From 31d30f41bfc7d054bf78b18acff4aedd4e04662b Mon Sep 17 00:00:00 2001 From: Jean Pihet Date: Mon, 7 Jul 2014 14:45:09 +0100 Subject: [PATCH 1/1] ARM: perf: disable the pagefault handler when reading from user space Under perf, the fp unwinding scheme requires access to user space memory and can provoke a pagefault via call to __copy_from_user_inatomic from user_backtrace. This unwinding can take place in response to an interrupt (__perf_event_overflow). This is undesirable as we may already have mmap_sem held for write. One example being a process that calls mprotect just as a the PMU counters overflow. An example that can provoke this behaviour: perf record -e event:tocapture --call-graph fp ./application_to_test This patch addresses this issue by disabling pagefaults briefly in user_backtrace (as is done in the other architectures: ARM64, x86, Sparc etc.). Without the patch a deadlock occurs when __perf_event_overflow is called while reading the data from the user space: [ INFO: possible recursive locking detected ] 3.16.0-rc2-00038-g0ed7ff6 #46 Not tainted --------------------------------------------- stress/1634 is trying to acquire lock: (&mm->mmap_sem){++++++}, at: [] do_page_fault+0xa8/0x428 but task is already holding lock: (&mm->mmap_sem){++++++}, at: [] SyS_mprotect+0xa8/0x1c8 other info that might help us debug this: Possible unsafe locking scenario: CPU0 ---- lock(&mm->mmap_sem); lock(&mm->mmap_sem); *** DEADLOCK *** May be due to missing lock nesting notation 2 locks held by stress/1634: #0: (&mm->mmap_sem){++++++}, at: [] SyS_mprotect+0xa8/0x1c8 #1: (rcu_read_lock){......}, at: [] __perf_event_overflow+0x120/0x294 stack backtrace: CPU: 1 PID: 1634 Comm: stress Not tainted 3.16.0-rc2-00038-g0ed7ff6 #46 [] (unwind_backtrace) from [] (show_stack+0x20/0x24) [] (show_stack) from [] (dump_stack+0x7c/0x98) [] (dump_stack) from [] (__lock_acquire+0x1484/0x1cf0) [] (__lock_acquire) from [] (lock_acquire+0xa4/0x11c) [] (lock_acquire) from [] (down_read+0x40/0x7c) [] (down_read) from [] (do_page_fault+0xa8/0x428) [] (do_page_fault) from [] (do_DataAbort+0x44/0xa8) [] (do_DataAbort) from [] (__dabt_svc+0x3c/0x60) Exception stack(0xed7c5ae0 to 0xed7c5b28) 5ae0: ed7c5b5c b6dadff4 ffffffec 00000000 b6dadff4 ebc08000 00000000 ebc08000 5b00: 0000007e 00000000 ed7c4000 ed7c5b94 00000014 ed7c5b2c c001a438 c0236c60 5b20: 00000013 ffffffff [] (__dabt_svc) from [] (__copy_from_user+0xa4/0x3a4) Acked-by: Steve Capper Signed-off-by: Jean Pihet Signed-off-by: Will Deacon --- arch/arm/kernel/perf_event.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/arch/arm/kernel/perf_event.c b/arch/arm/kernel/perf_event.c index c024d37ab7c7..3d865f3f6363 100644 --- a/arch/arm/kernel/perf_event.c +++ b/arch/arm/kernel/perf_event.c @@ -778,11 +778,16 @@ user_backtrace(struct frame_tail __user *tail, struct perf_callchain_entry *entry) { struct frame_tail buftail; + unsigned long err; - /* Also check accessibility of one struct frame_tail beyond */ if (!access_ok(VERIFY_READ, tail, sizeof(buftail))) return NULL; - if (__copy_from_user_inatomic(&buftail, tail, sizeof(buftail))) + + pagefault_disable(); + err = __copy_from_user_inatomic(&buftail, tail, sizeof(buftail)); + pagefault_enable(); + + if (err) return NULL; perf_callchain_store(entry, buftail.lr); -- 2.39.2