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Date: Thu, 7 Jan 2021 10:58:39 +1100 From: Alexey Kardashevskiy <aik@...abs.ru> To: Jan Kara <jack@...e.cz> Cc: Christoph Hellwig <hch@....de>, Alexander Viro <viro@...iv.linux.org.uk>, Hannes Reinecke <hare@...e.de>, Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>, linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [RFC PATCH kernel] block: initialize block_device::bd_bdi for bdev_cache On 06/01/2021 21:41, Jan Kara wrote: > On Wed 06-01-21 20:29:00, Alexey Kardashevskiy wrote: >> This is a workaround to fix a null derefence crash: >> >> [c00000000b01f840] c00000000b01f880 (unreliable) >> [c00000000b01f880] c000000000769a3c bdev_evict_inode+0x21c/0x370 >> [c00000000b01f8c0] c00000000070bacc evict+0x11c/0x230 >> [c00000000b01f900] c00000000070c138 iput+0x2a8/0x4a0 >> [c00000000b01f970] c0000000006ff030 dentry_unlink_inode+0x220/0x250 >> [c00000000b01f9b0] c0000000007001c0 __dentry_kill+0x190/0x320 >> [c00000000b01fa00] c000000000701fb8 dput+0x5e8/0x860 >> [c00000000b01fa80] c000000000705848 shrink_dcache_for_umount+0x58/0x100 >> [c00000000b01fb00] c0000000006cf864 generic_shutdown_super+0x54/0x200 >> [c00000000b01fb80] c0000000006cfd48 kill_anon_super+0x38/0x60 >> [c00000000b01fbc0] c0000000006d12cc deactivate_locked_super+0xbc/0x110 >> [c00000000b01fbf0] c0000000006d13bc deactivate_super+0x9c/0xc0 >> [c00000000b01fc20] c00000000071a340 cleanup_mnt+0x1b0/0x250 >> [c00000000b01fc80] c000000000278fa8 task_work_run+0xf8/0x180 >> [c00000000b01fcd0] c00000000002b4ac do_notify_resume+0x4dc/0x5d0 >> [c00000000b01fda0] c00000000004ba0c syscall_exit_prepare+0x28c/0x370 >> [c00000000b01fe10] c00000000000e06c system_call_common+0xfc/0x27c >> --- Exception: c00 (System Call) at 0000000010034890 >> >> Is this fixed properly already somewhere? Thanks, >> >> Fixes: e6cb53827ed6 ("block: initialize struct block_device in bdev_alloc") > > I don't think it's fixed anywhere and I've seen the syzbot report and I was > wondering how this can happen when bdev_alloc() initializes bdev->bd_bdi > and it also wasn't clear to me whether bd_bdi is really the only field that > is problematic - if we can get to bdev_evict_inode() without going through > bdev_alloc(), we are probably missing initialization of other fields in > that place as well... > > But now I've realized that probably the inode is a root inode for bdev > superblock which is allocated by VFS through new_inode() and thus doesn't > undergo the initialization in bdev_alloc(). yup, this is the case. > And AFAICT the root inode on > bdev superblock can get only to bdev_evict_inode() and bdev_free_inode(). > Looking at bdev_evict_inode() the only thing that's used there from struct > block_device is really bd_bdi. bdev_free_inode() will also access > bdev->bd_stats and bdev->bd_meta_info. So we need to at least initialize > these to NULL as well. These are all NULL. > IMO the most logical place for all these > initializations is in bdev_alloc_inode()... This works. We can also check for NULL where it crashes. But I do not know the code to make an informed decision... > > Honza > >> --- >> fs/block_dev.c | 2 ++ >> 1 file changed, 2 insertions(+) >> >> diff --git a/fs/block_dev.c b/fs/block_dev.c >> index 3e5b02f6606c..86fdc28d565e 100644 >> --- a/fs/block_dev.c >> +++ b/fs/block_dev.c >> @@ -792,8 +792,10 @@ static void bdev_free_inode(struct inode *inode) >> static void init_once(void *data) >> { >> struct bdev_inode *ei = data; >> + struct block_device *bdev = &ei->bdev; >> >> inode_init_once(&ei->vfs_inode); >> + bdev->bd_bdi = &noop_backing_dev_info; >> } >> >> static void bdev_evict_inode(struct inode *inode) >> -- >> 2.17.1 >> -- Alexey
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