Re: [PATCH] fs: fix a data race in i_size_write/i_size_read

[Al Viro]

On Tue, Feb 18, 2020 at 11:04:26PM -0500, Qian Cai wrote:
> inode::i_size could be accessed concurently as noticed by KCSAN,
> 
>  BUG: KCSAN: data-race in iomap_do_writepage / iomap_write_end
> 
>  write to 0xffff8bf68fc0cac0 of 8 bytes by task 7484 on cpu 71:
>   iomap_write_end+0xea/0x530
>   i_size_write at include/linux/fs.h:888
>   (inlined by) iomap_write_end at fs/iomap/buffered-io.c:782
>   iomap_write_actor+0x132/0x200
>   iomap_apply+0x245/0x8a5
>   iomap_file_buffered_write+0xbd/0xf0
>   xfs_file_buffered_aio_write+0x1c2/0x790 [xfs]
>   xfs_file_write_iter+0x232/0x2d0 [xfs]
>   new_sync_write+0x29c/0x3b0
>   __vfs_write+0x92/0xa0
>   vfs_write+0x103/0x260
>   ksys_write+0x9d/0x130
>   __x64_sys_write+0x4c/0x60
>   do_syscall_64+0x91/0xb05
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
>  read to 0xffff8bf68fc0cac0 of 8 bytes by task 5901 on cpu 70:
>   iomap_do_writepage+0xf4/0x450
>   i_size_read at include/linux/fs.h:866
>   (inlined by) iomap_do_writepage at fs/iomap/buffered-io.c:1558
>   write_cache_pages+0x523/0xb20
>   iomap_writepages+0x47/0x80
>   xfs_vm_writepages+0xc7/0x100 [xfs]
>   do_writepages+0x5e/0x130
>   __writeback_single_inode+0xd5/0xb20
>   writeback_sb_inodes+0x429/0x910
>   __writeback_inodes_wb+0xc4/0x150
>   wb_writeback+0x47b/0x830
>   wb_workfn+0x688/0x930
>   process_one_work+0x54f/0xb90
>   worker_thread+0x80/0x5f0
>   kthread+0x1cd/0x1f0
>   ret_from_fork+0x27/0x50
> 
>  Reported by Kernel Concurrency Sanitizer on:
>  CPU: 70 PID: 5901 Comm: kworker/u257:2 Tainted: G             L    5.6.0-rc2-next-20200218+ #2
>  Hardware name: HPE ProLiant DL385 Gen10/ProLiant DL385 Gen10, BIOS A40 07/10/2019
>  Workqueue: writeback wb_workfn (flush-254:0)
> 
> The write is protected by exclusive inode::i_rwsem (in
> xfs_file_buffered_aio_write()) but the read is not. A shattered value
> could introduce a logic bug. Fixed it using a pair of WRITE/READ_ONCE().

If aligned 64bit stores on 64bit host (note the BITS_PER_LONG ifdefs) end up
being split, the kernel is FUBAR anyway.  Details, please - how could that
end up happening?