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Subject: Re: [RESEND PATCH v3 2/4] iommu/iova: Avoid double-negatives in
 magazine helpers
To:     John Garry <john.garry@huawei.com>, <robin.murphy@arm.com>,
        <joro@8bytes.org>, <will@kernel.org>
CC:     <linuxarm@huawei.com>, <linux-kernel@vger.kernel.org>,
        <iommu@lists.linux-foundation.org>, <xiyou.wangcong@gmail.com>
References: <1605608734-84416-1-git-send-email-john.garry@huawei.com>
 <1605608734-84416-3-git-send-email-john.garry@huawei.com>
From:   "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
Message-ID: <7eb70f4b-b050-24ca-f1fa-d8f3c9ddce65@huawei.com>
Date:   Wed, 9 Dec 2020 17:03:24 +0800
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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@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 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.

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 {
>