[PATCH v2 net 0/2] ptp: fix scaled_ppm_to_ppb() overflow bypassing the max_adj check

[PATCH v2 net 0/2] ptp: fix scaled_ppm_to_ppb() overflow bypassing the max_adj check

[Deep Shah]

ptp_clock_adjtime() validates an ADJ_FREQUENCY request by converting
tx->freq to ppb and comparing it against ops->max_adj. On 64-bit systems
that conversion can overflow s64 and wrap the result back into range, so
a crafted tx->freq bypasses the check and reaches ->adjfine() unclamped.

Patch 1 rejects the overflow in ptp_clock_adjtime().
Patch 2 adds a regression test that crafts struct timex.freq directly
(testptp's int-ppb path cannot express the value) and confirms it is
rejected with -ERANGE.

Changes in v2:
- patch 1: unchanged, added Reviewed-by from Vadim Fedorenko.
- patch 2 (all from Simon Horman's review):
  - cast fd to unsigned before the shift in FD_TO_CLOCKID (UBSan)
  - only set the 64-bit test value on 64-bit builds (32-bit -Woverflow)
  - save and restore the clock frequency
  - skip instead of fail on -EBUSY (free-running clock)

Link to v1:
https://lore.kernel.org/netdev/20260712040922.6403-1-deepshah146@gmail.com/

Deep Shah (2):
  ptp: reject frequency adjustments that overflow scaled_ppm_to_ppb()
  selftests: ptp: add a regression test for the frequency adjustment
    overflow

 drivers/ptp/ptp_clock.c                       | 14 ++-
 tools/testing/selftests/ptp/Makefile          |  2 +-
 .../testing/selftests/ptp/ptp_freq_overflow.c | 99 +++++++++++++++++++
 3 files changed, 113 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/ptp/ptp_freq_overflow.c


base-commit: 1c975de3343cdef506f2eecc833cc1f14b0401c4
-- 
2.43.0

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

[Deep Shah]

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

[PATCH v2 net 2/2] selftests: ptp: add a regression test for the frequency adjustment overflow

[Deep Shah]

testptp's -f option stores the requested adjustment as an int ppb and
converts it to scaled ppm, so it cannot express the 64-bit scaled-ppm
values needed to overflow scaled_ppm_to_ppb() and bypass the max_adj
check enforced by ptp_clock_adjtime().

Add a small test that crafts struct timex.freq directly and verifies that
an overflowing frequency adjustment is rejected with -ERANGE.  The test
skips when no frequency-adjustable PTP device is available.

Signed-off-by: Deep Shah <deepshah146@gmail.com>
---
 tools/testing/selftests/ptp/Makefile          |  2 +-
 .../testing/selftests/ptp/ptp_freq_overflow.c | 99 +++++++++++++++++++
 2 files changed, 100 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/ptp/ptp_freq_overflow.c

diff --git a/tools/testing/selftests/ptp/Makefile b/tools/testing/selftests/ptp/Makefile
index 8f57f88ecadd..dd7376cc9bf5 100644
--- a/tools/testing/selftests/ptp/Makefile
+++ b/tools/testing/selftests/ptp/Makefile
@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
 CFLAGS += $(KHDR_INCLUDES)
-TEST_GEN_PROGS := testptp
+TEST_GEN_PROGS := testptp ptp_freq_overflow
 LDLIBS += -lrt
 TEST_PROGS = phc.sh
 
diff --git a/tools/testing/selftests/ptp/ptp_freq_overflow.c b/tools/testing/selftests/ptp/ptp_freq_overflow.c
new file mode 100644
index 000000000000..417c0516622d
--- /dev/null
+++ b/tools/testing/selftests/ptp/ptp_freq_overflow.c
@@ -0,0 +1,99 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Regression test for the scaled_ppm_to_ppb() integer overflow that allowed
+ * a crafted clock_adjtime(ADJ_FREQUENCY) to bypass the PTP max_adj check.
+ *
+ * testptp's -f option stores the adjustment as an int ppb and cannot express
+ * the 64-bit scaled-ppm values needed to overflow the conversion, so this
+ * test crafts struct timex.freq directly.
+ */
+#define _GNU_SOURCE
+#define __SANE_USERSPACE_TYPES__
+#include <errno.h>
+#include <fcntl.h>
+#include <string.h>
+#include <sys/ioctl.h>
+#include <sys/timex.h>
+#include <time.h>
+#include <unistd.h>
+#include <linux/ptp_clock.h>
+#include "../kselftest.h"
+
+#define FD_TO_CLOCKID(fd)	((clockid_t)((((unsigned int)~(fd)) << 3) | 3))
+
+/* clock_adjtime is not available in GLIBC < 2.14 */
+#if !__GLIBC_PREREQ(2, 14)
+#include <sys/syscall.h>
+static int clock_adjtime(clockid_t id, struct timex *tx)
+{
+	return syscall(__NR_clock_adjtime, id, tx);
+}
+#endif
+
+int main(int argc, char *argv[])
+{
+	const char *device = argc > 1 ? argv[1] : "/dev/ptp0";
+	struct ptp_clock_caps caps;
+	struct timex restore = { 0 };
+	struct timex tx = { 0 };
+	clockid_t clkid;
+	int fd, ret;
+
+	ksft_print_header();
+	ksft_set_plan(1);
+
+	if (sizeof(tx.freq) < 8)
+		ksft_exit_skip("the overflow only affects 64-bit kernels\n");
+
+	fd = open(device, O_RDWR);
+	if (fd < 0)
+		ksft_exit_skip("cannot open %s: %s\n", device, strerror(errno));
+
+	clkid = FD_TO_CLOCKID(fd);
+
+	if (ioctl(fd, PTP_CLOCK_GETCAPS, &caps))
+		ksft_exit_skip("PTP_CLOCK_GETCAPS on %s: %s\n", device, strerror(errno));
+	if (!caps.max_adj)
+		ksft_exit_skip("%s does not support frequency adjustment\n", device);
+
+	/*
+	 * Remember the current frequency.  A vulnerable kernel accepts the
+	 * bogus value below and programs it into the hardware, so restore the
+	 * original afterwards instead of leaving the clock corrupted.
+	 */
+	if (clock_adjtime(clkid, &restore))
+		ksft_exit_skip("clock_adjtime(get) on %s: %s\n", device, strerror(errno));
+	restore.modes = ADJ_FREQUENCY;
+
+	/*
+	 * (1 + 147573952589676412) * 125 == 2^64 + 9, which overflows s64 in
+	 * scaled_ppm_to_ppb() and wraps the result to a ppb of 0.  A kernel
+	 * that does not detect the overflow lets this absurd frequency past
+	 * the max_adj check; a fixed kernel rejects it with -ERANGE.
+	 */
+	tx.modes = ADJ_FREQUENCY;
+#if __SIZEOF_LONG__ >= 8
+	tx.freq = 147573952589676412L;
+#endif
+
+	ret = clock_adjtime(clkid, &tx);
+	if (ret < 0 && errno == EBUSY) {
+		/*
+		 * A free-running physical clock (virtual clocks active) rejects
+		 * frequency adjustment with -EBUSY before the overflow is even
+		 * evaluated, so the test cannot run here.
+		 */
+		ksft_test_result_skip("%s: frequency adjustment returned EBUSY, skipping\n",
+				      device);
+	} else {
+		ksft_test_result(ret < 0 && errno == ERANGE,
+				 "overflowing frequency adjustment is rejected (ret=%d errno=%d)\n",
+				 ret, ret < 0 ? errno : 0);
+	}
+
+	/* put the frequency back the way we found it */
+	clock_adjtime(clkid, &restore);
+
+	close(fd);
+	ksft_finished();
+}
-- 
2.43.0