CVE-2025-38373: IB/mlx5: Fix potential deadlock in MR deregistration
In the Linux kernel, the following vulnerability has been resolved:

IB/mlx5: Fix potential deadlock in MR deregistration

The issue arises when kzalloc() is invoked while holding umem_mutex or
any other lock acquired under umem_mutex. This is problematic because
kzalloc() can trigger fs_reclaim_aqcuire(), which may, in turn, invoke
mmu_notifier_invalidate_range_start(). This function can lead to
mlx5_ib_invalidate_range(), which attempts to acquire umem_mutex again,
resulting in a deadlock.

The problematic flow:
             CPU0                      |              CPU1
---------------------------------------|------------------------------------------------
mlx5_ib_dereg_mr()                     |
 → revoke_mr()                         |
   → mutex_lock(&umem_odp->umem_mutex) |
                                       | mlx5_mkey_cache_init()
                                       |  → mutex_lock(&dev->cache.rb_lock)
                                       |  → mlx5r_cache_create_ent_locked()
                                       |    → kzalloc(GFP_KERNEL)
                                       |      → fs_reclaim()
                                       |        → mmu_notifier_invalidate_range_start()
                                       |          → mlx5_ib_invalidate_range()
                                       |            → mutex_lock(&umem_odp->umem_mutex)
   → cache_ent_find_and_store()        |
     → mutex_lock(&dev->cache.rb_lock) |

Additionally, when kzalloc() is called from within
cache_ent_find_and_store(), we encounter the same deadlock due to
re-acquisition of umem_mutex.

Solve by releasing umem_mutex in dereg_mr() after umr_revoke_mr()
and before acquiring rb_lock. This ensures that we don't hold
umem_mutex while performing memory allocations that could trigger
the reclaim path.

This change prevents the deadlock by ensuring proper lock ordering and
avoiding holding locks during memory allocation operations that could
trigger the reclaim path.

The following lockdep warning demonstrates the deadlock:

 python3/20557 is trying to acquire lock:
 ffff888387542128 (&umem_odp->umem_mutex){+.+.}-{4:4}, at:
 mlx5_ib_invalidate_range+0x5b/0x550 [mlx5_ib]

 but task is already holding lock:
 ffffffff82f6b840 (mmu_notifier_invalidate_range_start){+.+.}-{0:0}, at:
 unmap_vmas+0x7b/0x1a0

 which lock already depends on the new lock.

 the existing dependency chain (in reverse order) is:

 -> #3 (mmu_notifier_invalidate_range_start){+.+.}-{0:0}:
       fs_reclaim_acquire+0x60/0xd0
       mem_cgroup_css_alloc+0x6f/0x9b0
       cgroup_init_subsys+0xa4/0x240
       cgroup_init+0x1c8/0x510
       start_kernel+0x747/0x760
       x86_64_start_reservations+0x25/0x30
       x86_64_start_kernel+0x73/0x80
       common_startup_64+0x129/0x138

 -> #2 (fs_reclaim){+.+.}-{0:0}:
       fs_reclaim_acquire+0x91/0xd0
       __kmalloc_cache_noprof+0x4d/0x4c0
       mlx5r_cache_create_ent_locked+0x75/0x620 [mlx5_ib]
       mlx5_mkey_cache_init+0x186/0x360 [mlx5_ib]
       mlx5_ib_stage_post_ib_reg_umr_init+0x3c/0x60 [mlx5_ib]
       __mlx5_ib_add+0x4b/0x190 [mlx5_ib]
       mlx5r_probe+0xd9/0x320 [mlx5_ib]
       auxiliary_bus_probe+0x42/0x70
       really_probe+0xdb/0x360
       __driver_probe_device+0x8f/0x130
       driver_probe_device+0x1f/0xb0
       __driver_attach+0xd4/0x1f0
       bus_for_each_dev+0x79/0xd0
       bus_add_driver+0xf0/0x200
       driver_register+0x6e/0xc0
       __auxiliary_driver_register+0x6a/0xc0
       do_one_initcall+0x5e/0x390
       do_init_module+0x88/0x240
       init_module_from_file+0x85/0xc0
       idempotent_init_module+0x104/0x300
       __x64_sys_finit_module+0x68/0xc0
       do_syscall_64+0x6d/0x140
       entry_SYSCALL_64_after_hwframe+0x4b/0x53

 -> #1 (&dev->cache.rb_lock){+.+.}-{4:4}:
       __mutex_lock+0x98/0xf10
       __mlx5_ib_dereg_mr+0x6f2/0x890 [mlx5_ib]
       mlx5_ib_dereg_mr+0x21/0x110 [mlx5_ib]
       ib_dereg_mr_user+0x85/0x1f0 [ib_core]
  
---truncated---
CVSS v3.1: 5.5