linux-kernel - Re: [PATCH] smb/client: avoid possible NULL dereference in cifs_free

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Message-ID: <893f2ebb-2979-4e34-bdab-a7cbb0c7e7b8@stanley.mountain>
Date: Fri, 9 Aug 2024 18:00:26 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Su Hui <suhui@...china.com>, David Howells <dhowells@...hat.com>
Cc: sfrench@...ba.org, pc@...guebit.com, ronniesahlberg@...il.com,
	sprasad@...rosoft.com, tom@...pey.com, bharathsm@...rosoft.com,
	nathan@...nel.org, ndesaulniers@...gle.com, morbo@...gle.com,
	justinstitt@...gle.com, linux-cifs@...r.kernel.org,
	samba-technical@...ts.samba.org, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] smb/client: avoid possible NULL dereference in
 cifs_free_subrequest()

On Thu, Aug 08, 2024 at 08:23:32PM +0800, Su Hui wrote:
> Clang static checker (scan-build) warning:
> 	cifsglob.h:line 890, column 3
> 	Access to field 'ops' results in a dereference of a null pointer.
> 
> Commit 519be989717c ("cifs: Add a tracepoint to track credits involved in
> R/W requests") adds a check for 'rdata->server', and let clang throw this
> warning about NULL dereference.
> 
> When 'rdata->credits.value != 0 && rdata->server == NULL' happens,
> add_credits_and_wake_if() will call rdata->server->ops->add_credits().
> This will cause NULL dereference problem. Add a check for 'rdata->server'
> to avoid NULL dereference.
> 
> Signed-off-by: Su Hui <suhui@...china.com>

Needs a Fixes tag.

Also when you add a Fixes tag, then get_maintainer will add the David Howells
automatically.  I've added him manually.

> ---
>  fs/smb/client/file.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/smb/client/file.c b/fs/smb/client/file.c
> index b2405dd4d4d4..45459af5044d 100644
> --- a/fs/smb/client/file.c
> +++ b/fs/smb/client/file.c
> @@ -315,7 +315,7 @@ static void cifs_free_subrequest(struct netfs_io_subrequest *subreq)
>  #endif
>  	}
>  
> -	if (rdata->credits.value != 0)
> +	if (rdata->credits.value != 0) {
>  		trace_smb3_rw_credits(rdata->rreq->debug_id,
>  				      rdata->subreq.debug_index,
>  				      rdata->credits.value,
> @@ -323,8 +323,12 @@ static void cifs_free_subrequest(struct netfs_io_subrequest *subreq)
>  				      rdata->server ? rdata->server->in_flight : 0,
>  				      -rdata->credits.value,
>  				      cifs_trace_rw_credits_free_subreq);
> +		if (rdata->server)
> +			add_credits_and_wake_if(rdata->server, &rdata->credits, 0);
> +		else
> +			rdata->credits.value = 0;
                        ^^^^^^^^^^^^^^^^^^^^^^^^^
Why do this?

regards,
dan carpenter