[PATCH v2 net 1/2] ptp: reject frequency adjustments that overflow scaled_ppm_to_ppb()

[Deep Shah <deepshah146@gmail.com>]

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.

Fixes: d39a743511cd ("ptp: validate the requested frequency adjustment.")
Signed-off-by: Deep Shah <deepshah146@gmail.com>
Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
---
 drivers/ptp/ptp_clock.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/drivers/ptp/ptp_clock.c b/drivers/ptp/ptp_clock.c
index d6f54ccaf93b..4111342d64f0 100644
--- a/drivers/ptp/ptp_clock.c
+++ b/drivers/ptp/ptp_clock.c
@@ -9,6 +9,7 @@
 #include <linux/init.h>
 #include <linux/kernel.h>
 #include <linux/module.h>
+#include <linux/overflow.h>
 #include <linux/posix-clock.h>
 #include <linux/pps_kernel.h>
 #include <linux/property.h>
@@ -159,7 +160,18 @@ static int ptp_clock_adjtime(struct posix_clock *pc, struct __kernel_timex *tx)
 		delta = ktime_to_ns(kt);
 		err = ops->adjtime(ops, delta);
 	} else if (tx->modes & ADJ_FREQUENCY) {
-		long ppb = scaled_ppm_to_ppb(tx->freq);
+		long ppb;
+		s64 tmp;
+
+		/*
+		 * scaled_ppm_to_ppb() multiplies (1 + freq) by 125 in s64;
+		 * reject a ->freq large enough to overflow that, which would
+		 * otherwise wrap the result back into the max_adj range.
+		 */
+		if (check_add_overflow((s64)tx->freq, (s64)1, &tmp) ||
+		    check_mul_overflow(tmp, (s64)125, &tmp))
+			return -ERANGE;
+		ppb = scaled_ppm_to_ppb(tx->freq);
 		if (ppb > ops->max_adj || ppb < -ops->max_adj)
 			return -ERANGE;
 		err = ops->adjfine(ops, tx->freq);
-- 
2.43.0