Re: [PATCH net-next] net: stmmac: ptp: limit n_per_out

[Simon Horman <horms@kernel.org>]

On Mon, Feb 23, 2026 at 12:20:47PM +0000, Russell King (Oracle) wrote:
> ptp_clock_ops.n_per_out sets the number of PPS outputs, which the PTP
> subsystem uses to validate userspace input, such as the index number
> used in a PTP_CLK_REQ_PEROUT request.
> 
> stmmac_enable() uses this to index the priv->pps array, which is an
> array of size STMMAC_PPS_MAX. ptp_clock_ops.n_per_out is initialised
> using priv->dma_cap.pps_out_num, which is a three bit field read from
> hardware.
> 
> Documentation that I've checked suggests that values >= 5 are reserved,
> but that doesn't mean such values won't appear, and if they do, we
> can overrun the priv->pps array in stmmac_enable().
> 
> stmmac_ptp_register() has protection against this in its loop, but it
> doesn't act to limit ptp_clock_ops.n_per_out.
> 
> Fix this by introducing a local variable, pps_out_num which is limited
> to STMMAC_PPS_MAX, and use that when initialising the array and setting
> priv->ptp_clock_ops.n_per_out.
> 
> Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
> ---
> 
> This could be a user exploitable bug (although one has to be root
> so the gun is already pointing at one's foot.) This is the commit
> which introduced the problem:

Hi Russell,

From the description I assumed that for this problem to manifest
out-of-range values would need to be turned by hardware.
But maybe I misunderstand things.

Could you elaborate on the vector you have in mind?

> 
> Fixes: 9a8a02c9d46d ("net: stmmac: Add Flexible PPS support")

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