Many devices implement highly accurate clocks, which the kernel manages
as PTP Hardware Clocks (PHCs). Userspace applications rely on these
clocks to timestamp events, trace workload execution, correlate
timescales across devices, and keep various clocks in sync.

The kernel’s current implementation of PTP clocks does not enforce file
permissions checks for most device operations except for POSIX clock
operations, where file mode is verified in the POSIX layer before
forwarding the call to the PTP subsystem. Consequently, it is common
practice to not give unprivileged userspace applications any access to
PTP clocks whatsoever by giving the PTP chardevs 600 permissions. An
example of users running into this limitation is documented in [1].

Add permission checks for functions that modify the state of a PTP
device. Continue enforcing permission checks for POSIX clock operations
(settime, adjtime) in the POSIX layer. One limitation remains: querying
the adjusted frequency of a PTP device (using adjtime() with an empty
modes field) is not supported for chardevs opened without WRITE
permissions, as the POSIX layer mandates WRITE access for any adjtime
operation.

[1] https://lists.nwtime.org/sympa/arc/linuxptp-users/2024-01/msg00036.html

Signed-off-by: Wojtek Wasko <redacted>
---
 drivers/ptp/ptp_chardev.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/drivers/ptp/ptp_chardev.c b/drivers/ptp/ptp_chardev.c
index bf6468c56419..4380e6ddb849 100644
--- a/drivers/ptp/ptp_chardev.c
+++ b/drivers/ptp/ptp_chardev.c

@@ -205,6 +205,10 @@ long ptp_ioctl(struct posix_clock_context *pccontext, unsigned int cmd,
 
 	case PTP_EXTTS_REQUEST:
 	case PTP_EXTTS_REQUEST2:
+		if ((pccontext->fp->f_mode & FMODE_WRITE) == 0) {
+			err = -EACCES;
+			break;
+		}
 		memset(&req, 0, sizeof(req));
 
 		if (copy_from_user(&req.extts, (void __user *)arg,

@@ -246,6 +250,10 @@ long ptp_ioctl(struct posix_clock_context *pccontext, unsigned int cmd,
 
 	case PTP_PEROUT_REQUEST:
 	case PTP_PEROUT_REQUEST2:
+		if ((pccontext->fp->f_mode & FMODE_WRITE) == 0) {
+			err = -EACCES;
+			break;
+		}
 		memset(&req, 0, sizeof(req));
 
 		if (copy_from_user(&req.perout, (void __user *)arg,

@@ -314,6 +322,10 @@ long ptp_ioctl(struct posix_clock_context *pccontext, unsigned int cmd,
 
 	case PTP_ENABLE_PPS:
 	case PTP_ENABLE_PPS2:
+		if ((pccontext->fp->f_mode & FMODE_WRITE) == 0) {
+			err = -EACCES;
+			break;
+		}
 		memset(&req, 0, sizeof(req));
 
 		if (!capable(CAP_SYS_TIME))

@@ -456,6 +468,10 @@ long ptp_ioctl(struct posix_clock_context *pccontext, unsigned int cmd,
 
 	case PTP_PIN_SETFUNC:
 	case PTP_PIN_SETFUNC2:
+		if ((pccontext->fp->f_mode & FMODE_WRITE) == 0) {
+			err = -EACCES;
+			break;
+		}
 		if (copy_from_user(&pd, (void __user *)arg, sizeof(pd))) {
 			err = -EFAULT;
 			break;

-- 
2.39.3