Re: [PATCH] iommu/iommufd: Fix NULL pointer deref in iommufd_ioas_change_process when racing with iopt_map_file_pages

Hey curious to know,

On 8/6/2026 10:20 AM, Peiyang He wrote:
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> iommufd_ioas_change_process() iterates every IOAS area while only
> holding every IOAS iova_rwsem, so it assumes every area has a non-NULL
> pages pointer. That assumption can be false when it runs concurrently
> with iopt_map_file_pages().
>
> iopt_map_pages() executes in two phases. It first creates the area and
> inserts it into the interval tree under iova_rwsem, with area->pages
> still NULL. It then drops iova_rwsem and later fills area->pages
> under domains_rwsem. This leaves a window between area creation and
> area->pages fill where a concurrent iommufd_ioas_change_process()
> can observe the area and dereference a NULL area->pages pointer,
> leading to a NULL pointer dereference:
AFAIU, the flow must be

iopt_map_file_pages()
         ->iopt_map_common()
             ->iopt_map_pages() <- here it does create and write pages ..


iommufd_ioas_change_process() <-the page pointer is being accessed..

1. Why IOCTL got triggered before setting the respective functionality ?

2. Since here unlocked_ioctl() used which means driver should have 
manage locking as user space will not hold global Big Kernel Lock  (BKL).
      We have pages->mutex - do you think we need it some where instead 
of "-EBUSY" return ? - if yes can you try and repro ?

snip:
  *
  * The locking order is domains_rwsem -> iova_rwsem -> pages::mutex
  */
struct io_pagetable {

---

         down_read(&iopt->domains_rwsem);
         rc = iopt_fill_domains_pages(pages_list);
         if (rc)
                 goto out_unlock_domains;

         down_write(&iopt->iova_rwsem);
         list_for_each_entry(elm, pages_list, next) {
                 /*
                  * area->pages must be set inside the domains_rwsem to 
ensure
                  * any newly added domains will get filled. Moves the 
reference
                  * in from the list.
                  */
                 elm->area->pages = elm->pages;   <- maybe page mutex to 
protect this ?
                 elm->pages = NULL;
                 elm->area = NULL;
         }
         up_write(&iopt->iova_rwsem);
out_unlock_domains:
         up_read(&iopt->domains_rwsem);
         return rc;


Thanks,

Tushar Nimkar

>
> BUG: kernel NULL pointer dereference, address: 00000000000000c0
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 4b655067 P4D 4b655067 PUD 0
> Oops: Oops: 0000 [#1] SMP NOPTI
> CPU: 0 UID: 0 PID: 11841 Comm: syz.1.628 Not tainted 7.1.0 #3 PREEMPT(full)
> Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
> RIP: 0010:iommufd_ioas_change_process+0x419/0xd50 drivers/iommu/iommufd/ioas.c:538
> Code: 48 89 c3 48 85 c0 0f 84 cc 00 00 00 e8 10 f5 cb fd 48 8d 7b 68 e8 a7 b5 eb fd 48 8b 6b 68 48 8d bd c0 00 00 00 e8 17 b2 eb fd <8b> ad c0 00 00 00 bf 01 00 00 00 89 ee e8 85 ef cb fd 83 fd 01 74
> RSP: 0018:ffffc90015c17d28 EFLAGS: 00010246
> RAX: ffff8880186d5328 RBX: ffff88801d25e240 RCX: 0000000080000000
> RDX: 00000000000002d7 RSI: ffffffff83ba9e10 RDI: 00000000000000c0
> RBP: 0000000000000000 R08: ffffffff8e781eb8 R09: 0000000000000000
> R10: 00000000000000c0 R11: ffffffff83ba9e29 R12: ffff88802e216008
> R13: ffff88802e216000 R14: 0000000000000001 R15: 0000000000000000
> FS:  00007f4aea3f66c0(0000) GS:ffff8880b1fa1000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00000000000000c0 CR3: 000000004b75c000 CR4: 0000000000350ef0
> Call Trace:
>   <TASK>
>   iommufd_fops_ioctl+0x287/0x400 drivers/iommu/iommufd/main.c:533
>   vfs_ioctl fs/ioctl.c:51 [inline]
>   __do_sys_ioctl fs/ioctl.c:597 [inline]
>   __se_sys_ioctl fs/ioctl.c:583 [inline]
>   __x64_sys_ioctl+0x120/0x170 fs/ioctl.c:583
>   x64_sys_call+0x1092/0x1fb0 arch/x86/include/generated/asm/syscalls_64.h:17
>   do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
>   do_syscall_64+0x10a/0x680 arch/x86/entry/syscall_64.c:94
>   entry_SYSCALL_64_after_hwframe+0x77/0x7f
> RIP: 0033:0x7f4aec1a82bd
> Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007f4aea3f6018 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 00007f4aec436090 RCX: 00007f4aec1a82bd
> RDX: 0000200000000180 RSI: 0000000000003b92 RDI: 0000000000000003
> RBP: 00007f4aec250295 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007f4aec436128 R14: 00007f4aec436090 R15: 00007ffd04ef23e0
>   </TASK>
> Modules linked in:
> CR2: 00000000000000c0
> ---[ end trace 0000000000000000 ]---
> RIP: 0010:iommufd_ioas_change_process+0x419/0xd50 drivers/iommu/iommufd/ioas.c:538
> Code: 48 89 c3 48 85 c0 0f 84 cc 00 00 00 e8 10 f5 cb fd 48 8d 7b 68 e8 a7 b5 eb fd 48 8b 6b 68 48 8d bd c0 00 00 00 e8 17 b2 eb fd <8b> ad c0 00 00 00 bf 01 00 00 00 89 ee e8 85 ef cb fd 83 fd 01 74
> RSP: 0018:ffffc90015c17d28 EFLAGS: 00010246
> RAX: ffff8880186d5328 RBX: ffff88801d25e240 RCX: 0000000080000000
> RDX: 00000000000002d7 RSI: ffffffff83ba9e10 RDI: 00000000000000c0
> RBP: 0000000000000000 R08: ffffffff8e781eb8 R09: 0000000000000000
> R10: 00000000000000c0 R11: ffffffff83ba9e29 R12: ffff88802e216008
> R13: ffff88802e216000 R14: 0000000000000001 R15: 0000000000000000
> FS:  00007f4aea3f66c0(0000) GS:ffff8880b1fa1000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00000000000000c0 CR3: 000000004b75c000 CR4: 0000000000350ef0
> ----------------
> Code disassembly (best guess):
>     0:   48 89 c3                mov    %rax,%rbx
>     3:   48 85 c0                test   %rax,%rax
>     6:   0f 84 cc 00 00 00       je     0xd8
>     c:   e8 10 f5 cb fd          call   0xfdcbf521
>    11:   48 8d 7b 68             lea    0x68(%rbx),%rdi
>    15:   e8 a7 b5 eb fd          call   0xfdebb5c1
>    1a:   48 8b 6b 68             mov    0x68(%rbx),%rbp
>    1e:   48 8d bd c0 00 00 00    lea    0xc0(%rbp),%rdi
>    25:   e8 17 b2 eb fd          call   0xfdebb241
> * 2a:   8b ad c0 00 00 00       mov    0xc0(%rbp),%ebp <-- trapping instruction
>    30:   bf 01 00 00 00          mov    $0x1,%edi
>    35:   89 ee                   mov    %ebp,%esi
>    37:   e8 85 ef cb fd          call   0xfdcbefc1
>    3c:   83 fd 01                cmp    $0x1,%ebp
>    3f:   74                      .byte 0x74
>
> Fix by mirroring the handling logic in iopt_unmap_iova_range(),
> i.e. return -EBUSY if area->pages is NULL.
>
> Fixes: 829ed626499c ("iommufd: Add IOMMU_IOAS_CHANGE_PROCESS")
> Cc: [email protected]
> Signed-off-by: Peiyang He <[email protected]>
> ---
>   drivers/iommu/iommufd/ioas.c | 4 ++++
>   1 file changed, 4 insertions(+)
>
> diff --git a/drivers/iommu/iommufd/ioas.c b/drivers/iommu/iommufd/ioas.c
> index fed06c2b728e..71bffece84b5 100644
> --- a/drivers/iommu/iommufd/ioas.c
> +++ b/drivers/iommu/iommufd/ioas.c
> @@ -535,6 +535,10 @@ int iommufd_ioas_change_process(struct iommufd_ucmd *ucmd)
>                  return rc;
>
>          for_each_ioas_area(&ioas_list, index, ioas, area)  {
> +               if (!area->pages) {
> +                       rc = -EBUSY;
> +                       goto out;
> +               }
>                  if (area->pages->type != IOPT_ADDRESS_FILE) {
>                          rc = -EINVAL;
>                          goto out;
> --
> 2.43.0
>
>