Messages in this thread Patch in this message | | | Date | Thu, 05 Sep 2019 14:03:43 +0200 | From | Thomas Gleixner <> | Subject | [patch 4/6] posix-cpu-timers: Restrict clock_gettime() permissions |
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Similar to creating timers on a process there is no restriction at all to read the Posix CPU clocks of any process in the system. Per thread CPU clock access is limited to threads in the same thread group.
The per process CPU clocks can be used to observe activity of tasks and reading them can affect the execution of the process to which they are attached as reading can require to lock sighand lock and sum up the fine grained accounting for all threads in the process.
Restrict it by checking ptrace MODE_READ permissions of the reader on the target process.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de> --- kernel/time/posix-cpu-timers.c | 17 ++++++++++++----- 1 file changed, 12 insertions(+), 5 deletions(-)
--- a/kernel/time/posix-cpu-timers.c +++ b/kernel/time/posix-cpu-timers.c @@ -71,9 +71,18 @@ static struct task_struct *lookup_task(c /* * For clock_gettime() the task does not need to be the * actual group leader. tsk->sighand gives access to the - * group's clock. + * group's clock. current can obviously access itself, so + * spare the ptrace check below. */ - return (p == current || thread_group_leader(p)) ? p : NULL; + if (p == current) + return p; + + if (!thread_group_leader(p)) + return NULL; + + if (!ptrace_may_access(p, PTRACE_MODE_READ_REALCREDS)) + return NULL; + return p; } /* @@ -92,9 +101,7 @@ static struct task_struct *lookup_task(c * Creating timers on processes which cannot be ptraced is not * permitted: */ - if (!ptrace_may_access(p, PTRACE_MODE_ATTACH_REALCREDS)) - return NULL; - return p; + return ptrace_may_access(p, PTRACE_MODE_ATTACH_REALCREDS) ? p : NULL; } static struct task_struct *__get_task_for_clock(const clockid_t clock,
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