Subject: Re: [PATCH net-next] net: mdio: Add netlink interface

From: "Russell King (Oracle)" <[email-masked]>
Date: Mon, 6 Mar 2023 22:48:48 +0000

On Mon, Mar 06, 2023 at 03:45:16PM -0500, Sean Anderson wrote:
> +static int mdio_nl_eval(struct mdio_nl_xfer *xfer)
> +{
> +	struct mdio_nl_insn *insn;
> +	unsigned long timeout;
> +	u16 regs[8] = { 0 };
> +	int pc, ret = 0;

So "pc" is signed.

> +	int phy_id, reg, prtad, devad, val;
> +
> +	timeout = jiffies + msecs_to_jiffies(xfer->timeout_ms);
> +
> +	mutex_lock(&xfer->mdio->mdio_lock);
> +
> +	for (insn = xfer->prog, pc = 0;
> +	     pc < xfer->prog_len;

xfer->prog_len is signed, so this is a signed comparison.

> +		case MDIO_NL_OP_JEQ:
> +			if (__arg_ri(insn->arg0, regs) ==
> +			    __arg_ri(insn->arg1, regs))
> +				pc += (s16)__arg_i(insn->arg2);

This adds a signed 16-bit integer to pc, which can make pc negative.

And so the question becomes... what prevents pc becoming negative
and then trying to use a negative number as an index?

I think prog_len and pc should both be unsigned, then the test you
have will be unsigned, and thus wrapping "pc" around zero makes it
a very large integer which fails the test - preventing at least
access outside of the array. Better still would be a validator
that checks that the program is in fact safe to execute.

-- 
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