Messages in this thread | | | Date | Wed, 15 Feb 2023 11:53:40 +0000 | Subject | Re: [PATCH] ext4: reject 1k block fs on the first block of disk | From | Tudor Ambarus <> |
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On 2/15/23 11:46, Tudor Ambarus wrote: > Hi, Ted! > > On 2/15/23 04:32, Theodore Ts'o wrote: >> On Wed, Jan 04, 2023 at 09:58:03AM +0800, Jun Nie wrote: >>> Darrick J. Wong <djwong@kernel.org> 于2023年1月4日周三 03:17写道: >>>> >>>> On Thu, Dec 29, 2022 at 09:45:02AM +0800, Jun Nie wrote: >>>>> For 1k-block filesystems, the filesystem starts at block 1, not >>>>> block 0. >>>>> If start_fsb is 0, it will be bump up to s_first_data_block. Then >>>>> ext4_get_group_no_and_offset don't know what to do and return garbage >>>>> results (blockgroup 2^32-1). The underflow make index >>>>> exceed es->s_groups_count in ext4_get_group_info() and trigger the >>>>> BUG_ON. >>>>> >>>>> Fixes: 4a4956249dac0 ("ext4: fix off-by-one fsmap error on 1k block >>>>> filesystems") >>>>> Link: >>>>> https://syzkaller.appspot.com/bug?id=79d5768e9bfe362911ac1a5057a36fc6b5c30002 >>>>> Reported-by: syzbot+6be2b977c89f79b6b153@syzkaller.appspotmail.com >>>>> Signed-off-by: Jun Nie <jun.nie@linaro.org> >>>>> --- >>>>> fs/ext4/fsmap.c | 6 ++++++ >>>>> 1 file changed, 6 insertions(+) >>>>> >>>>> diff --git a/fs/ext4/fsmap.c b/fs/ext4/fsmap.c >>>>> index 4493ef0c715e..1aef127b0634 100644 >>>>> --- a/fs/ext4/fsmap.c >>>>> +++ b/fs/ext4/fsmap.c >>>>> @@ -702,6 +702,12 @@ int ext4_getfsmap(struct super_block *sb, >>>>> struct ext4_fsmap_head *head, >>>>> if (handlers[i].gfd_dev > head->fmh_keys[0].fmr_device) >>>>> memset(&dkeys[0], 0, sizeof(struct >>>>> ext4_fsmap)); >>>>> >>>>> + /* >>>>> + * Re-check the range after above limit operation and >>>>> reject >>>>> + * 1K fs on block 0 as fs should start block 1. */ >>>>> + if (dkeys[0].fmr_physical ==0 && >>>>> dkeys[1].fmr_physical == 0) >>>>> + continue; >>>> >>>> ...and if this filesystem has 4k blocks, and therefore *does* define a >>>> block 0? >>> >>> Yes, this is a real corner case test :-) >> >> So I'm really nervous about this change. I don't understand the code; >> and I don't understand how the reproducer works. I can certainly >> reproduce it using the reproducer found here[1], but it seems to >> require running multiple processes all creating loop devices and then >> running FS_IOC_GETMAP. >> >> [1] >> https://syzkaller.appspot.com/bug?id=79d5768e9bfe362911ac1a5057a36fc6b5c30002 >> >> If I change the reproducer to just run the execute_one() once, it >> doesn't trigger the bug. It seems to only trigger when you have >> multiple processes all racing to create a loop device, mount the file >> system, try running FS_IOC_GETMAP --- and then delete the loop device >> without actually unmounting the file system. Which is **weird***. >> >> I've tried taking the image, and just running "xfs_io -c fsmap /mnt", >> and that doesn't trigger it either. >> >> And I don't understand the reply to Darrick's question about why it's >> safe to add the check since for 4k block file systems, block 0 *is* >> valid. >> >> So if someone can explain to me what is going on here with this code >> (there are too many abstractions and what's going on with keys is just >> making my head hurt), *and* what the change actually does, and how to >> reproduce the problem with a ***simple*** reproducer -- the syzbot >> mess doesn't count, that would be great. But applying a change that I >> don't understand to code I don't understand, to fix a reproducer which >> I also doesn't understand, just doesn't make me feel comfortable. >> > > Let me share what I understood until now. The low key is zeroed. The > high key is defined and uses a fmr_physical of value zero, which is > smaller than the first data block for the 1k-block ext4 fs (which starts > at offset 1024). > > -> ext4_getfsmap_datadev() > keys[0].fmr_physical = 0, keys[1].fmr_physical = 0 > bofs = le32_to_cpu(sbi->s_es->s_first_data_block) = 1, eofs = 256 > start_fsb = keys[0].fmr_physical = 1, end_fsb = keys[1].fmr_physical = 0 > -> ext4_get_group_no_and_offset() > blocknr = 1, le32_to_cpu(es->s_first_data_block) =1 > start_ag = 0, first_cluster = 0 > -> > blocknr = 0, le32_to_cpu(es->s_first_data_block) =1 > end_ag = 4294967295, last_cluster = 8191
because of poor key validation we get a wrong end_ag which eventually causes the BUG_ON.
> > Then there's a loop that stops when info->gfi_agno <= end_ag; that > will trigger the BUG_ON in ext4_get_group_info() as the group nr exceeds > EXT4_SB(sb)->s_groups_count) > -> ext4_mballoc_query_range() > -> ext4_mb_load_buddy() > -> ext4_mb_load_buddy_gfp() > -> ext4_get_group_info() > > It's an out of bounds request and Darrick suggested to not return any > mapping for the byte range 0-1023 for the 1k-block filesystem. The > alternative would be to return -EINVAL when the high key starts at > fmr_phisical of value zero for the 1k-block fs. > > In order to reproduce this one would have to create an 1k-block ext4 fs > and to pass a high key with fmr_physical of value zero, thus I would > expect to reproduce it with something like this: > xfs_io -c 'fsmap -d 0 0' /mnt/scratch > > However when doing this I notice that in > xfsprogs-dev/io/fsmap.c l->fmr_device and h->fmr_device will have value > zero, FS_IOC_GETFSMAP is called and then we receive no entries > (head->fmh_entries = 0). Now I'm trying to see what I do wrong, and how > to reproduce the bug. > > Cheers, > ta
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