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Ingo Molnar authored
commit b2f73922 upstream. So the /proc/PID/stat 'wchan' field (the 30th field, which contains the absolute kernel address of the kernel function a task is blocked in) leaks absolute kernel addresses to unprivileged user-space: seq_put_decimal_ull(m, ' ', wchan); The absolute address might also leak via /proc/PID/wchan as well, if KALLSYMS is turned off or if the symbol lookup fails for some reason: static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns, struct pid *pid, struct task_struct *task) { unsigned long wchan; char symname[KSYM_NAME_LEN]; wchan = get_wchan(task); if (lookup_symbol_name(wchan, symname) < 0) { if (!ptrace_may_access(task, PTRACE_MODE_READ)) return 0; seq_printf(m, "%lu", wchan); } else { seq_printf(m, "%s", symname); } return 0; } This isn't ideal, because for example it trivially leaks the KASLR offset to any local attacker: fomalhaut:~> printf "%016lx\n" $(cat /proc/$$/stat | cut -d' ' -f35) ffffffff8123b380 Most real-life uses of wchan are symbolic: ps -eo pid:10,tid:10,wchan:30,comm and procps uses /proc/PID/wchan, not the absolute address in /proc/PID/stat: triton:~/tip> strace -f ps -eo pid:10,tid:10,wchan:30,comm 2>&1 | grep wchan | tail -1 open("/proc/30833/wchan", O_RDONLY) = 6 There's one compatibility quirk here: procps relies on whether the absolute value is non-zero - and we can provide that functionality by outputing "0" or "1" depending on whether the task is blocked (whether there's a wchan address). These days there appears to be very little legitimate reason user-space would be interested in the absolute address. The absolute address is mostly historic: from the days when we didn't have kallsyms and user-space procps had to do the decoding itself via the System.map. So this patch sets all numeric output to "0" or "1" and keeps only symbolic output, in /proc/PID/wchan. ( The absolute sleep address can generally still be profiled via perf, by tasks with sufficient privileges. ) Reviewed-by:
Thomas Gleixner <tglx@linutronix.de> Acked-by:
Kees Cook <keescook@chromium.org> Acked-by:
Linus Torvalds <torvalds@linux-foundation.org> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: Alexander Potapenko <glider@google.com> Cc: Andrey Konovalov <andreyknvl@google.com> Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com> Cc: Andy Lutomirski <luto@amacapital.net> Cc: Andy Lutomirski <luto@kernel.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Denys Vlasenko <dvlasenk@redhat.com> Cc: Dmitry Vyukov <dvyukov@google.com> Cc: Kostya Serebryany <kcc@google.com> Cc: Mike Galbraith <efault@gmx.de> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Sasha Levin <sasha.levin@oracle.com> Cc: kasan-dev <kasan-dev@googlegroups.com> Cc: linux-kernel@vger.kernel.org Link: http://lkml.kernel.org/r/20150930135917.GA3285@gmail.comSigned-off-by:
Ingo Molnar <mingo@kernel.org> [ kamal: backport to 3.16-stable: proc_pid_wchan context ] Signed-off-by:
Kamal Mostafa <kamal@canonical.com> Signed-off-by:
Luis Henriques <luis.henriques@canonical.com>
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