bcachefs: Fix UAF in bchfs_read()

Commit 3ba0240a8789 fixed a bug in the read retry path in __bch2_read(),
and changed bchfs_read() to match - to avoid a landmine if
bch2_read_extent() ever starts returning transaction restarts.

But that was incorrect, because bchfs_read() doesn't use a separate
stack allocated bvec_iter, it uses the one in the rbio being submitted.

Add a comment explaining the issue, and revert the buggy change.

Fixes: 3ba0240a8789 ("bcachefs: Fix silent short reads in data read retry path")
Reported-by: syzbot+2deb10b8dc9aae6fab67@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

diff --git a/fs/bcachefs/fs-io-buffered.c b/fs/bcachefs/fs-io-buffered.c
index 19d4599918dc59ad79901746f957e2630db33f0b..e3a75dcca60c819f1d3cf8a9726ac6ff0c2d6029 100644 (file)
--- a/fs/bcachefs/fs-io-buffered.c
+++ b/fs/bcachefs/fs-io-buffered.c
@@ -225,11 +225,26 @@ static void bchfs_read(struct btree_trans *trans,
 
                bch2_read_extent(trans, rbio, iter.pos,
                                 data_btree, k, offset_into_extent, flags);
-               swap(rbio->bio.bi_iter.bi_size, bytes);
+               /*
+                * Careful there's a landmine here if bch2_read_extent() ever
+                * starts returning transaction restarts here.
+                *
+                * We've changed rbio->bi_iter.bi_size to be "bytes we can read
+                * from this extent" with the swap call, and we restore it
+                * below. That restore needs to come before checking for
+                * errors.
+                *
+                * But unlike __bch2_read(), we use the rbio bvec iter, not one
+                * on the stack, so we can't do the restore right after the
+                * bch2_read_extent() call: we don't own that iterator anymore
+                * if BCH_READ_last_fragment is set, since we may have submitted
+                * that rbio instead of cloning it.
+                */
 
                if (flags & BCH_READ_last_fragment)
                        break;
 
+               swap(rbio->bio.bi_iter.bi_size, bytes);
                bio_advance(&rbio->bio, bytes);
 err:
                if (ret &&