Re: [PATCH 6.1 1/2] erofs: handle overlapped pclusters out of crafted images properly

[Gao Xiang]

Hi Fedor,

On 2025/3/2 18:56, Fedor Pchelkin wrote:
> On Fri, 28. Feb 19:51, Alexey Panov wrote:
> > From: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>
> >
> > commit 9e2f9d34dd12e6e5b244ec488bcebd0c2d566c50 upstream.
> >
> > syzbot reported a task hang issue due to a deadlock case where it is
> > waiting for the folio lock of a cached folio that will be used for
> > cache I/Os.
> >
> > After looking into the crafted fuzzed image, I found it's formed with
> > several overlapped big pclusters as below:
> >
> >   Ext:   logical offset   |  length :     physical offset    |  length
> >     0:        0..   16384 |   16384 :     151552..    167936 |   16384
> >     1:    16384..   32768 |   16384 :     155648..    172032 |   16384
> >     2:    32768..   49152 |   16384 :  537223168.. 537239552 |   16384
> > ...
> >
> > Here, extent 0/1 are physically overlapped although it's entirely
> > _impossible_ for normal filesystem images generated by mkfs.
> >
> > First, managed folios containing compressed data will be marked as
> > up-to-date and then unlocked immediately (unlike in-place folios) when
> > compressed I/Os are complete.  If physical blocks are not submitted in
> > the incremental order, there should be separate BIOs to avoid dependency
> > issues.  However, the current code mis-arranges z_erofs_fill_bio_vec()
> > and BIO submission which causes unexpected BIO waits.
> >
> > Second, managed folios will be connected to their own pclusters for
> > efficient inter-queries.  However, this is somewhat hard to implement
> > easily if overlapped big pclusters exist.  Again, these only appear in
> > fuzzed images so let's simply fall back to temporary short-lived pages
> > for correctness.
> >
> > Additionally, it justifies that referenced managed folios cannot be
> > truncated for now and reverts part of commit 2080ca1ed3e4 ("erofs: tidy
> > up `struct z_erofs_bvec`") for simplicity although it shouldn't be any
> > difference.
> >
> > Reported-by: syzbot+4fc98ed414ae63d1ada2@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Reported-by: syzbot+de04e06b28cfecf2281c@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Reported-by: syzbot+c8c8238b394be4a1087d@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Tested-by: syzbot+4fc98ed414ae63d1ada2@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://lore.kernel.org/r/0000000000002fda01061e334873@xxxxxxxxxx
> > Fixes: 8e6c8fa9f2e9 ("erofs: enable big pcluster feature")
> > Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>
> > Link: https://lore.kernel.org/r/20240910070847.3356592-1-hsiangkao@xxxxxxxxxxxxxxxxx
> > [Alexey: minor fix to resolve merge conflict]
>
> Urgh, it doesn't look so minor indeed. Backward struct folio -> struct
> page conversions can be tricky sometimes. Please see several comments
> below.

I manually backported it for Linux 6.6.y, see
https://web.git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-6.6.y&id=1bf7e414cac303c9aec1be67872e19be8b64980c

Actually I had a very similiar backport for Linux 6.1.y,
but I forgot to send it out due to other ongoing stuffs.

I think this backport patch is all good, but you could
also mention it follows linux 6.6.y conflict changes
instead of "minor fix to resolve merge conflict".

> > Signed-off-by: Alexey Panov <apanov@xxxxxxxxxxxxx>
> > ---
> > Backport fix for CVE-2024-47736
> >
> >   fs/erofs/zdata.c | 59 +++++++++++++++++++++++++-----------------------
> >   1 file changed, 31 insertions(+), 28 deletions(-)
> >
> > diff --git a/fs/erofs/zdata.c b/fs/erofs/zdata.c
> > index 94e9e0bf3bbd..ac01c0ede7f7 100644
>
> I'm looking at the diff of upstream commit and the first thing it does
> is to remove zeroing out the folio/page private field here:
>
>    // upstream commit 9e2f9d34dd12 ("erofs: handle overlapped pclusters out of crafted images properly")
>    @@ -1450,7 +1451,6 @@ static void z_erofs_fill_bio_vec(struct bio_vec *bvec,
>             * file-backed folios will be used instead.
>             */
>            if (folio->private == (void *)Z_EROFS_PREALLOCATED_PAGE) {
>    -               folio->private = 0;
>                    tocache = true;
>                    goto out_tocache;
>            }
>
> while in 6.1.129 the corresponding fragment seems untouched with the
> backport patch. Is it intended?

Yes, because it was added in
commit 2080ca1ed3e4 ("erofs: tidy up `struct z_erofs_bvec`")
and dropped again.

But for Linux 6.6.y and 6.1.y, we don't need to backport
2080ca1ed3e4.

Thanks,
Gao Xiang