linux-kernel - RE: [PATCH] Remove hardcoded static string length

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Message-ID: <30d65ea9170d4f60bd76ed516541cb46@AcuMS.aculab.com>
Date:   Thu, 25 May 2023 09:14:53 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Kenny Ho' <y2kenny@...il.com>, Andrew Lunn <andrew@...n.ch>
CC:     Marc Dionne <marc.dionne@...istor.com>,
        Kenny Ho <Kenny.Ho@....com>,
        "David Howells" <dhowells@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        "Eric Dumazet" <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "linux-afs@...ts.infradead.org" <linux-afs@...ts.infradead.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] Remove hardcoded static string length

From: Kenny Ho
> Sent: 24 May 2023 19:01
> 
> On Wed, May 24, 2023 at 1:43 PM Andrew Lunn <andrew@...n.ch> wrote:
> >
> > The other end of the socket should not blow up, because that would be
> > an obvious DOS or buffer overwrite attack vector. So you need to
> > decide, do you want to expose such issues and see if anything does
> > actually blow up, or do you want to do a bit more work and correctly
> > terminate the string when capped?
> 
> Right... I guess it's not clear to me that existing implementations
> null-terminate correctly when UTS_RELEASE causes the string to exceed
> the 65 byte size of rxrpc_version_string.  We can of course do better,
> but I hesitate to do strncpy because I am not familiar with this code
> base enough to tell if this function is part of some hot path where
> strncpy matters.

The whole thing looks like it is expecting a max of 64 characters
and a terminating '\0'.
Since UTE_RELEASE goes in between two fixed strings truncating
the whole thing to 64/65 chars/bytes doesn't seem ideal.

I does rather beg the question as what is in UTS_RELEASE when
it exceeds (IIRC) about 48 characters?

If UTS_RELEASE is getting that long, it might easily exceed
the 64 characters returned by uname().

I suspect that you need to truncate UTS_RELEASE to limit
the string to 64 characters - so something like:
	static char id[65];
	if (!id[0])
		snprintf(id, sizeof id, "xxx-%.48s-yyy", UTS_RELEASE);

Using an on-stack buffer almost certainly wouldn't matter.

	David

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