In legacy mode, SSPTPTR is ignored if TT is not 00b or 01b. SSPTPTR
maybe uninitialized or zero in that case and may cause oops like:

 Oops: general protection fault, probably for non-canonical address 0xf00087d3f000f000: 0000 [#1] SMP NOPTI
 CPU: 2 UID: 0 PID: 786 Comm: cat Not tainted 6.16.0 #191 PREEMPT(voluntary)
 Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.17.0-5.fc42 04/01/2014
 RIP: 0010:pgtable_walk_level+0x98/0x150
 RSP: 0018:ffffc90000f279c0 EFLAGS: 00010206
 RAX: 0000000040000000 RBX: ffffc90000f27ab0 RCX: 000000000000001e
 RDX: 0000000000000003 RSI: f00087d3f000f000 RDI: f00087d3f0010000
 RBP: ffffc90000f27a00 R08: ffffc90000f27a98 R09: 0000000000000002
 R10: 0000000000000000 R11: 0000000000000000 R12: f00087d3f000f000
 R13: 0000000000000000 R14: 0000000040000000 R15: ffffc90000f27a98
 FS:  0000764566dcb740(0000) GS:ffff8881f812c000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000764566d44000 CR3: 0000000109d81003 CR4: 0000000000772ef0
 PKRU: 55555554
 Call Trace:
  <TASK>
  pgtable_walk_level+0x88/0x150
  domain_translation_struct_show.isra.0+0x2d9/0x300
  dev_domain_translation_struct_show+0x20/0x40
  seq_read_iter+0x12d/0x490
...

Avoid walking the page table if TT is not 00b or 01b.

Signed-off-by: Vineeth Pillai (Google) <vineeth@bitbyteword.org>
---
 drivers/iommu/intel/debugfs.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/drivers/iommu/intel/debugfs.c b/drivers/iommu/intel/debugfs.c
index affbf4a1558de..5aa7f46a420b5 100644
--- a/drivers/iommu/intel/debugfs.c
+++ b/drivers/iommu/intel/debugfs.c
@@ -435,8 +435,21 @@ static int domain_translation_struct_show(struct seq_file *m,
 			}
 			pgd &= VTD_PAGE_MASK;
 		} else { /* legacy mode */
-			pgd = context->lo & VTD_PAGE_MASK;
-			agaw = context->hi & 7;
+			u8 tt = (u8)(context->lo & GENMASK_ULL(3, 2)) >> 2;
+
+			/*
+			 * According to Translation Type(TT),
+			 * get the page table pointer(SSPTPTR).
+			 */
+			switch (tt) {
+			case CONTEXT_TT_MULTI_LEVEL:
+			case CONTEXT_TT_DEV_IOTLB:
+				pgd = context->lo & VTD_PAGE_MASK;
+				agaw = context->hi & 7;
+				break;
+			default:
+				goto iommu_unlock;
+			}
 		}
 
 		seq_printf(m, "Device %04x:%02x:%02x.%x ",
-- 
2.50.1

> From: Vineeth Pillai (Google) <vineeth@bitbyteword.org>
> Sent: Friday, August 15, 2025 12:32 AM
> 
> In legacy mode, SSPTPTR is ignored if TT is not 00b or 01b. SSPTPTR
> maybe uninitialized or zero in that case and may cause oops like:
> 
>  Oops: general protection fault, probably for non-canonical address
> 0xf00087d3f000f000: 0000 [#1] SMP NOPTI
>  CPU: 2 UID: 0 PID: 786 Comm: cat Not tainted 6.16.0 #191
> PREEMPT(voluntary)
>  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.17.0-5.fc42
> 04/01/2014
>  RIP: 0010:pgtable_walk_level+0x98/0x150
>  RSP: 0018:ffffc90000f279c0 EFLAGS: 00010206
>  RAX: 0000000040000000 RBX: ffffc90000f27ab0 RCX: 000000000000001e
>  RDX: 0000000000000003 RSI: f00087d3f000f000 RDI: f00087d3f0010000
>  RBP: ffffc90000f27a00 R08: ffffc90000f27a98 R09: 0000000000000002
>  R10: 0000000000000000 R11: 0000000000000000 R12: f00087d3f000f000
>  R13: 0000000000000000 R14: 0000000040000000 R15: ffffc90000f27a98
>  FS:  0000764566dcb740(0000) GS:ffff8881f812c000(0000)
> knlGS:0000000000000000
>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>  CR2: 0000764566d44000 CR3: 0000000109d81003 CR4: 0000000000772ef0
>  PKRU: 55555554
>  Call Trace:
>   <TASK>
>   pgtable_walk_level+0x88/0x150
>   domain_translation_struct_show.isra.0+0x2d9/0x300
>   dev_domain_translation_struct_show+0x20/0x40
>   seq_read_iter+0x12d/0x490
> ...
> 
> Avoid walking the page table if TT is not 00b or 01b.
> 
> Signed-off-by: Vineeth Pillai (Google) <vineeth@bitbyteword.org>

Reviewed-by: Kevin Tian <kevin.tian@intel.com>