• Peter Zijlstra's avatar
    perf: Fix race in perf_event_exec() · c1274499
    Peter Zijlstra authored
    I managed to tickle this warning:
    
      [ 2338.884942] ------------[ cut here ]------------
      [ 2338.890112] WARNING: CPU: 13 PID: 35162 at ../kernel/events/core.c:2702 task_ctx_sched_out+0x6b/0x80()
      [ 2338.900504] Modules linked in:
      [ 2338.903933] CPU: 13 PID: 35162 Comm: bash Not tainted 4.4.0-rc4-dirty #244
      [ 2338.911610] Hardware name: Intel Corporation S2600GZ/S2600GZ, BIOS SE5C600.86B.02.02.0002.122320131210 12/23/2013
      [ 2338.923071]  ffffffff81f1468e ffff8807c6457cb8 ffffffff815c680c 0000000000000000
      [ 2338.931382]  ffff8807c6457cf0 ffffffff810c8a56 ffffe8ffff8c1bd0 ffff8808132ed400
      [ 2338.939678]  0000000000000286 ffff880813170380 ffff8808132ed400 ffff8807c6457d00
      [ 2338.947987] Call Trace:
      [ 2338.950726]  [<ffffffff815c680c>] dump_stack+0x4e/0x82
      [ 2338.956474]  [<ffffffff810c8a56>] warn_slowpath_common+0x86/0xc0
      [ 2338.963195]  [<ffffffff810c8b4a>] warn_slowpath_null+0x1a/0x20
      [ 2338.969720]  [<ffffffff811a49cb>] task_ctx_sched_out+0x6b/0x80
      [ 2338.976244]  [<ffffffff811a62d2>] perf_event_exec+0xe2/0x180
      [ 2338.982575]  [<ffffffff8121fb6f>] setup_new_exec+0x6f/0x1b0
      [ 2338.988810]  [<ffffffff8126de83>] load_elf_binary+0x393/0x1660
      [ 2338.995339]  [<ffffffff811dc772>] ? get_user_pages+0x52/0x60
      [ 2339.001669]  [<ffffffff8121e297>] search_binary_handler+0x97/0x200
      [ 2339.008581]  [<ffffffff8121f8b3>] do_execveat_common.isra.33+0x543/0x6e0
      [ 2339.016072]  [<ffffffff8121fcea>] SyS_execve+0x3a/0x50
      [ 2339.021819]  [<ffffffff819fc165>] stub_execve+0x5/0x5
      [ 2339.027469]  [<ffffffff819fbeb2>] ? entry_SYSCALL_64_fastpath+0x12/0x71
      [ 2339.034860] ---[ end trace ee1337c59a0ddeac ]---
    
    Which is a WARN_ON_ONCE() indicating that cpuctx->task_ctx is not
    what we expected it to be.
    
    This is because context switches can swap the task_struct::perf_event_ctxp[]
    pointer around. Therefore you have to either disable preemption when looking
    at current, or hold ctx->lock.
    
    Fix perf_event_enable_on_exec(), it loads current->perf_event_ctxp[]
    before disabling interrupts, therefore a preemption in the right place
    can swap contexts around and we're using the wrong one.
    Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Alexander Potapenko <glider@google.com>
    Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
    Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
    Cc: Dmitry Vyukov <dvyukov@google.com>
    Cc: Eric Dumazet <edumazet@google.com>
    Cc: Jiri Olsa <jolsa@redhat.com>
    Cc: Kostya Serebryany <kcc@google.com>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Sasha Levin <sasha.levin@oracle.com>
    Cc: Stephane Eranian <eranian@google.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Vince Weaver <vincent.weaver@maine.edu>
    Cc: syzkaller <syzkaller@googlegroups.com>
    Link: http://lkml.kernel.org/r/20151210195740.GG6357@twins.programming.kicks-ass.netSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    c1274499
core.c 221 KB