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Date: Wed, 9 Dec 2020 11:39:39 +0000
From: John Garry <john.garry@...wei.com>
To: "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>,
"robin.murphy@....com" <robin.murphy@....com>,
"joro@...tes.org" <joro@...tes.org>,
"will@...nel.org" <will@...nel.org>
CC: Linuxarm <linuxarm@...wei.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
"xiyou.wangcong@...il.com" <xiyou.wangcong@...il.com>
Subject: Re: [RESEND PATCH v3 2/4] iommu/iova: Avoid double-negatives in
magazine helpers
On 09/12/2020 09:03, Leizhen (ThunderTown) wrote:
>
>
> On 2020/11/17 18:25, John Garry wrote:
>> 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:
>> ESR = 0x96000004
>> EC = 0x25: DABT (current EL), IL = 32 bits
>> SET = 0, FnV = 0
>> EA = 0, S1PTW = 0
>> Data abort info:
>> ISV = 0, ISS = 0x00000004
>> CM = 0, WnR = 0
>> [0000000000000000] user address but active_mm is swapper
>> Internal error: Oops: 96000004 [#1] PREEMPT SMP
>> Modules linked in:
>> CPU: 11 PID: 696 Comm: irq/40-hisi_sas Not tainted 5.9.0-rc7-dirty #109
>> Hardware name: Huawei D06 /D06, BIOS Hisilicon D06 UEFI RC0 - V1.16.01 03/15/2019
>> Call trace:
>> 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
>>
>> Code: f9400060 f102001f 54000981 d4210000 (f9400043)
>>
>> ---[ end trace 4afcbdfc61b60467 ]---
>>
>> The issue is that expression !iova_magazine_full(NULL) evaluates true; this
>> falls over in 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@...wei.com>
Thanks for checking here...
>> ---
>> 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 81b7399dd5e8..1f3f0f8b12e0 100644
>> --- a/drivers/iommu/iova.c
>> +++ b/drivers/iommu/iova.c
>> @@ -827,14 +827,18 @@ 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,
>> @@ -843,7 +847,7 @@ 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--)
>> @@ -859,7 +863,7 @@ 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;
>> }
>> @@ -905,9 +909,9 @@ static bool __iova_rcache_insert(struct iova_domain *iovad,
>> 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 {
>> @@ -916,8 +920,9 @@ static bool __iova_rcache_insert(struct iova_domain *iovad,
>> 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)
>
> Looks like it just needs to change this place. Compiler ensures that mag->size
> will not be accessed when mag is NULL.
Not sure about that. We would get a crash prior to touching this codepath:
So if cpu_rcache->loaded == NULL at entry, then
!iova_magazine_full(cpu_rcache->loaded (==NULL) ) evaluates true, *
above, so can_insert is set true, and we then attempt
iova_magazine_push(cpu_rcache->loaded (==NULL), iova_pfn), which is not
safe.
ok?
Cheers,
John
>
> static bool iova_magazine_full(struct iova_magazine *mag)
> {
> return (mag && mag->size == IOVA_MAG_SIZE);
> }
>
> static bool iova_magazine_empty(struct iova_magazine *mag)
> {
> return (!mag || mag->size == 0);
> }
>
>
>> + rcache->depot[rcache->depot_size++] =
>> + cpu_rcache->loaded;
>> } else {
>> mag_to_free = cpu_rcache->loaded;
>> }
>> @@ -968,9 +973,9 @@ 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 {
>>
>
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