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

From: Ryusuke Konishi
Date: Fri Nov 18 2022 - 20:34:47 EST

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Hi, ZhangPeng,

On Tue, Nov 15, 2022 at 3:39 AM Ryusuke Konishi wrote:
>
> On Mon, Nov 14, 2022 at 12:39 PM Peng Zhang wrote:
> >
> > From: ZhangPeng <zhangpeng362@xxxxxxxxxx>
> >
> > Syzbot reported a null-ptr-deref bug:
> >
> > NILFS (loop0): segctord starting. Construction interval = 5 seconds, CP
> > frequency < 30 seconds
> > general protection fault, probably for non-canonical address
> > 0xdffffc0000000002: 0000 [#1] PREEMPT SMP KASAN
> > KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
> > CPU: 1 PID: 3603 Comm: segctord Not tainted
> > 6.1.0-rc2-syzkaller-00105-gb229b6ca5abb #0
> > Hardware name: Google Compute Engine/Google Compute Engine, BIOS Google
> > 10/11/2022
> > RIP: 0010:nilfs_palloc_commit_free_entry+0xe5/0x6b0
> > fs/nilfs2/alloc.c:608
> > Code: 00 00 00 00 fc ff df 80 3c 02 00 0f 85 cd 05 00 00 48 b8 00 00 00
> > 00 00 fc ff df 4c 8b 73 08 49 8d 7e 10 48 89 fa 48 c1 ea 03 <80> 3c 02
> > 00 0f 85 26 05 00 00 49 8b 46 10 be a6 00 00 00 48 c7 c7
> > RSP: 0018:ffffc90003dff830 EFLAGS: 00010212
> > RAX: dffffc0000000000 RBX: ffff88802594e218 RCX: 000000000000000d
> > RDX: 0000000000000002 RSI: 0000000000002000 RDI: 0000000000000010
> > RBP: ffff888071880222 R08: 0000000000000005 R09: 000000000000003f
> > R10: 000000000000000d R11: 0000000000000000 R12: ffff888071880158
> > R13: ffff88802594e220 R14: 0000000000000000 R15: 0000000000000004
> > FS: 0000000000000000(0000) GS:ffff8880b9b00000(0000)
> > knlGS:0000000000000000
> > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007fb1c08316a8 CR3: 0000000018560000 CR4: 0000000000350ee0
> > Call Trace:
> > <TASK>
> > nilfs_dat_commit_free fs/nilfs2/dat.c:114 [inline]
> > nilfs_dat_commit_end+0x464/0x5f0 fs/nilfs2/dat.c:193
> > nilfs_dat_commit_update+0x26/0x40 fs/nilfs2/dat.c:236
> > nilfs_btree_commit_update_v+0x87/0x4a0 fs/nilfs2/btree.c:1940
> > nilfs_btree_commit_propagate_v fs/nilfs2/btree.c:2016 [inline]
> > nilfs_btree_propagate_v fs/nilfs2/btree.c:2046 [inline]
> > nilfs_btree_propagate+0xa00/0xd60 fs/nilfs2/btree.c:2088
> > nilfs_bmap_propagate+0x73/0x170 fs/nilfs2/bmap.c:337
> > nilfs_collect_file_data+0x45/0xd0 fs/nilfs2/segment.c:568
> > nilfs_segctor_apply_buffers+0x14a/0x470 fs/nilfs2/segment.c:1018
> > nilfs_segctor_scan_file+0x3f4/0x6f0 fs/nilfs2/segment.c:1067
> > nilfs_segctor_collect_blocks fs/nilfs2/segment.c:1197 [inline]
> > nilfs_segctor_collect fs/nilfs2/segment.c:1503 [inline]
> > nilfs_segctor_do_construct+0x12fc/0x6af0 fs/nilfs2/segment.c:2045
> > nilfs_segctor_construct+0x8e3/0xb30 fs/nilfs2/segment.c:2379
> > nilfs_segctor_thread_construct fs/nilfs2/segment.c:2487 [inline]
> > nilfs_segctor_thread+0x3c3/0xf30 fs/nilfs2/segment.c:2570
> > kthread+0x2e4/0x3a0 kernel/kthread.c:376
> > ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:306
> > </TASK>
> > Modules linked in:
> > ---[ end trace 0000000000000000 ]---
> > RIP: 0010:nilfs_palloc_commit_free_entry+0xe5/0x6b0
> > fs/nilfs2/alloc.c:608
> > Code: 00 00 00 00 fc ff df 80 3c 02 00 0f 85 cd 05 00 00 48 b8 00 00 00
> > 00 00 fc ff df 4c 8b 73 08 49 8d 7e 10 48 89 fa 48 c1 ea 03 <80> 3c 02
> > 00 0f 85 26 05 00 00 49 8b 46 10 be a6 00 00 00 48 c7 c7
> > RSP: 0018:ffffc90003dff830 EFLAGS: 00010212
> > RAX: dffffc0000000000 RBX: ffff88802594e218 RCX: 000000000000000d
> > RDX: 0000000000000002 RSI: 0000000000002000 RDI: 0000000000000010
> > RBP: ffff888071880222 R08: 0000000000000005 R09: 000000000000003f
> > R10: 000000000000000d R11: 0000000000000000 R12: ffff888071880158
> > R13: ffff88802594e220 R14: 0000000000000000 R15: 0000000000000004
> > FS: 0000000000000000(0000) GS:ffff8880b9b00000(0000)
> > knlGS:0000000000000000
> > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007fb1c08316a8 CR3: 0000000018560000 CR4: 0000000000350ee0
> > ----------------
> > Code disassembly (best guess), 7 bytes skipped:
> > 0: 80 3c 02 00 cmpb $0x0,(%rdx,%rax,1)
> > 4: 0f 85 cd 05 00 00 jne 0x5d7
> > a: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax
> > 11: fc ff df
> > 14: 4c 8b 73 08 mov 0x8(%rbx),%r14
> > 18: 49 8d 7e 10 lea 0x10(%r14),%rdi
> > 1c: 48 89 fa mov %rdi,%rdx
> > 1f: 48 c1 ea 03 shr $0x3,%rdx
> > * 23: 80 3c 02 00 cmpb $0x0,(%rdx,%rax,1) <-- trapping
> > instruction
> > 27: 0f 85 26 05 00 00 jne 0x553
> > 2d: 49 8b 46 10 mov 0x10(%r14),%rax
> > 31: be a6 00 00 00 mov $0xa6,%esi
> > 36: 48 rex.W
> > 37: c7 .byte 0xc7
> > 38: c7 .byte 0xc7
> >
> > When maxlevelp is 1, there is a case where req->pr_desc_bh is NULL and
> > blocknr is 0, because nilfs_dat_commit_alloc() will modify the blocknr
> > of oldreq at one level higher to 0. And we don't have a NULL check on
> > req->pr_desc_bh and req->pr_bitmap_bh in
> > nilfs_palloc_commit_free_entry() function, so when req->pr_desc_bh is
> > NULL and kmap() dereferences a NULL pointer, it leads to above crash.
> > Fix this by adding a NULL check on req->pr_desc_bh and req->pr_bitmap_bh
> > before nilfs_palloc_commit_free_entry() in nilfs_dat_commit_free().
> >
> > Reported-by: syzbot+ebe05ee8e98f755f61d0@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Fixes: bd8169efae8b ("nilfs2: add update functions of virtual block address to dat")
> > Signed-off-by: ZhangPeng <zhangpeng362@xxxxxxxxxx>
> > ---
> > fs/nilfs2/dat.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > diff --git a/fs/nilfs2/dat.c b/fs/nilfs2/dat.c
> > index 3b55e239705f..84ce050a8fa3 100644
> > --- a/fs/nilfs2/dat.c
> > +++ b/fs/nilfs2/dat.c
> > @@ -111,6 +111,9 @@ static void nilfs_dat_commit_free(struct inode *dat,
> > kunmap_atomic(kaddr);
> >
> > nilfs_dat_commit_entry(dat, req);
> > +
> > + if (req->pr_desc_bh == NULL || req->pr_bitmap_bh == NULL)
> > + return;
> > nilfs_palloc_commit_free_entry(dat, req);
> > }
> >
> > --
> > 2.25.1
> >
>
> Thank you for your help.
>
> This patch actually fixes the NULL pointer dereference, and doesn't
> seem to cause any regressions so far.
> But, if de_blocknr is 0 in the first place, it assumes that
> req->pr_desc_bh and req->pr_bitmap_bh are set properly in the
> corresponding "prepare" operation, nilfs_dat_prepare_end(), so I'd
> like to review in a little more detail why that is broken.
>
> Thanks,
> Ryusuke Konishi

What was happening here was complicated.
There were two state inconsistencies in DAT:
1) A virtual block number (vblocknr == 6145) in the report, was used
twice in a btree.
Because of this, nilfs_dat_commit_end() on that address was called twice.
2) For the virtual block number (vblocknr == 6145), the DAT bitmap
status was free (second anomaly), and nilfs_dat_commit_alloc() was
performed at the same time as the second nilfs_dat_commit_end() above.

As I mentioned, when nilfs_dat_commit_end() frees a virtual block
number in the bitmap of DAT, nilfs_dat_prepare_end() usually allocates
buffer heads of the bitmap block and the descriptor block, and sets
them to the request struct. But, the above two anomalies overlap, as
you pointed out, de_block is initialized to 0 by
nilfs_dat_prepare_alloc() in the middle of "prepare" and "commit", and
nilfs_dat_commit_end() tries to use unallocated buffer heads.

As a workaround, your patch only needs two minor fixes that I can handle:
1) The following part should use unlikely() annotation
+ if (req->pr_desc_bh == NULL || req->pr_bitmap_bh == NULL)
like
+ if (unlikely(req->pr_desc_bh == NULL || req->pr_bitmap_bh == NULL))
because this usually doesn't happen.
2) The "Fixes" tag should be removed, because it's not the cause of this bug.

The problem is that this fix doesn't eliminate the double usage of the
virtual block number, and it leaves de_start and de_end of the virtual
block number broken, which leads to disk GC failure.

So, at least, we should call nilfs_error() to signal that the
filesystem is fatally flawed and prevent further operations.

Please give me a few more days to consider how to deal with it. I
would like to think about whether there is a good solution including
the GC problem.

As for the current outlook, it seems difficult to eliminate
duplication of the virtual block number, so I think we will call
nilfs_error() as follows:

nilfs_dat_commit_entry(dat, req);
+ if (unlikely(req->pr_desc_bh == NULL || req->pr_bitmap_bh == NULL)) {
+ nilfs_error(dat->i_sb,
+ "state inconsistency due to duplicate use
of vblocknr = %llu",
+ (unsigned long long)req->pr_entry_nr);
+ return;
+ }
nilfs_palloc_commit_free_entry(dat, req);

In that case, I would like to modify your patch and send it upstream
as your patch. Is that OK ?
Or do you want to fix it more by yourself ?

Thanks,
Ryusuke Konishi

Next message: Gao Xiang: "Re: [PATCH v4 2/2] erofs: switch to prepare_ondemand_read() in fscache mode"
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