13 Aug 2025 13:00:21 +0200
From: "Danilo Krummrich" <dakr@...nel.org>
To: "Timur Tabi" <ttabi@...dia.com>
Cc: "rongqianfeng@...o.com" <rongqianfeng@...o.com>, "airlied@...hat.com"
 <airlied@...hat.com>, "simona@...ll.ch" <simona@...ll.ch>,
 "lyude@...hat.com" <lyude@...hat.com>, "dri-devel@...ts.freedesktop.org"
 <dri-devel@...ts.freedesktop.org>, "Zhi Wang" <zhiw@...dia.com>,
 "nouveau@...ts.freedesktop.org" <nouveau@...ts.freedesktop.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/nouveau/gsp: fix mismatched alloc/free for
 kvmalloc()

On Wed Aug 13, 2025 at 12:52 AM CEST, Timur Tabi wrote:
> On Mon, 2025-08-11 at 17:19 +0800, Qianfeng Rong wrote:
>> Replace kfree() with kvfree() for memory allocated by kvmalloc().
>> 
>> Compile-tested only.
>> 
>> Signed-off-by: Qianfeng Rong <rongqianfeng@...o.com>
>
> Reviewed-by: Timur Tabi <ttabi@...dia.com>
>
> This does fix a real bug.
>
> However, I think the real problem is that it's really confusing that
> r535_gsp_msgq_recv_one_elem(gsp, &info) returns info.gsp_rpc_buf instead of just success/failure. 
> r535_gsp_msgq_recv() does this:
>
> 	buf = kvmalloc(max_t(u32, rpc->length, expected), GFP_KERNEL);
> ...
> 	info.gsp_rpc_buf = buf;
> ... 
> 	buf = r535_gsp_msgq_recv_one_elem(gsp, &info);
>
> You wouldn't know it, but this does not change the value of 'buf' unless
> r535_gsp_msgq_recv_one_elem() fails.  If it does fail, the code does this:

Ick! That makes no sense, r535_gsp_msgq_recv_one_elem() should just return an
int and we shouldn't overwrite buf -- that's a footgun.

> 	if (IS_ERR(buf)) {
> 		kvfree(info.gsp_rpc_buf);

Should just be

	if (ret) {
		kvfree(buf);
		return ERR_PTR(ret);
	}

It also doesn't need the info.gsp_rpc_buf = NULL; assignment, info is local
anyways.

> It would be a lot clearer if we could kvfree(buf) here, but we can't because 'buf' no longer points
> to the buffer, even though the buffer still exists.

Agreed.

Zhi, Timur do you want to send a follow-up patch for this?