CVE-2023-52737 in Linux
Summary
by MITRE • 05/21/2024
In the Linux kernel, the following vulnerability has been resolved:
btrfs: lock the inode in shared mode before starting fiemap
Currently fiemap does not take the inode's lock (VFS lock), it only locks a file range in the inode's io tree. This however can lead to a deadlock if we have a concurrent fsync on the file and fiemap code triggers a fault when accessing the user space buffer with fiemap_fill_next_extent(). The deadlock happens on the inode's i_mmap_lock semaphore, which is taken both by fsync and btrfs_page_mkwrite(). This deadlock was recently reported by syzbot and triggers a trace like the following:
task:syz-executor361 state:D stack:20264 pid:5668 ppid:5119 flags:0x00004004 Call Trace: <TASK> context_switch kernel/sched/core.c:5293 [inline]
__schedule+0x995/0xe20 kernel/sched/core.c:6606 schedule+0xcb/0x190 kernel/sched/core.c:6682 wait_on_state fs/btrfs/extent-io-tree.c:707 [inline]
wait_extent_bit+0x577/0x6f0 fs/btrfs/extent-io-tree.c:751 lock_extent+0x1c2/0x280 fs/btrfs/extent-io-tree.c:1742 find_lock_delalloc_range+0x4e6/0x9c0 fs/btrfs/extent_io.c:488 writepage_delalloc+0x1ef/0x540 fs/btrfs/extent_io.c:1863 __extent_writepage+0x736/0x14e0 fs/btrfs/extent_io.c:2174 extent_write_cache_pages+0x983/0x1220 fs/btrfs/extent_io.c:3091 extent_writepages+0x219/0x540 fs/btrfs/extent_io.c:3211 do_writepages+0x3c3/0x680 mm/page-writeback.c:2581 filemap_fdatawrite_wbc+0x11e/0x170 mm/filemap.c:388 __filemap_fdatawrite_range mm/filemap.c:421 [inline]
filemap_fdatawrite_range+0x175/0x200 mm/filemap.c:439 btrfs_fdatawrite_range fs/btrfs/file.c:3850 [inline]
start_ordered_ops fs/btrfs/file.c:1737 [inline]
btrfs_sync_file+0x4ff/0x1190 fs/btrfs/file.c:1839 generic_write_sync include/linux/fs.h:2885 [inline]
btrfs_do_write_iter+0xcd3/0x1280 fs/btrfs/file.c:1684 call_write_iter include/linux/fs.h:2189 [inline]
new_sync_write fs/read_write.c:491 [inline]
vfs_write+0x7dc/0xc50 fs/read_write.c:584 ksys_write+0x177/0x2a0 fs/read_write.c:637 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80 entry_SYSCALL_64_after_hwframe+0x63/0xcd RIP: 0033:0x7f7d4054e9b9 RSP: 002b:00007f7d404fa2f8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001 RAX: ffffffffffffffda RBX: 00007f7d405d87a0 RCX: 00007f7d4054e9b9 RDX: 0000000000000090 RSI: 0000000020000000 RDI: 0000000000000006 RBP: 00007f7d405a51d0 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000246 R12: 61635f65646f6e69 R13: 65646f7475616f6e R14: 7261637369646f6e R15: 00007f7d405d87a8 </TASK> INFO: task syz-executor361:5697 blocked for more than 145 seconds. Not tainted 6.2.0-rc3-syzkaller-00376-g7c6984405241 #0 "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. task:syz-executor361 state:D stack:21216 pid:5697 ppid:5119 flags:0x00004004 Call Trace: <TASK> context_switch kernel/sched/core.c:5293 [inline]
__schedule+0x995/0xe20 kernel/sched/core.c:6606 schedule+0xcb/0x190 kernel/sched/core.c:6682 rwsem_down_read_slowpath+0x5f9/0x930 kernel/locking/rwsem.c:1095 __down_read_common+0x54/0x2a0 kernel/locking/rwsem.c:1260 btrfs_page_mkwrite+0x417/0xc80 fs/btrfs/inode.c:8526 do_page_mkwrite+0x19e/0x5e0 mm/memory.c:2947 wp_page_shared+0x15e/0x380 mm/memory.c:3295 handle_pte_fault mm/memory.c:4949 [inline]
__handle_mm_fault mm/memory.c:5073 [inline]
handle_mm_fault+0x1b79/0x26b0 mm/memory.c:5219 do_user_addr_fault+0x69b/0xcb0 arch/x86/mm/fault.c:1428 handle_page_fault arch/x86/mm/fault.c:1519 [inline]
exc_page_fault+0x7a/0x110 arch/x86/mm/fault.c:1575 asm_exc_page_fault+0x22/0x30 arch/x86/include/asm/idtentry.h:570 RIP: 0010:copy_user_short_string+0xd/0x40 arch/x86/lib/copy_user_64.S:233 Code: 74 0a 89 (...) RSP: 0018:ffffc9000570f330 EFLAGS: 000502 ---truncated---
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Analysis
by VulDB Data Team • 06/01/2026
The vulnerability CVE-2023-52737 affects the Linux kernel's Btrfs file system implementation and stems from an improper locking mechanism during fiemap operations. The issue arises when the fiemap functionality does not acquire the inode's VFS lock before initiating operations on the file's extent mapping. This design flaw creates a potential deadlock condition that can occur during concurrent file system operations. The deadlock specifically manifests when a fiemap operation is executed simultaneously with an fsync operation on the same file. The root cause lies in the shared use of the inode's i_mmap_lock semaphore, which is acquired by both the fsync code path and the btrfs_page_mkwrite function during page write operations. This concurrency issue was identified by the syzbot fuzzer, which is a key tool in kernel security research and is often used to discover race conditions and deadlocks in kernel code.
The technical execution of this vulnerability involves a complex interplay of kernel subsystems including the virtual file system layer, the Btrfs file system implementation, and the memory management subsystem. When fiemap attempts to fill the user space buffer with extent information, it triggers a page fault that leads to the btrfs_page_mkwrite function being invoked. This function requires the i_mmap_lock semaphore, which is already held by the fsync operation, creating a circular wait condition. The deadlock occurs in the extent I/O tree management code where the lock_extent function attempts to acquire a lock that is already held by another thread. The stack trace demonstrates that the blocking thread is waiting on the rwsem_down_read_slowpath function, indicating that the read-write semaphore acquisition has failed due to the conflicting lock acquisition pattern.
The operational impact of this vulnerability is significant as it can result in a system-wide deadlock or hang, where the affected process becomes unresponsive and cannot make progress. This condition can persist for extended periods, as indicated by the hung task timeout message showing a process blocked for over 145 seconds. The vulnerability affects systems running Btrfs file systems and can potentially be exploited to cause denial of service conditions, particularly in environments where concurrent file I/O operations are common. The issue is particularly concerning in high-performance computing environments or storage systems where Btrfs is heavily utilized, as the deadlock can propagate through the system and affect multiple processes. According to CWE classification, this vulnerability aligns with CWE-367, which describes a Time-of-Check to Time-of-Use (TOCTOU) race condition, and also relates to CWE-121, which covers stack-based buffer overflow conditions that can result from improper locking.
The recommended mitigation for this vulnerability involves applying the kernel patch that ensures the inode is locked in shared mode before beginning fiemap operations. This fix aligns with the ATT&CK framework's technique T1499.004, which covers the exploitation of resource exhaustion conditions, and T1565.001, which deals with the manipulation of files and directories. The patch ensures that proper locking order is maintained between the file system's extent management code and the VFS layer, preventing the circular wait condition. System administrators should update their kernel versions to include this fix and monitor for any potential performance regressions that may occur due to the additional locking overhead. Additionally, organizations using Btrfs should implement monitoring for hung task conditions and consider implementing automated restart procedures for critical services that might be affected by such deadlocks. The fix represents a standard defensive programming approach that enforces proper locking semantics and prevents race conditions in concurrent access scenarios.