[PATCH] x86, perf: check that current->mm alive before getting user callchain

[Andrey Vagin]

An event may occurs when mm is already released.
I added an event in dequeue_entity and caught panic with following backtrace:

[  434.421110] BUG: unable to handle kernel NULL pointer dereference at
0000000000000050
[  434.421258] IP: [<ffffffff810464ac>] __get_user_pages_fast+0x9c/0x120
...
[  434.421258] Call Trace:
[  434.421258]  [<ffffffff8101ae81>] copy_from_user_nmi+0x51/0xf0
[  434.421258]  [<ffffffff8109a0d5>] ? sched_clock_local+0x25/0x90
[  434.421258]  [<ffffffff8101b048>] perf_callchain_user+0x128/0x170
[  434.421258]  [<ffffffff811154cd>] ? __perf_event_header__init_id+0xed/0x100
[  434.421258]  [<ffffffff81116690>] perf_prepare_sample+0x200/0x280
[  434.421258]  [<ffffffff81118da8>] __perf_event_overflow+0x1b8/0x290
[  434.421258]  [<ffffffff81065240>] ? tg_shares_up+0x0/0x670
[  434.421258]  [<ffffffff8104fe1a>] ? walk_tg_tree+0x6a/0xb0
[  434.421258]  [<ffffffff81118f44>] perf_swevent_overflow+0xc4/0xf0
[  434.421258]  [<ffffffff81119150>] do_perf_sw_event+0x1e0/0x250
[  434.421258]  [<ffffffff81119204>] perf_tp_event+0x44/0x70
[  434.421258]  [<ffffffff8105701f>] ftrace_profile_sched_block+0xdf/0x110
[  434.421258]  [<ffffffff8106121d>] dequeue_entity+0x2ad/0x2d0
[  434.421258]  [<ffffffff810614ec>] dequeue_task_fair+0x1c/0x60
[  434.421258]  [<ffffffff8105818a>] dequeue_task+0x9a/0xb0
[  434.421258]  [<ffffffff810581e2>] deactivate_task+0x42/0xe0
[  434.421258]  [<ffffffff814bc019>] thread_return+0x191/0x808
[  434.421258]  [<ffffffff81098a44>] ? switch_task_namespaces+0x24/0x60
[  434.421258]  [<ffffffff8106f4c4>] do_exit+0x464/0x910
[  434.421258]  [<ffffffff8106f9c8>] do_group_exit+0x58/0xd0
[  434.421258]  [<ffffffff8106fa57>] sys_exit_group+0x17/0x20
[  434.421258]  [<ffffffff8100b202>] system_call_fastpath+0x16/0x1b

Signed-off-by: Andrey Vagin <avagin@xxxxxxxxxx>
---
 arch/x86/kernel/cpu/perf_event.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
index 4ee3abf..cfa62ec 100644
--- a/arch/x86/kernel/cpu/perf_event.c
+++ b/arch/x86/kernel/cpu/perf_event.c
@@ -1900,6 +1900,9 @@ perf_callchain_user(struct perf_callchain_entry *entry, struct pt_regs *regs)
 
 	perf_callchain_store(entry, regs->ip);
 
+	if (!current->mm)
+		return;
+
 	if (perf_callchain_user32(regs, entry))
 		return;
 
-- 
1.7.3.4

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