Re: [PATCH] Uprobes: Fix deadlock between delayed_uprobe_lock and fs_reclaim

Oleg Nesterov
2019-02-06T13:36:08Z

Ravi, I am on vacation till the end of this week, can't read your patch
carefully.

I am not sure I fully understand the problem, but shouldn't we change
binder_alloc_free_page() to use mmput_async() ? Like it does if trylock
fails.

In any case, I don't think memalloc_nofs_save() is what we need, see below.

On 02/04, Ravi Bangoria wrote:
>
> There can be a deadlock between delayed_uprobe_lock and
> fs_reclaim like:
>
> CPU0 CPU1
> ---- ----
> lock(fs_reclaim);
> lock(delayed_uprobe_lock);
> lock(fs_reclaim);
> lock(delayed_uprobe_lock);
>
> Here CPU0 is a file system code path which results in
> mmput()->__mmput()->uprobe_clear_state() with fs_reclaim
> locked. And, CPU1 is a uprobe event creation path.

But this is false positive, right? if CPU1 calls update_ref_ctr() then
either ->mm_users is already zero so binder_alloc_free_page()->mmget_not_zero()
will fail, or the caller of update_ref_ctr() has a reference and thus
binder_alloc_free_page()->mmput() can't trigger __mmput() ?

>
> This was reported by syzbot at [1]. Though, the reproducer
> is nither available by syzbot nor I can reproduce it locally.
>
> Callchains from syzbot report:
>
> -> #1 (fs_reclaim){+.+.}:
> __fs_reclaim_acquire mm/page_alloc.c:3730 [inline]
> fs_reclaim_acquire.part.97+0x24/0x30 mm/page_alloc.c:3741
> fs_reclaim_acquire+0x14/0x20 mm/page_alloc.c:3742
> slab_pre_alloc_hook mm/slab.h:418 [inline]
> slab_alloc mm/slab.c:3378 [inline]
> kmem_cache_alloc_trace+0x2d/0x750 mm/slab.c:3618
> kmalloc include/linux/slab.h:546 [inline]
> kzalloc include/linux/slab.h:741 [inline]
> delayed_uprobe_add kernel/events/uprobes.c:313 [inline]
> update_ref_ctr+0x36f/0x590 kernel/events/uprobes.c:447
> uprobe_write_opcode+0x94b/0xc50 kernel/events/uprobes.c:496
> set_swbp+0x2a/0x40
> install_breakpoint.isra.24+0x161/0x840 kernel/events/uprobes.c:885
> register_for_each_vma+0xa38/0xee0 kernel/events/uprobes.c:1041
> uprobe_apply+0xee/0x140 kernel/events/uprobes.c:1192
> uprobe_perf_open kernel/trace/trace_uprobe.c:1087 [inline]
> trace_uprobe_register+0x771/0xcf0 kernel/trace/trace_uprobe.c:1227
> perf_trace_event_open kernel/trace/trace_event_perf.c:181 [inline]
> perf_trace_event_init+0x1a5/0x990 kernel/trace/trace_event_perf.c:203
> perf_uprobe_init+0x1f1/0x280 kernel/trace/trace_event_perf.c:329
> perf_uprobe_event_init+0x106/0x1a0 kernel/events/core.c:8503
> perf_try_init_event+0x137/0x2f0 kernel/events/core.c:9770
> perf_init_event kernel/events/core.c:9801 [inline]
> perf_event_alloc.part.94+0x1d54/0x3740 kernel/events/core.c:10074
> perf_event_alloc kernel/events/core.c:10430 [inline]
> __do_sys_perf_event_open+0xada/0x3020 kernel/events/core.c:10531
> __se_sys_perf_event_open kernel/events/core.c:10420 [inline]
> __x64_sys_perf_event_open+0xbe/0x150 kernel/events/core.c:10420
> do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> -> #0 (delayed_uprobe_lock){+.+.}:
> lock_acquire+0x1ed/0x520 kernel/locking/lockdep.c:3844
> __mutex_lock_common kernel/locking/mutex.c:925 [inline]
> __mutex_lock+0x166/0x16f0 kernel/locking/mutex.c:1072
> mutex_lock_nested+0x16/0x20 kernel/locking/mutex.c:1087
> uprobe_clear_state+0xb4/0x390 kernel/events/uprobes.c:1511
> __mmput kernel/fork.c:1041 [inline]
> mmput+0x1bc/0x610 kernel/fork.c:1066
> binder_alloc_free_page+0x5ab/0x1520 drivers/android/binder_alloc.c:983
> __list_lru_walk_one+0x29d/0x8c0 mm/list_lru.c:234
> list_lru_walk_one+0xa5/0xe0 mm/list_lru.c:278
> list_lru_walk_node+0x43/0x280 mm/list_lru.c:307
> list_lru_walk include/linux/list_lru.h:214 [inline]
> binder_shrink_scan+0x164/0x220 drivers/android/binder_alloc.c:1019
> do_shrink_slab+0x501/0xd30 mm/vmscan.c:557
> shrink_slab+0x389/0x8c0 mm/vmscan.c:706
> shrink_node+0x431/0x16b0 mm/vmscan.c:2758
> shrink_zones mm/vmscan.c:2987 [inline]
> do_try_to_free_pages+0x3e7/0x1290 mm/vmscan.c:3049
> try_to_free_pages+0x4d0/0xb90 mm/vmscan.c:3264
> __perform_reclaim mm/page_alloc.c:3773 [inline]
> __alloc_pages_direct_reclaim mm/page_alloc.c:3795 [inline]
> __alloc_pages_slowpath+0xa48/0x2de0 mm/page_alloc.c:4185
> __alloc_pages_nodemask+0xad8/0xea0 mm/page_alloc.c:4393
> __alloc_pages include/linux/gfp.h:473 [inline]
> __alloc_pages_node include/linux/gfp.h:486 [inline]
> khugepaged_alloc_page+0x95/0x190 mm/khugepaged.c:773
> collapse_huge_page mm/khugepaged.c:963 [inline]
> khugepaged_scan_pmd+0x1715/0x3d60 mm/khugepaged.c:1216
> khugepaged_scan_mm_slot mm/khugepaged.c:1725 [inline]
> khugepaged_do_scan mm/khugepaged.c:1806 [inline]
> khugepaged+0xf20/0x1750 mm/khugepaged.c:1851
> kthread+0x35a/0x440 kernel/kthread.c:246
> ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:352
>
> [1] https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1876397.html
>
> Reported-by: syzbot+1068f09c44d151250c33@syzkaller.appspotmail.com
> Suggested-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
> ---
> kernel/events/uprobes.c | 24 ++++++++++++++++++++++++
> 1 file changed, 24 insertions(+)
>
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 8aef47ee7bfa..8be39a83d83a 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -95,6 +95,11 @@ struct delayed_uprobe {
> struct mm_struct *mm;
> };
>
> +/*
> + * Any memory allocation happening within lock(delayed_uprobe_lock)
> + * must use memalloc_nofs_save()/memalloc_nofs_restore() to avoid
> + * calling file system code from memory allocation code.
> + */
> static DEFINE_MUTEX(delayed_uprobe_lock);
> static LIST_HEAD(delayed_uprobe_list);
>
> @@ -429,6 +434,7 @@ static int update_ref_ctr(struct uprobe *uprobe, struct mm_struct *mm,
> struct vm_area_struct *rc_vma;
> unsigned long rc_vaddr;
> int ret = 0;
> + unsigned int nofs_flags;
>
> rc_vma = find_ref_ctr_vma(uprobe, mm);
>
> @@ -442,12 +448,30 @@ static int update_ref_ctr(struct uprobe *uprobe, struct mm_struct *mm,
> return ret;
> }
>
> + /*
> + * There is a possibility of deadlock here:
> + *
> + * CPU0 CPU1
> + * ---- ----
> + * lock(fs_reclaim);
> + * lock(delayed_uprobe_lock);
> + * lock(fs_reclaim);
> + * lock(delayed_uprobe_lock);
> + *
> + * Here CPU0 is a file system code path which results in
> + * mmput()->__mmput()->uprobe_clear_state() with fs_reclaim locked.
> + * And, CPU1 is a uprobe event creation path.
> + *
> + * Avoid calling into filesystem code inside lock(delayed_uprobe_lock).
> + */
> + nofs_flags = memalloc_nofs_save();
> mutex_lock(&delayed_uprobe_lock);
> if (d > 0)
> ret = delayed_uprobe_add(uprobe, mm);
> else
> delayed_uprobe_remove(uprobe, mm);
> mutex_unlock(&delayed_uprobe_lock);
> + memalloc_nofs_restore(nofs_flags);

PF_MEMALLOC_NOFS is only needed when we are going to call delayed_uprobe_add()
which does kzalloc(GFP_KERNEL). Can't we simply change it tuse use use GFP_NOFS
instead?

Oleg.