On Tue, Sep 19, 2023 at 10:44:58AM +0530, Ritesh Harjani wrote: > "Darrick J. Wong" <djwong@xxxxxxxxxx> writes: > > > From: Darrick J. Wong <djwong@xxxxxxxxxx> > > > > Prior to commit a01b8f225248e, we would always read in the contents of a > > !uptodate folio prior to writing userspace data into the folio, > > allocated a folio state object, etc. Ritesh introduced an optimization > > that skips all of that if the write would cover the entire folio. > > > > Unfortunately, the optimization misses the unshare case, where we always > > have to read in the folio contents since there isn't a data buffer > > supplied by userspace. This can result in stale kernel memory exposure > > if userspace issues a FALLOC_FL_UNSHARE_RANGE call on part of a shared > > file that isn't already cached. > > > > This was caught by observing fstests regressions in the "unshare around" > > mechanism that is used for unaligned writes to a reflinked realtime > > volume when the realtime extent size is larger than 1FSB, > > I was wondering what is testcase that you are referring here to? > Can you please tell the testcase no. and the mkfs / mount config options > which I can use to observe the regression please? https://lore.kernel.org/linux-fsdevel/169507871947.772278.5767091361086740046.stgit@frogsfrogsfrogs/T/#m8081f74f4f1fcb862399aa1544be082aabe56765 (any xfs config with reflink enabled) --D > > though I think it applies to any shared file. > > > > Cc: ritesh.list@xxxxxxxxx, willy@xxxxxxxxxxxxx > > Fixes: a01b8f225248e ("iomap: Allocate ifs in ->write_begin() early") > > Signed-off-by: Darrick J. Wong <djwong@xxxxxxxxxx> > > --- > > fs/iomap/buffered-io.c | 6 ++++-- > > 1 file changed, 4 insertions(+), 2 deletions(-) > > > > > > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c > > index ae8673ce08b1..0350830fc989 100644 > > --- a/fs/iomap/buffered-io.c > > +++ b/fs/iomap/buffered-io.c > > @@ -640,11 +640,13 @@ static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos, > > size_t poff, plen; > > > > /* > > - * If the write completely overlaps the current folio, then > > + * If the write or zeroing completely overlaps the current folio, then > > * entire folio will be dirtied so there is no need for > > * per-block state tracking structures to be attached to this folio. > > + * For the unshare case, we must read in the ondisk contents because we > > + * are not changing pagecache contents. > > */ > > - if (pos <= folio_pos(folio) && > > + if (!(iter->flags & IOMAP_UNSHARE) && pos <= folio_pos(folio) && > > pos + len >= folio_pos(folio) + folio_size(folio)) > > return 0; > >
