Re: [PATCH net-next v3 1/2] ptp: reject frequency adjustments that overflow scaled_ppm_to_ppb()

On Sat, Aug 01, 2026 at 10:29:22PM +0000, Deep Shah wrote:

ptp_clock_adjtime() validates an ADJ_FREQUENCY request by converting the
requested scaled ppm to ppb and comparing it against ops->max_adj:

	long ppb = scaled_ppm_to_ppb(tx->freq);
	if (ppb > ops->max_adj || ppb < -ops->max_adj)
		return -ERANGE;

scaled_ppm_to_ppb() computes (1 + ppm) * 125 >> 13 in s64.  For a
sufficiently large tx->freq the multiplication overflows s64 and wraps,
so the resulting ppb can fall back within [-max_adj, max_adj] and pass
the check.  The unclamped tx->freq is then handed to ->adjfine(), where
drivers scale it again (e.g. scaled_ppm * 762939453125 in ptp_idt82p33)
and program a bogus frequency word.

For example tx->freq = 147573952589676412 makes (1 + ppm) * 125 equal
2^64 + 9, which wraps to ppb == 0 and is accepted.

The caller already has write access to the PHC, so this hardens the
max_adj sanity check rather than crossing a privilege boundary, and
well-behaved user space (e.g. ptp4l) never requests such values.  It is
a follow-up to commit 475b92f93216 ("ptp: improve max_adj check against
unreasonable values"), which handled the analogous s32 narrowing but not
this multiplication overflow.

Detect the overflow with check_*_overflow() and reject the request in
ptp_clock_adjtime() instead of acting on the wrapped value.

Signed-off-by: Deep Shah <redacted>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>

Acked-by: Richard Cochran <richardcochran@gmail.com>