We report the crash: KASAN: use-after-free in raw_cmd_done This crash was found in v4.19-rc2 using Alexkidd-fuzzer (a modified version of Syzkaller). Our analysis shows that the race arose between a kernel thread (kworker) and the invoked ioctl syscall. kernel config: https://kt0755.github.io/etc/config-4.19-rc2 repro: https://kt0755.github.io/etc/repro.92596.c Analysis: When there is a race between "raw_cmd_done" (driver/block/floppy.c:3054) and "raw_cmd_free" (driver/block/floppy.c:3115), a pointer variable "raw_cmd" in raw_cmd_done may point to the freed memory area. When raw_cmd_done dereferences this pointer, use-after-free occurs. <Thread-0 (raw_cmd_ioctl)> <Thread-1 (floppy_shutdown)> 1. raw_cmd_copyin(&my_raw_cmd) // allocate my_raw_cmd 2. raw_cmd = my_raw_cmd 3. cont = &raw_cmd_cont 4. wait_til_done(floppy_start) ------------------------------------------------------------------------------------------- 5. cont->done(0) // call raw_cmd_done() ------------------------------------------------------------------------------------------ 6. raw_cmd_free(&my_raw_cmd) // kfree(my_raw_cmd) ------------------------------------------------------------------------------------------ 7. rmd_cmd->flag // Use-after-free # Call Sequence <Thread-0> fd_ioctl fd_locked_ioctl raw_cmd_ioctl raw_cmd_free <Thread-1> worker_thread process_one_work floppy_shutdown raw_cmd_done We observed that raw_cmd_ioctl (say T0) is invoked by fd_locked_ioctl (FDRAWCMD). It first allocates "raw_cmd" (driver/block/floppy.c:3212), and then wait until floppy_start is done. In the meantime, floppy_shutdown scheduled by floppy_start is called (say T1) and invokes raw_cmd_done. Unfortunately, before raw_cmd_done accesses raw_cmd, T0 waked up and deallocated raw_cmd in raw_cmd_free (driver/block/floppy.c:3225). So the access to raw_cmd (by T1) fails. We suspect that the insufficient locking between raw_cmd_done and raw_cnd_free, is the cause of the crash. Crash log ================================================================== BUG: KASAN: use-after-free in raw_cmd_done+0x58a/0x5e0 drivers/block/floppy.c:3054 Read of size 4 at addr ffff880119b35cc0 by task kworker/u4:0/7 CPU: 0 PID: 7 Comm: kworker/u4:0 Not tainted 4.19.0-rc2 #2 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011 Workqueue: floppy floppy_shutdown Call Trace: __dump_stack lib/dump_stack.c:77 [inline] dump_stack+0xd2/0x148 lib/dump_stack.c:113 print_address_description+0x7f/0x2c0 mm/kasan/report.c:256 kasan_report_error mm/kasan/report.c:354 [inline] kasan_report+0x271/0x3a0 mm/kasan/report.c:412 __asan_report_load4_noabort+0x19/0x20 mm/kasan/report.c:432 raw_cmd_done+0x58a/0x5e0 drivers/block/floppy.c:3054 floppy_shutdown+0x127/0x1f0 drivers/block/floppy.c:1851 process_one_work+0x923/0x1690 kernel/workqueue.c:2153 worker_thread+0x8f/0xcf0 kernel/workqueue.c:2296 kthread+0x3a3/0x470 kernel/kthread.c:246 ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:413 Allocated by task 16211: save_stack+0x43/0xd0 mm/kasan/kasan.c:448 set_track mm/kasan/kasan.c:460 [inline] kasan_kmalloc+0xc5/0xf0 mm/kasan/kasan.c:553 kmem_cache_alloc_trace+0x14a/0x2b0 mm/slub.c:2733 kmalloc include/linux/slab.h:513 [inline] raw_cmd_copyin drivers/block/floppy.c:3130 [inline] raw_cmd_ioctl drivers/block/floppy.c:3206 [inline] fd_locked_ioctl+0x608/0x21b0 drivers/block/floppy.c:3544 fd_ioctl+0x40/0x60 drivers/block/floppy.c:3571 __blkdev_driver_ioctl block/ioctl.c:303 [inline] blkdev_ioctl+0xb48/0x1a50 block/ioctl.c:601 block_ioctl+0x105/0x150 fs/block_dev.c:1883 vfs_ioctl fs/ioctl.c:46 [inline] do_vfs_ioctl+0x1c0/0x1150 fs/ioctl.c:687 ksys_ioctl+0x9e/0xb0 fs/ioctl.c:702 __do_sys_ioctl fs/ioctl.c:709 [inline] __se_sys_ioctl fs/ioctl.c:707 [inline] __x64_sys_ioctl+0x7e/0xc0 fs/ioctl.c:707 do_syscall_64+0xc4/0x4e0 arch/x86/entry/common.c:290 entry_SYSCALL_64_after_hwframe+0x49/0xbe Freed by task 16211: save_stack+0x43/0xd0 mm/kasan/kasan.c:448 set_track mm/kasan/kasan.c:460 [inline] __kasan_slab_free+0x14d/0x1a0 mm/kasan/kasan.c:521 kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528 slab_free_hook mm/slub.c:1371 [inline] slab_free_freelist_hook mm/slub.c:1398 [inline] slab_free mm/slub.c:2953 [inline] kfree+0x109/0x2f0 mm/slub.c:3906 raw_cmd_free+0x82/0x170 drivers/block/floppy.c:3115 raw_cmd_ioctl drivers/block/floppy.c:3225 [inline] fd_locked_ioctl+0x1d37/0x21b0 drivers/block/floppy.c:3544 fd_ioctl+0x40/0x60 drivers/block/floppy.c:3571 __blkdev_driver_ioctl block/ioctl.c:303 [inline] blkdev_ioctl+0xb48/0x1a50 block/ioctl.c:601 block_ioctl+0x105/0x150 fs/block_dev.c:1883 vfs_ioctl fs/ioctl.c:46 [inline] do_vfs_ioctl+0x1c0/0x1150 fs/ioctl.c:687 ksys_ioctl+0x9e/0xb0 fs/ioctl.c:702 __do_sys_ioctl fs/ioctl.c:709 [inline] __se_sys_ioctl fs/ioctl.c:707 [inline] __x64_sys_ioctl+0x7e/0xc0 fs/ioctl.c:707 do_syscall_64+0xc4/0x4e0 arch/x86/entry/common.c:290 entry_SYSCALL_64_after_hwframe+0x49/0xbe The buggy address belongs to the object at ffff880119b35cc0 which belongs to the cache kmalloc-128 of size 128 The buggy address is located 0 bytes inside of 128-byte region [ffff880119b35cc0, ffff880119b35d40) The buggy address belongs to the page: page:ffffea000466cd40 count:1 mapcount:0 mapping:ffff88011a403500 index:0x0 flags: 0x200000000000100(slab) raw: 0200000000000100 ffffea0004042700 0000000700000007 ffff88011a403500 raw: 0000000000000000 0000000000150015 00000001ffffffff 0000000000000000 page dumped because: kasan: bad access detected Memory state around the buggy address: ffff880119b35b80: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc ffff880119b35c00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb >ffff880119b35c80: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb ^ ffff880119b35d00: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc ffff880119b35d80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb ==================================================================
