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powerpc/mm_iommu: Fix potential deadlock
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Currently mm_iommu_do_alloc() is called in 2 cases:
- VFIO_IOMMU_SPAPR_REGISTER_MEMORY ioctl() for normal memory;
- vfio_pci_nvgpu_regops::mmap() for GPU memory.

One of the differences here is that the mmap() is called with mm::mmap_sem
help and mm_iommu_do_alloc() locks mm::mmap_sem itself (when adjusting
locked_vm and when pinning pages) which can potentially cause a deadlock.

We did not hit this yet because the mmap() path does not adjust
locked_vm and does not pin pages. However with CONFIG_DEBUG_LOCKDEP=y
there is an annoying warning (below, it is slightly confusing).

This makes a few changes to reduce the amount of time spent under a lock.

This holds mem_list_mutex only when looking or changing the mem list.
This means the list is checked twice now for the normal memory case -
before starting pinning and before adding the item to the list.

This changes mm_iommu_do_alloc() to only allocate and add an iommu memory
descriptor (used to deal with both normal and GPU memory in a rather
messy way).

This cleans the code in a way that mm_iommu_new() and mm_iommu_do_alloc()
do not need to test for (dev_hpa != MM_IOMMU_TABLE_INVALID_HPA) which
makes the code simpler.

This moves locked_vm decrementing from under mem_list_mutex for the same
reasons.

This is one of the lockdep warnings:
======================================================
WARNING: possible circular locking dependency detected
5.1.0-rc2-le_nv2_aikATfstn1-p1 torvalds#363 Not tainted
------------------------------------------------------
qemu-system-ppc/8038 is trying to acquire lock:
000000002ec6c453 (mem_list_mutex){+.+.}, at: mm_iommu_do_alloc+0x70/0x490

but task is already holding lock:
00000000fd7da97f (&mm->mmap_sem){++++}, at: vm_mmap_pgoff+0xf0/0x160

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (&mm->mmap_sem){++++}:
       lock_acquire+0xf8/0x260
       down_write+0x44/0xa0
       mm_iommu_adjust_locked_vm.part.1+0x4c/0x190
       mm_iommu_do_alloc+0x310/0x490
       tce_iommu_ioctl.part.9+0xb84/0x1150 [vfio_iommu_spapr_tce]
       vfio_fops_unl_ioctl+0x94/0x430 [vfio]
       do_vfs_ioctl+0xe4/0x930
       ksys_ioctl+0xc4/0x110
       sys_ioctl+0x28/0x80
       system_call+0x5c/0x70

-> #0 (mem_list_mutex){+.+.}:
       __lock_acquire+0x1484/0x1900
       lock_acquire+0xf8/0x260
       __mutex_lock+0x88/0xa70
       mm_iommu_do_alloc+0x70/0x490
       vfio_pci_nvgpu_mmap+0xc0/0x130 [vfio_pci]
       vfio_pci_mmap+0x198/0x2a0 [vfio_pci]
       vfio_device_fops_mmap+0x44/0x70 [vfio]
       mmap_region+0x5d4/0x770
       do_mmap+0x42c/0x650
       vm_mmap_pgoff+0x124/0x160
       ksys_mmap_pgoff+0xdc/0x2f0
       sys_mmap+0x40/0x80
       system_call+0x5c/0x70

other info that might help us debug this:

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&mm->mmap_sem);
                               lock(mem_list_mutex);
                               lock(&mm->mmap_sem);
  lock(mem_list_mutex);

 *** DEADLOCK ***

1 lock held by qemu-system-ppc/8038:
 #0: 00000000fd7da97f (&mm->mmap_sem){++++}, at: vm_mmap_pgoff+0xf0/0x160

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
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@aik @intel-lab-lkp
aik authored and intel-lab-lkp committed Mar 29, 2019
1 parent a31753f commit 4852b5e
Showing 1 changed file with 64 additions and 62 deletions.
126 changes: 64 additions & 62 deletions arch/powerpc/mm/mmu_context_iommu.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -96,50 +96,59 @@ static bool mm_iommu_find(struct mm_struct *mm, unsigned long ua,
return false;
}

static long mm_iommu_do_alloc(struct mm_struct *mm, unsigned long ua,
unsigned long entries, unsigned long dev_hpa,
struct mm_iommu_table_group_mem_t **pmem)
/* Must be called with &mem_list_mutex held */
static int mm_iommu_do_alloc(struct mm_struct *mm, unsigned long ua,
unsigned long entries, unsigned long dev_hpa,
unsigned int mem_pageshift, phys_addr_t *hpas,
struct mm_iommu_table_group_mem_t **pmem)
{
struct mm_iommu_table_group_mem_t *mem;
long i, ret, locked_entries = 0;

if (mm_iommu_find(mm, ua, entries))
return -EINVAL;

mem = kzalloc(sizeof(*mem), GFP_KERNEL);
if (!mem)
return -ENOMEM;

atomic64_set(&mem->mapped, 1);
mem->dev_hpa = dev_hpa;
mem->used = 1;
mem->ua = ua;
mem->entries = entries;
mem->pageshift = mem_pageshift;
mem->hpas = hpas;
list_add_rcu(&mem->next, &mm->context.iommu_group_mem_list);
*pmem = mem;

return 0;
}

long mm_iommu_new(struct mm_struct *mm, unsigned long ua, unsigned long entries,
struct mm_iommu_table_group_mem_t **pmem)
{
long i, ret = 0, locked_entries = 0;
unsigned int pageshift, mem_pageshift;
struct page **hpages;
phys_addr_t *hpas;

mutex_lock(&mem_list_mutex);

if (mm_iommu_find(mm, ua, entries)) {
ret = -EINVAL;
goto unlock_exit;
}

if (dev_hpa == MM_IOMMU_TABLE_INVALID_HPA) {
ret = mm_iommu_adjust_locked_vm(mm, entries, true);
if (ret)
goto unlock_exit;

locked_entries = entries;
mutex_unlock(&mem_list_mutex);
return -EINVAL;
}
mutex_unlock(&mem_list_mutex);

mem = kzalloc(sizeof(*mem), GFP_KERNEL);
if (!mem) {
ret = -ENOMEM;
goto unlock_exit;
}
ret = mm_iommu_adjust_locked_vm(mm, entries, true);
if (ret)
return ret;

if (dev_hpa != MM_IOMMU_TABLE_INVALID_HPA) {
mem_pageshift = __ffs(dev_hpa | (entries << PAGE_SHIFT));
hpas = NULL;
mem->dev_hpa = dev_hpa;
goto good_exit;
}
mem->dev_hpa = MM_IOMMU_TABLE_INVALID_HPA;
locked_entries = entries;

hpages = vzalloc(array_size(entries, sizeof(hpages[0])));
if (!hpages) {
kfree(mem);
ret = -ENOMEM;
goto unlock_exit;
goto cleanup_exit;
}

down_read(&mm->mmap_sem);
Expand All @@ -149,11 +158,8 @@ static long mm_iommu_do_alloc(struct mm_struct *mm, unsigned long ua,
/* free the reference taken */
for (i = 0; i < ret; i++)
put_page(hpages[i]);

vfree(hpages);
kfree(mem);
ret = -EFAULT;
goto unlock_exit;
goto cleanup_exit;
}

/*
Expand Down Expand Up @@ -184,40 +190,35 @@ static long mm_iommu_do_alloc(struct mm_struct *mm, unsigned long ua,
hpas[i] = page_to_pfn(page) << PAGE_SHIFT;
}

good_exit:
ret = 0;
atomic64_set(&mem->mapped, 1);
mem->used = 1;
mem->ua = ua;
mem->entries = entries;
mem->hpas = hpas;
mem->pageshift = mem_pageshift;
*pmem = mem;

list_add_rcu(&mem->next, &mm->context.iommu_group_mem_list);
mutex_lock(&mem_list_mutex);
ret = mm_iommu_do_alloc(mm, ua, entries, MM_IOMMU_TABLE_INVALID_HPA,
mem_pageshift, hpas, pmem);
mutex_unlock(&mem_list_mutex);

unlock_exit:
if (locked_entries && ret)
mm_iommu_adjust_locked_vm(mm, locked_entries, false);
if (ret)
goto cleanup_exit;
return 0;

mutex_unlock(&mem_list_mutex);
cleanup_exit:
mm_iommu_adjust_locked_vm(mm, locked_entries, false);
vfree(hpages);

return ret;
}

long mm_iommu_new(struct mm_struct *mm, unsigned long ua, unsigned long entries,
struct mm_iommu_table_group_mem_t **pmem)
{
return mm_iommu_do_alloc(mm, ua, entries, MM_IOMMU_TABLE_INVALID_HPA,
pmem);
}
EXPORT_SYMBOL_GPL(mm_iommu_new);

long mm_iommu_newdev(struct mm_struct *mm, unsigned long ua,
unsigned long entries, unsigned long dev_hpa,
struct mm_iommu_table_group_mem_t **pmem)
{
return mm_iommu_do_alloc(mm, ua, entries, dev_hpa, pmem);
int ret;

mutex_lock(&mem_list_mutex);
ret = mm_iommu_do_alloc(mm, ua, entries, dev_hpa,
__ffs(dev_hpa | (entries << PAGE_SHIFT)), NULL, pmem);
mutex_unlock(&mem_list_mutex);

return ret;
}
EXPORT_SYMBOL_GPL(mm_iommu_newdev);

Expand Down Expand Up @@ -270,10 +271,13 @@ static void mm_iommu_release(struct mm_iommu_table_group_mem_t *mem)
long mm_iommu_put(struct mm_struct *mm, struct mm_iommu_table_group_mem_t *mem)
{
long ret = 0;
unsigned long entries, dev_hpa;
unsigned long unlock_entries = 0;

mutex_lock(&mem_list_mutex);

if (mem->dev_hpa == MM_IOMMU_TABLE_INVALID_HPA)
unlock_entries = mem->entries;

if (mem->used == 0) {
ret = -ENOENT;
goto unlock_exit;
Expand All @@ -292,16 +296,14 @@ long mm_iommu_put(struct mm_struct *mm, struct mm_iommu_table_group_mem_t *mem)
}

/* @mapped became 0 so now mappings are disabled, release the region */
entries = mem->entries;
dev_hpa = mem->dev_hpa;
mm_iommu_release(mem);

if (dev_hpa == MM_IOMMU_TABLE_INVALID_HPA)
mm_iommu_adjust_locked_vm(mm, entries, false);

unlock_exit:
mutex_unlock(&mem_list_mutex);

if (!ret)
mm_iommu_adjust_locked_vm(mm, unlock_entries, false);

return ret;
}
EXPORT_SYMBOL_GPL(mm_iommu_put);
Expand Down

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