From fb822e6076d972691c5dd33431c10f82dda94ae9 Mon Sep 17 00:00:00 2001 From: Ravi Bangoria Date: Wed, 2 Mar 2016 15:25:17 +0530 Subject: [PATCH] powerpc/hw_breakpoint: Fix oops when destroying hw_breakpoint event When destroying a hw_breakpoint event, the kernel oopses as follows: Unable to handle kernel paging request for data at address 0x00000c07 NIP [c0000000000291d0] arch_unregister_hw_breakpoint+0x40/0x60 LR [c00000000020b6b4] release_bp_slot+0x44/0x80 Call chain: hw_breakpoint_event_init() bp->destroy = bp_perf_event_destroy; do_exit() perf_event_exit_task() perf_event_exit_task_context() WRITE_ONCE(child_ctx->task, TASK_TOMBSTONE); perf_event_exit_event() free_event() _free_event() bp_perf_event_destroy() // event->destroy(event); release_bp_slot() arch_unregister_hw_breakpoint() perf_event_exit_task_context() sets child_ctx->task as TASK_TOMBSTONE which is (void *)-1. arch_unregister_hw_breakpoint() tries to fetch 'thread' attribute of 'task' resulting in oops. Peterz points out that the code shouldn't be using bp->ctx anyway, but fixing that will require a decent amount of rework. So for now to fix the oops, check if bp->ctx->task has been set to (void *)-1, before dereferencing it. We don't use TASK_TOMBSTONE, because that would require exporting it and it's supposed to be an internal detail. Fixes: 63b6da39bb38 ("perf: Fix perf_event_exit_task() race") Signed-off-by: Ravi Bangoria Acked-by: Peter Zijlstra (Intel) Signed-off-by: Michael Ellerman --- arch/powerpc/kernel/hw_breakpoint.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/arch/powerpc/kernel/hw_breakpoint.c b/arch/powerpc/kernel/hw_breakpoint.c index 05e804cdecaa..aec9a1b1d25b 100644 --- a/arch/powerpc/kernel/hw_breakpoint.c +++ b/arch/powerpc/kernel/hw_breakpoint.c @@ -109,8 +109,9 @@ void arch_unregister_hw_breakpoint(struct perf_event *bp) * If the breakpoint is unregistered between a hw_breakpoint_handler() * and the single_step_dabr_instruction(), then cleanup the breakpoint * restoration variables to prevent dangling pointers. + * FIXME, this should not be using bp->ctx at all! Sayeth peterz. */ - if (bp->ctx && bp->ctx->task) + if (bp->ctx && bp->ctx->task && bp->ctx->task != ((void *)-1L)) bp->ctx->task->thread.last_hit_ubp = NULL; } -- 2.20.1