ntp: fix non privileged system time shifting

The ADJ_SETOFFSET bit added in commit 094aa188 ("ntp: Add ADJ_SETOFFSET
mode bit") also introduced a way for any user to change the system time.
Sneaky or buggy calls to adjtimex() could set

    ADJ_OFFSET_SS_READ | ADJ_SETOFFSET

which would result in a successful call to timekeeping_inject_offset().
This patch fixes the issue by adding the capability check.

Signed-off-by: Richard Cochran <richard.cochran@omicron.at>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 5f1bb8e2008f..f6117a4c7cb8 100644
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -652,6 +652,8 @@ int do_adjtimex(struct timex *txc)
                 struct timespec delta;
                 delta.tv_sec  = txc->time.tv_sec;
                 delta.tv_nsec = txc->time.tv_usec;
+                if (!capable(CAP_SYS_TIME))
+                        return -EPERM;
                 if (!(txc->modes & ADJ_NANO))
                         delta.tv_nsec *= 1000;
                 result = timekeeping_inject_offset(&delta);