Re: [PATCH] nilfs2: fix NULL pointer dereference in nilfs_segctor_prepare_write()

From: Liu Shixin
Date: Tue Nov 08 2022 - 22:26:38 EST



On 2022/11/9 1:50, Ryusuke Konishi wrote:
> Hi Liu Shixin,
>
> I'm sorry for my repeated emails.
>
> On Wed, Nov 9, 2022 at 12:13 AM Ryusuke Konishi wrote:
>>> I think the most likely cause is metadata corruption. If so, we
>>> should fix it by adding a proper sanity check, yes.
>>> However, there is still the possibility that the error retry logic
>>> after detecting errors has a flaw. (I believe at least this is not a
>>> problem with normal paths.)
>>> The sufile state inconsistency above is hypothetical for now.
>>> Either way, I'd like to make sure what's actually happening (and where
>>> the anomaly is coming from) so we can decide how to fix it.
>> I noticed that this syzbot report has a disk image "mount_0.gz", so I
>> tried to mount it read-only.
>> The result was as follows:
>>
>> $ sudo mount -t nilfs2 -r ./mount_0 /mnt/test
>> $ lssu
>> SEGNUM DATE TIME STAT NBLOCKS
>> 0 26760730-10-29 19:40:00 -de 527958016
>> 1 26760730-11-01 20:29:04 -de 527958016
>> 2 1176433641-11-01 02:01:52 -de 2983038235
>> 3 -1158249729-11-01 04:57:19 a-- 25375
>> 4 1970-01-02 21:24:32 a-- 0
>> 5 -1415215929-01-02 00:19:15 --e 1631451365
>> 6 841067190-01-02 13:02:59 -d- 3101461260
>> 7 1495233207-01-02 01:31:11 -de 1697748441
>> 8 1875753393-01-02 21:54:14 -de 337757600
>> 9 648878284-01-02 21:06:08 --- 2133388152
>> 10 2122778986-01-02 17:49:43 --e 874605800
>> 11 55846197-01-02 09:50:53 -de 4262365368
>> 12 1872396026-01-02 06:45:18 --- 1051768258
>> 13 820920473-01-02 19:28:35 --e 2932336675
>> 14 2128306755-01-02 10:17:45 --- 3568501216
>> 15 1457063063-01-02 01:24:17 --e 2330511560
>> 16 586864775-01-02 16:08:15 --- 355283425
>> 17 -824870041-01-02 22:47:26 -d- 4177999057
>> 18 1562176264-01-02 08:06:45 --- 1312597355
>> 19 -392925420-01-02 17:08:27 -d- 3811075948
>> 20 1927575458-01-02 21:26:51 -de 1384562525
>> 21 2139923505-01-02 08:16:04 -d- 41861305
>>
>> Here, we can see that neither segment #3 (= ns_segnum) nor #4 (=
>> ns_nextnum) has the dirty flag
>> ("d" in STAT). So, as expected, this seems to be the root cause of
>> the duplicate assignments and oops.
>> The proper correction would be, therefore, to check and reject (or
>> fix) this anomaly while outputting an error message
>> (or warning if fix it at the same time).
>> It should be inserted in mount time logic because the segments that
>> nilfs2 itself allocates are always marked dirty with
>> nilfs_sufile_alloc().
> I will change my opinion.
>
> Considering the possibility of sufile corruption at runtime, it seems
> that the sanity check should be done on every nilfs_sufile_alloc()
> call.
>
> I now think nilfs_sufile_alloc() should call nilfs_error() and return
> -EIO if the number of a newly found vacant segment matches
> nilfs->ns_segnum or nilfs->ns_nextnum.
Before we add the first segbuf into sci->sc_segbufs in nilfs_segctor_begin_construction(),
there is no possibility existing duplicate segnum. And the subsequent process should
not be affected by sufile corruption. So I think it's enough to only check for case alloc==0.
I don't find any other possible wrong scenarios.

diff --git a/fs/nilfs2/segment.c b/fs/nilfs2/segment.c
index 7be632c15f91..7b91c790b798 100644
--- a/fs/nilfs2/segment.c
+++ b/fs/nilfs2/segment.c
@@ -1326,7 +1326,13 @@ static int nilfs_segctor_begin_construction(struct nilfs_sc_info *sci,
err = nilfs_sufile_alloc(nilfs->ns_sufile, &nextnum);
if (err)
goto failed;
+ } else {
+ /* Check the next segment has alreadly been allocated */
+ err = nilfs_sufile_test_allocated(nilfs->ns_sufile, nextnum);
+ if (err)
+ goto failed;
}
+
nilfs_segbuf_set_next_segnum(segbuf, nextnum, nilfs);

BUG_ON(!list_empty(&sci->sc_segbufs));
diff --git a/fs/nilfs2/sufile.c b/fs/nilfs2/sufile.c
index 853a8212114f..8dff12c56891 100644
--- a/fs/nilfs2/sufile.c
+++ b/fs/nilfs2/sufile.c
@@ -399,6 +399,36 @@ int nilfs_sufile_alloc(struct inode *sufile, __u64 *segnump)
return ret;
}

+int nilfs_sufile_test_allocated(struct inode *sufile, __u64 *segnump)
+{
+ struct buffer_head *su_bh;
+ struct nilfs_segment_usage *su;
+ void *kaddr;
+ int ret;
+
+ down_write(&NILFS_MDT(sufile)->mi_sem);
+
+ ret = nilfs_sufile_get_segment_usage_block(sufile, segnump, 1,
+ &su_bh);
+ if (ret < 0)
+ goto out_sem;
+
+ kaddr = kmap_atomic(su_bh->b_page);
+ su = nilfs_sufile_block_get_segment_usage(
+ sufile, segnump, su_bh, kaddr);
+
+ if (nilfs_segment_usage_clean(su))
+ ret = -EIO;
+
+ kunmap_atomic(kaddr);
+
+ brelse(su_bh);
+
+out_sem:
+ up_write(&NILFS_MDT(sufile)->mi_sem);
+ return ret;
+}
+
void nilfs_sufile_do_cancel_free(struct inode *sufile, __u64 segnum,
struct buffer_head *header_bh,
struct buffer_head *su_bh)
diff --git a/fs/nilfs2/sufile.h b/fs/nilfs2/sufile.h
index 8e8a1a5a0402..02b61ca6f318 100644
--- a/fs/nilfs2/sufile.h
+++ b/fs/nilfs2/sufile.h
@@ -24,6 +24,7 @@ unsigned long nilfs_sufile_get_ncleansegs(struct inode *sufile);

int nilfs_sufile_set_alloc_range(struct inode *sufile, __u64 start, __u64 end);
int nilfs_sufile_alloc(struct inode *, __u64 *);
+int nilfs_sufile_test_allocated(struct inode *, __u64 *);
int nilfs_sufile_mark_dirty(struct inode *sufile, __u64 segnum);
int nilfs_sufile_set_segment_usage(struct inode *sufile, __u64 segnum,
unsigned long nblocks, time64_t modtime);

>
> This test should not need to be done for every segbuf's sb_segnum
> because metadata blocks that contain the information about allocated
> segments are kept in memory because of its dirty state and will not be
> destroyed until they are finally written out.
>
> One correction then. Just checking the output of the lssu command
> wasn't enough because the following test is done on all flags except
> the active flag "a". (this flag is not persistent and visible only
> when seeing via ioctl.)
>
>> if (!nilfs_segment_usage_clean(su))
>> continue;
>> /* found a clean segment */
> We also had to see the invisible flags as well to confirm if that is
> the root cause, but lssu doens't show them all. So I checked the dump
> data of the mount_0 file.
> As a result, the segment at ns_segnum was not clean, but that of
> ns_nextnum was clean, which means it's the root cause as expected.
Thanks for your confirmation.

Regards,
Liu Shixin
.
>
> Regards,
> Ryusuke Konishi
> .
>