linux-kernel - Re: [PATCH v2] drm/panthor: Fix kernel panic on partial unmap of a GPU VA region

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Message-ID: <4c681d1f-cdfd-4bad-a859-4181e2b91871@arm.com>
Date: Tue, 21 Oct 2025 08:14:45 +0100
From: Akash Goel <akash.goel@....com>
To: Steven Price <steven.price@....com>, boris.brezillon@...labora.com,
 liviu.dudau@....com
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 maarten.lankhorst@...ux.intel.com, mripard@...nel.org, tzimmermann@...e.de,
 airlied@...il.com, daniel@...ll.ch, nd@....com
Subject: Re: [PATCH v2] drm/panthor: Fix kernel panic on partial unmap of a
 GPU VA region



On 10/20/25 16:44, Steven Price wrote:
> Hi Akash,
> 
> On 20/10/2025 07:00, Akash Goel wrote:
>> This commit address a kernel panic issue that can happen if Userspace
>> tries to partially unmap a GPU virtual region (aka drm_gpuva).
>> The VM_BIND interface allows partial unmapping of a BO.
>>
>> Panthor driver pre-allocates memory for the new drm_gpuva structures
>> that would be needed for the map/unmap operation, done using drm_gpuvm
>> layer. It expected that only one new drm_gpuva would be needed on umap
>> but a partial unmap can require 2 new drm_gpuva and that's why it
>> ended up doing a NULL pointer dereference causing a kernel panic.
>>
>> Following dump was seen when partial unmap was exercised.
>>   Unable to handle kernel NULL pointer dereference at virtual address 0000000000000078
>>   Mem abort info:
>>     ESR = 0x0000000096000046
>>     EC = 0x25: DABT (current EL), IL = 32 bits
>>     SET = 0, FnV = 0
>>     EA = 0, S1PTW = 0
>>     FSC = 0x06: level 2 translation fault
>>   Data abort info:
>>     ISV = 0, ISS = 0x00000046, ISS2 = 0x00000000
>>     CM = 0, WnR = 1, TnD = 0, TagAccess = 0
>>     GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
>>   user pgtable: 4k pages, 48-bit VAs, pgdp=000000088a863000
>>   [000000000000078] pgd=080000088a842003, p4d=080000088a842003, pud=0800000884bf5003, pmd=0000000000000000
>>   Internal error: Oops: 0000000096000046 [#1] PREEMPT SMP
>>   <snip>
>>   pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>>   pc : panthor_gpuva_sm_step_remap+0xe4/0x330 [panthor]
>>   lr : panthor_gpuva_sm_step_remap+0x6c/0x330 [panthor]
>>   sp : ffff800085d43970
>>   x29: ffff800085d43970 x28: ffff00080363e440 x27: ffff0008090c6000
>>   x26: 0000000000000030 x25: ffff800085d439f8 x24: ffff00080d402000
>>   x23: ffff800085d43b60 x22: ffff800085d439e0 x21: ffff00080abdb180
>>   x20: 0000000000000000 x19: 0000000000000000 x18: 0000000000000010
>>   x17: 6e656c202c303030 x16: 3666666666646466 x15: 393d61766f69202c
>>   x14: 312d3d7361203a70 x13: 303030323d6e656c x12: ffff80008324bf58
>>   x11: 0000000000000003 x10: 0000000000000002 x9 : ffff8000801a6a9c
>>   x8 : ffff00080360b300 x7 : 0000000000000000 x6 : 000000088aa35fc7
>>   x5 : fff1000080000000 x4 : ffff8000842ddd30 x3 : 0000000000000001
>>   x2 : 0000000100000000 x1 : 0000000000000001 x0 : 0000000000000078
>>   Call trace:
>>    panthor_gpuva_sm_step_remap+0xe4/0x330 [panthor]
>>    op_remap_cb.isra.22+0x50/0x80
>>    __drm_gpuvm_sm_unmap+0x10c/0x1c8
>>    drm_gpuvm_sm_unmap+0x40/0x60
>>    panthor_vm_exec_op+0xb4/0x3d0 [panthor]
>>    panthor_vm_bind_exec_sync_op+0x154/0x278 [panthor]
>>    panthor_ioctl_vm_bind+0x160/0x4a0 [panthor]
>>    drm_ioctl_kernel+0xbc/0x138
>>    drm_ioctl+0x240/0x500
>>    __arm64_sys_ioctl+0xb0/0xf8
>>    invoke_syscall+0x4c/0x110
>>    el0_svc_common.constprop.1+0x98/0xf8
>>    do_el0_svc+0x24/0x38
>>    el0_svc+0x40/0xf8
>>    el0t_64_sync_handler+0xa0/0xc8
>>    el0t_64_sync+0x174/0x178
>>
>> Changes in v2:
>>      - Add R-bs and fixes tags
> 
> Sorry, perhaps I wasn't very clear. I've already pushed v1 to
> drm-misc-fixes (with the fixes line and the R-bs):
> 
> 4eabd0d8791e ("drm/panthor: Fix kernel panic on partial unmap of a GPU VA region")
> 
> Usually there's no need to post another version just to update the tags
> as we can do that as part of applying the series.
> 


I am sorry Steve, I am not wel versed with the upstreaming process.


> Thanks,
> Steve
> 
>>
>> Fixes: 647810ec2476 ("drm/panthor: Add the MMU/VM logical block")
>> Signed-off-by: Akash Goel <akash.goel@....com>
>> Reviewed-by: Boris Brezillon <boris.brezillon@...labora.com>
>> Reviewed-by: Liviu Dudau <liviu.dudau@....com>
>> Reviewed-by: Steven Price <steven.price@....com>
>> ---
>>   drivers/gpu/drm/panthor/panthor_mmu.c | 10 +++++++---
>>   1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/panthor/panthor_mmu.c b/drivers/gpu/drm/panthor/panthor_mmu.c
>> index 6dec4354e378..7870e7dbaa5d 100644
>> --- a/drivers/gpu/drm/panthor/panthor_mmu.c
>> +++ b/drivers/gpu/drm/panthor/panthor_mmu.c
>> @@ -1175,10 +1175,14 @@ panthor_vm_op_ctx_prealloc_vmas(struct panthor_vm_op_ctx *op_ctx)
>>   		break;
>>   
>>   	case DRM_PANTHOR_VM_BIND_OP_TYPE_UNMAP:
>> -		/* Partial unmaps might trigger a remap with either a prev or a next VA,
>> -		 * but not both.
>> +		/* Two VMAs can be needed for an unmap, as an unmap can happen
>> +		 * in the middle of a drm_gpuva, requiring a remap with both
>> +		 * prev & next VA. Or an unmap can span more than one drm_gpuva
>> +		 * where the first and last ones are covered partially, requring
>> +		 * a remap for the first with a prev VA and remap for the last
>> +		 * with a next VA.
>>   		 */
>> -		vma_count = 1;
>> +		vma_count = 2;
>>   		break;
>>   
>>   	default:
>