linux-kernel - Re: [PATCH] SUNRPC: Output oversized frag reclen as ASCII if printable

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Date:   Fri, 19 Mar 2021 14:58:14 +0000
From:   Chuck Lever III <chuck.lever@...cle.com>
To:     Chris Down <chris@...isdown.name>
CC:     Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
        Linux-Net <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Bruce Fields <bfields@...hat.com>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] SUNRPC: Output oversized frag reclen as ASCII if
 printable

Hi Chris-

> On Mar 19, 2021, at 10:54 AM, Chris Down <chris@...isdown.name> wrote:
> 
> The reclen is taken directly from the first four bytes of the message
> with the highest bit stripped, which makes it ripe for protocol mixups.
> For example, if someone tries to send a HTTP GET request to us, we'll
> interpret it as a 1195725856-sized fragment (ie. (u32)'GET '), and print
> a ratelimited KERN_NOTICE with that number verbatim.
> 
> This can be confusing for downstream users, who don't know what messages
> like "fragment too large: 1195725856" actually mean, or that they
> indicate some misconfigured infrastructure elsewhere.

One wonders whether that error message is actually useful at all.
We could, for example, turn this into a tracepoint, or just get
rid of it.


> To allow users to more easily understand and debug these cases, add the
> number interpreted as ASCII if all characters are printable:
> 
>    RPC: fragment too large: 1195725856 (ASCII "GET ")
> 
> If demand grows elsewhere, a new printk format that takes a number and
> outputs it in various formats is also a possible solution. For now, it
> seems reasonable to put this here since this particular code path is the
> one that has repeatedly come up in production.
> 
> Signed-off-by: Chris Down <chris@...isdown.name>
> Cc: Chuck Lever <chuck.lever@...cle.com>
> Cc: J. Bruce Fields <bfields@...hat.com>
> Cc: Trond Myklebust <trond.myklebust@...merspace.com>
> Cc: David S. Miller <davem@...emloft.net>
> ---
> net/sunrpc/svcsock.c | 39 +++++++++++++++++++++++++++++++++++++--
> 1 file changed, 37 insertions(+), 2 deletions(-)
> 
> diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
> index 2e2f007dfc9f..046b1d104340 100644
> --- a/net/sunrpc/svcsock.c
> +++ b/net/sunrpc/svcsock.c
> @@ -46,6 +46,7 @@
> #include <linux/uaccess.h>
> #include <linux/highmem.h>
> #include <asm/ioctls.h>
> +#include <linux/ctype.h>
> 
> #include <linux/sunrpc/types.h>
> #include <linux/sunrpc/clnt.h>
> @@ -863,6 +864,34 @@ static void svc_tcp_clear_pages(struct svc_sock *svsk)
> 	svsk->sk_datalen = 0;
> }
> 
> +/* The reclen is taken directly from the first four bytes of the message with
> + * the highest bit stripped, which makes it ripe for protocol mixups. For
> + * example, if someone tries to send a HTTP GET request to us, we'll interpret
> + * it as a 1195725856-sized fragment (ie. (u32)'GET '), and print a ratelimited
> + * KERN_NOTICE with that number verbatim.
> + *
> + * To allow users to more easily understand and debug these cases, this
> + * function decodes the purported length as ASCII, and returns it if all
> + * characters were printable. Otherwise, we return NULL.
> + *
> + * WARNING: Since we reuse the u32 directly, the return value is not null
> + * terminated, and must be printed using %.*s with
> + * sizeof(svc_sock_reclen(svsk)).
> + */
> +static char *svc_sock_reclen_ascii(struct svc_sock *svsk)
> +{
> +	u32 len_be = cpu_to_be32(svc_sock_reclen(svsk));
> +	char *len_be_ascii = (char *)&len_be;
> +	size_t i;
> +
> +	for (i = 0; i < sizeof(len_be); i++) {
> +		if (!isprint(len_be_ascii[i]))
> +			return NULL;
> +	}
> +
> +	return len_be_ascii;
> +}
> +
> /*
>  * Receive fragment record header into sk_marker.
>  */
> @@ -870,6 +899,7 @@ static ssize_t svc_tcp_read_marker(struct svc_sock *svsk,
> 				   struct svc_rqst *rqstp)
> {
> 	ssize_t want, len;
> +	char *reclen_ascii;
> 
> 	/* If we haven't gotten the record length yet,
> 	 * get the next four bytes.
> @@ -898,9 +928,14 @@ static ssize_t svc_tcp_read_marker(struct svc_sock *svsk,
> 	return svc_sock_reclen(svsk);
> 
> err_too_large:
> -	net_notice_ratelimited("svc: %s %s RPC fragment too large: %d\n",
> +	reclen_ascii = svc_sock_reclen_ascii(svsk);
> +	net_notice_ratelimited("svc: %s %s RPC fragment too large: %d%s%.*s%s\n",
> 			       __func__, svsk->sk_xprt.xpt_server->sv_name,
> -			       svc_sock_reclen(svsk));
> +			       svc_sock_reclen(svsk),
> +			       reclen_ascii ? " (ASCII \"" : "",
> +			       (int)sizeof(u32),
> +			       reclen_ascii ?: "",
> +			       reclen_ascii ? "\")" : "");
> 	set_bit(XPT_CLOSE, &svsk->sk_xprt.xpt_flags);
> err_short:
> 	return -EAGAIN;
> -- 
> 2.30.2
> 

--
Chuck Lever