linux-kernel - Re: less size_t please (was Re: [PATCH net] xfrm: fix integer overflow in xfrm_replay_state_esn

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Message-ID: <1ee57015-a2c3-4dd1-99c2-53e9ff50a09f@stanley.mountain>
Date: Thu, 30 Jan 2025 19:15:15 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Alexey Dobriyan <adobriyan@...il.com>
Cc: Steffen Klassert <steffen.klassert@...unet.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: less size_t please (was Re: [PATCH net] xfrm: fix integer
 overflow in xfrm_replay_state_esn_len())

On Thu, Jan 30, 2025 at 04:44:42PM +0300, Alexey Dobriyan wrote:
> > -static inline unsigned int xfrm_replay_state_esn_len(struct xfrm_replay_state_esn *replay_esn)
> > +static inline size_t xfrm_replay_state_esn_len(struct xfrm_replay_state_esn *replay_esn)
> >  {
> > -	return sizeof(*replay_esn) + replay_esn->bmp_len * sizeof(__u32);
> > +	return size_add(sizeof(*replay_esn), size_mul(replay_esn->bmp_len, sizeof(__u32)));
> 
> Please don't do this.
> 
> You can (and should!) make calculations and check for overflow at the
> same time. It's very efficient.
> 
> > 1) Use size_add() and size_mul().  This change is necessary for 32bit systems.
> 
> This bloats code on 32-bit.
> 

I'm not sure I understand.  On 32-bit systems a size_t and an unsigned
int are the same size.  Did you mean to say 64-bit?

Declaring sizes as u32 leads to integer overflows like this one.  If
you look at integer overflows with security implications there is a
5 to 1 ratio of bugs that only affect 32-bit vs bugs that affect
everything because it's just so much easier to overflow a 32-bit size.

aab98e2dbd64 ("ksmbd: fix integer overflows on 32 bit systems")
16ebb6f5b629 ("nfp: bpf: prevent integer overflow in nfp_bpf_event_output()")
09c4a6101532 ("rtc: tps6594: Fix integer overflow on 32bit systems")
55cf2f4b945f ("binfmt_flat: Fix integer overflow bug on 32 bit systems")
fbbd84af6ba7 ("chelsio/chtls: prevent potential integer overflow on 32bit")
bd96a3935e89 ("rdma/cxgb4: Prevent potential integer overflow on 32bit")
d0257e089d1b ("RDMA/uverbs: Prevent integer overflow issue")
3c63d8946e57 ("svcrdma: Address an integer overflow")
7f33b92e5b18 ("NFSD: Prevent a potential integer overflow")

> 	int len;
> 	if (__builtin_mul_overflow(replay_esn->bmp_len, 4, &len)) {
> 		return true;
> 	}
> 	if (__builtin_add_overflow(len, sizeof(*replay_esn), &len)) {
> 		return true;
> 	}

This is so ugly...  :/  I'd prefer to just do open code the check at
that point.

static inline int xfrm_replay_state_esn_len(struct xfrm_replay_state_esn *replay_esn)
{
	if (replay_esn->bmp_len > (INT_MAX - sizeof(*replay_esn)) / sizeof(__u32))
		return -EINVAL;
	return sizeof(*replay_esn) + replay_esn->bmp_len * sizeof(__u32);
}

regards,
dan carpenter