CVE-2025-39885: Improper Locking

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

ocfs2: fix recursive semaphore deadlock in fiemap call

syzbot detected a OCFS2 hang due to a recursive semaphore on a FS_IOC_FIEMAP of the extent list on a specially crafted mmap file.

context_switch kernel/sched/core.c:5357 [inline] __schedule+0x1798/0x4cc0 kernel/sched/core.c:6961 __schedule_loop kernel/sched/core.c:7043 [inline] schedule+0x165/0x360 kernel/sched/core.c:7058 schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7115 rwsem_down_write_slowpath+0x872/0xfe0 kernel/locking/rwsem.c:1185 __down_write_common kernel/locking/rwsem.c:1317 [inline] __down_write kernel/locking/rwsem.c:1326 [inline] down_write+0x1ab/0x1f0 kernel/locking/rwsem.c:1591 ocfs2_page_mkwrite+0x2ff/0xc40 fs/ocfs2/mmap.c:142 do_page_mkwrite+0x14d/0x310 mm/memory.c:3361 wp_page_shared mm/memory.c:3762 [inline] do_wp_page+0x268d/0x5800 mm/memory.c:3981 handle_pte_fault mm/memory.c:6068 [inline] __handle_mm_fault+0x1033/0x5440 mm/memory.c:6195 handle_mm_fault+0x40a/0x8e0 mm/memory.c:6364 do_user_addr_fault+0x764/0x1390 arch/x86/mm/fault.c:1387 handle_page_fault arch/x86/mm/fault.c:1476 [inline] exc_page_fault+0x76/0xf0 arch/x86/mm/fault.c:1532 asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:623 RIP: 0010:copy_user_generic arch/x86/include/asm/uaccess_64.h:126 [inline] RIP: 0010:raw_copy_to_user arch/x86/include/asm/uaccess_64.h:147 [inline] RIP: 0010:_inline_copy_to_user include/linux/uaccess.h:197 [inline] RIP: 0010:_copy_to_user+0x85/0xb0 lib/usercopy.c:26 Code: e8 00 bc f7 fc 4d 39 fc 72 3d 4d 39 ec 77 38 e8 91 b9 f7 fc 4c 89 f7 89 de e8 47 25 5b fd 0f 01 cb 4c 89 ff 48 89 d9 4c 89 f6 <f3> a4 0f 1f 00 48 89 cb 0f 01 ca 48 89 d8 5b 41 5c 41 5d 41 5e 41 RSP: 0018:ffffc9000403f950 EFLAGS: 00050256 RAX: ffffffff84c7f101 RBX: 0000000000000038 RCX: 0000000000000038 RDX: 0000000000000000 RSI: ffffc9000403f9e0 RDI: 0000200000000060 RBP: ffffc9000403fa90 R08: ffffc9000403fa17 R09: 1ffff92000807f42 R10: dffffc0000000000 R11: fffff52000807f43 R12: 0000200000000098 R13: 00007ffffffff000 R14: ffffc9000403f9e0 R15: 0000200000000060 copy_to_user include/linux/uaccess.h:225 [inline] fiemap_fill_next_extent+0x1c0/0x390 fs/ioctl.c:145 ocfs2_fiemap+0x888/0xc90 fs/ocfs2/extent_map.c:806 ioctl_fiemap fs/ioctl.c:220 [inline] do_vfs_ioctl+0x1173/0x1430 fs/ioctl.c:532 __do_sys_ioctl fs/ioctl.c:596 [inline] __se_sys_ioctl+0x82/0x170 fs/ioctl.c:584 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x7f5f13850fd9 RSP: 002b:00007ffe3b3518b8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010 RAX: ffffffffffffffda RBX: 0000200000000000 RCX: 00007f5f13850fd9 RDX: 0000200000000040 RSI: 00000000c020660b RDI: 0000000000000004 RBP: 6165627472616568 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffe3b3518f0 R13: 00007ffe3b351b18 R14: 431bde82d7b634db R15: 00007f5f1389a03b

ocfs2_fiemap() takes a read lock of the ip_alloc_sem semaphore (since v2.6.22-527-g7307de80510a) and calls fiemap_fill_next_extent() to read the extent list of this running mmap executable. The user supplied buffer to hold the fiemap information page faults calling ocfs2_page_mkwrite() which will take a write lock (since v2.6.27-38-g00dc417fa3e7) of the same semaphore. This recursive semaphore will hold filesystem locks and causes a hang of the fileystem.

The ip_alloc_sem protects the inode extent list and size. Release the read semphore before calling fiemap_fill_next_extent() in ocfs2_fiemap() and ocfs2_fiemap_inline(). This does an unnecessary semaphore lock/unlock on the last extent but simplifies the error path.