btrfs: lock the inode in shared mode before starting fiemap
CVE: CVE-2023-52737
Component: kernel
CWE: CWE-667
Severity: Moderate
CVSS v3: 5.6/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Release date: 2023-01-01T00:00:00+00:00

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---


Remediation: