[PATCH 5/5] iommu/iova: Avoid double-negatives in magazine helpers

John Garry
2021-09-24T10:07:29Z

A similar crash to the following could be observed if initial CPU rcache
magazine allocations fail in init_iova_rcaches():

Unable to handle kernel NULL pointer dereference at virtual address 0000000000000000
Mem abort info:

free_iova_fast+0xfc/0x280
iommu_dma_free_iova+0x64/0x70
__iommu_dma_unmap+0x9c/0xf8
iommu_dma_unmap_sg+0xa8/0xc8
dma_unmap_sg_attrs+0x28/0x50
cq_thread_v3_hw+0x2dc/0x528
irq_thread_fn+0x2c/0xa0
irq_thread+0x130/0x1e0
kthread+0x154/0x158
ret_from_fork+0x10/0x34

The issue is that expression !iova_magazine_full(NULL) evaluates true; this
falls over in __iova_rcache_insert() when we attempt to cache a mag and
cpu_rcache->loaded == NULL:

if (!iova_magazine_full(cpu_rcache->loaded)) {
can_insert = true;
...

if (can_insert)
iova_magazine_push(cpu_rcache->loaded, iova_pfn);

As above, can_insert is evaluated true, which it shouldn't be, and we try
to insert pfns in a NULL mag, which is not safe.

To avoid this, stop using double-negatives, like !iova_magazine_full() and
!iova_magazine_empty(), and use positive tests, like
iova_magazine_has_space() and iova_magazine_has_pfns(), respectively; these
can safely deal with cpu_rcache->{loaded, prev} = NULL.

Signed-off-by: John Garry <john.garry@huawei.com>
Reviewed-by: Zhen Lei <thunder.leizhen@huawei.com>
---
drivers/iommu/iova.c | 29 +++++++++++++++++------------
1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index 30eb128b1581..978ff7921029 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -688,14 +688,18 @@ iova_magazine_free_pfns(struct iova_magazine *mag, struct iova_domain *iovad)
mag->size = 0;
}

-static bool iova_magazine_full(struct iova_magazine *mag)
+static bool iova_magazine_has_space(struct iova_magazine *mag)
{
- return (mag && mag->size == IOVA_MAG_SIZE);
+ if (!mag)
+ return false;
+ return mag->size < IOVA_MAG_SIZE;
}

-static bool iova_magazine_empty(struct iova_magazine *mag)
+static bool iova_magazine_has_pfns(struct iova_magazine *mag)
{
- return (!mag || mag->size == 0);
+ if (!mag)
+ return false;
+ return mag->size;
}

static unsigned long iova_magazine_pop(struct iova_magazine *mag,
@@ -704,7 +708,7 @@ static unsigned long iova_magazine_pop(struct iova_magazine *mag,
int i;
unsigned long pfn;

- BUG_ON(iova_magazine_empty(mag));
+ BUG_ON(!iova_magazine_has_pfns(mag));

/* Only fall back to the rbtree if we have no suitable pfns at all */
for (i = mag->size - 1; mag->pfns[i] > limit_pfn; i--)
@@ -720,7 +724,7 @@ static unsigned long iova_magazine_pop(struct iova_magazine *mag,

static void iova_magazine_push(struct iova_magazine *mag, unsigned long pfn)
{
- BUG_ON(iova_magazine_full(mag));
+ BUG_ON(!iova_magazine_has_space(mag));

mag->pfns[mag->size++] = pfn;
}
@@ -772,9 +776,9 @@ static bool __iova_rcache_insert(struct iova_caching_domain *rcached,
cpu_rcache = raw_cpu_ptr(rcache->cpu_rcaches);
spin_lock_irqsave(&cpu_rcache->lock, flags);

- if (!iova_magazine_full(cpu_rcache->loaded)) {
+ if (iova_magazine_has_space(cpu_rcache->loaded)) {
can_insert = true;
- } else if (!iova_magazine_full(cpu_rcache->prev)) {
+ } else if (iova_magazine_has_space(cpu_rcache->prev)) {
swap(cpu_rcache->prev, cpu_rcache->loaded);
can_insert = true;
} else {
@@ -783,8 +787,9 @@ static bool __iova_rcache_insert(struct iova_caching_domain *rcached,
if (new_mag) {
spin_lock(&rcache->lock);
if (rcache->depot_size < MAX_GLOBAL_MAGS) {
- rcache->depot[rcache->depot_size++] =
- cpu_rcache->loaded;
+ if (cpu_rcache->loaded)
+ rcache->depot[rcache->depot_size++] =
+ cpu_rcache->loaded;
} else {
mag_to_free = cpu_rcache->loaded;
}
@@ -835,9 +840,9 @@ static unsigned long __iova_rcache_get(struct iova_rcache *rcache,
cpu_rcache = raw_cpu_ptr(rcache->cpu_rcaches);
spin_lock_irqsave(&cpu_rcache->lock, flags);

- if (!iova_magazine_empty(cpu_rcache->loaded)) {
+ if (iova_magazine_has_pfns(cpu_rcache->loaded)) {
has_pfn = true;
- } else if (!iova_magazine_empty(cpu_rcache->prev)) {
+ } else if (iova_magazine_has_pfns(cpu_rcache->prev)) {
swap(cpu_rcache->prev, cpu_rcache->loaded);
has_pfn = true;
} else {
--
2.26.2