linux-cve-announce - CVE-2023-53795: iommufd: IOMMUFD_DESTROY should not increase the refcount

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Message-ID: <2025120941-CVE-2023-53795-f912@gregkh>
Date: Tue,  9 Dec 2025 09:02:16 +0900
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2023-53795: iommufd: IOMMUFD_DESTROY should not increase the refcount

From: Greg Kroah-Hartman <gregkh@...nel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

iommufd: IOMMUFD_DESTROY should not increase the refcount

syzkaller found a race where IOMMUFD_DESTROY increments the refcount:

       obj = iommufd_get_object(ucmd->ictx, cmd->id, IOMMUFD_OBJ_ANY);
       if (IS_ERR(obj))
               return PTR_ERR(obj);
       iommufd_ref_to_users(obj);
       /* See iommufd_ref_to_users() */
       if (!iommufd_object_destroy_user(ucmd->ictx, obj))

As part of the sequence to join the two existing primitives together.

Allowing the refcount the be elevated without holding the destroy_rwsem
violates the assumption that all temporary refcount elevations are
protected by destroy_rwsem. Racing IOMMUFD_DESTROY with
iommufd_object_destroy_user() will cause spurious failures:

  WARNING: CPU: 0 PID: 3076 at drivers/iommu/iommufd/device.c:477 iommufd_access_destroy+0x18/0x20 drivers/iommu/iommufd/device.c:478
  Modules linked in:
  CPU: 0 PID: 3076 Comm: syz-executor.0 Not tainted 6.3.0-rc1-syzkaller #0
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/03/2023
  RIP: 0010:iommufd_access_destroy+0x18/0x20 drivers/iommu/iommufd/device.c:477
  Code: e8 3d 4e 00 00 84 c0 74 01 c3 0f 0b c3 0f 1f 44 00 00 f3 0f 1e fa 48 89 fe 48 8b bf a8 00 00 00 e8 1d 4e 00 00 84 c0 74 01 c3 <0f> 0b c3 0f 1f 44 00 00 41 57 41 56 41 55 4c 8d ae d0 00 00 00 41
  RSP: 0018:ffffc90003067e08 EFLAGS: 00010246
  RAX: 0000000000000000 RBX: ffff888109ea0300 RCX: 0000000000000000
  RDX: 0000000000000001 RSI: 0000000000000000 RDI: 00000000ffffffff
  RBP: 0000000000000004 R08: 0000000000000000 R09: ffff88810bbb3500
  R10: ffff88810bbb3e48 R11: 0000000000000000 R12: ffffc90003067e88
  R13: ffffc90003067ea8 R14: ffff888101249800 R15: 00000000fffffffe
  FS:  00007ff7254fe6c0(0000) GS:ffff888237c00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000555557262da8 CR3: 000000010a6fd000 CR4: 0000000000350ef0
  Call Trace:
   <TASK>
   iommufd_test_create_access drivers/iommu/iommufd/selftest.c:596 [inline]
   iommufd_test+0x71c/0xcf0 drivers/iommu/iommufd/selftest.c:813
   iommufd_fops_ioctl+0x10f/0x1b0 drivers/iommu/iommufd/main.c:337
   vfs_ioctl fs/ioctl.c:51 [inline]
   __do_sys_ioctl fs/ioctl.c:870 [inline]
   __se_sys_ioctl fs/ioctl.c:856 [inline]
   __x64_sys_ioctl+0x84/0xc0 fs/ioctl.c:856
   do_syscall_x64 arch/x86/entry/common.c:50 [inline]
   do_syscall_64+0x38/0x80 arch/x86/entry/common.c:80
   entry_SYSCALL_64_after_hwframe+0x63/0xcd

The solution is to not increment the refcount on the IOMMUFD_DESTROY path
at all. Instead use the xa_lock to serialize everything. The refcount
check == 1 and xa_erase can be done under a single critical region. This
avoids the need for any refcount incrementing.

It has the downside that if userspace races destroy with other operations
it will get an EBUSY instead of waiting, but this is kind of racing is
already dangerous.

The Linux kernel CVE team has assigned CVE-2023-53795 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.2 with commit 2ff4bed7fee72ba1abfcff5f11ae8f8e570353f2 and fixed in 6.4.8 with commit 495b327435b0298e9b3b434f5834d459a93673ce
	Issue introduced in 6.2 with commit 2ff4bed7fee72ba1abfcff5f11ae8f8e570353f2 and fixed in 6.5 with commit 99f98a7c0d6985d5507c8130a981972e4b7b3bdc

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2023-53795
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/iommu/iommufd/device.c
	drivers/iommu/iommufd/iommufd_private.h
	drivers/iommu/iommufd/main.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/495b327435b0298e9b3b434f5834d459a93673ce
	https://git.kernel.org/stable/c/99f98a7c0d6985d5507c8130a981972e4b7b3bdc