Zero-initialize part of allocated bounce buffer which wasn't touched by subsequent bch2_key_sort_fix_overlapping to mitigate later uinit-value use KMSAN bug[1].
After applying the patch reproducer still triggers stack overflow[2] but it seems unrelated to the uninit-value use warning. After further investigation it was found that stack overflow occurs because KMSAN adds too many function calls[3]. Backtrace of where the stack magic number gets smashed was added as a reply to syzkaller thread[3]. It was confirmed that task's stack magic number gets smashed after the code path where KSMAN detects uninit-value use is executed, so it can be assumed that it doesn't contribute in any way to uninit-value use detection. [1] https://syzkaller.appspot.com/bug?extid=6f655a60d3244d0c6718 [2] https://lore.kernel.org/lkml/[email protected] [3] https://lore.kernel.org/all/rVaWgPULej8K7HqMPNIu8kVNyXNjjCiTB-QBtItLFBmk0alH6fV2tk4joVPk97Evnuv4ZRDd8HB5uDCkiFG6u81xKdzDj-KrtIMJSlF6Kt8=@proton.me Reported-by: [email protected] Closes: https://syzkaller.appspot.com/bug?extid=6f655a60d3244d0c6718 Fixes: ec4edd7b9d20 ("bcachefs: Prep work for variable size btree node buffers") Suggested-by: Kent Overstreet <[email protected]> Signed-off-by: Piotr Zalewski <[email protected]> --- Notes: changes in v2: - instead of adding __GFP_ZERO flag to bounce buffer alloc, zero initialize only the part untouched by subsequent bch2_key_sort_fix_overlapping to make the solution more performant link to v1: https://lore.kernel.org/linux-bcachefs/[email protected]/ --- fs/bcachefs/btree_io.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/fs/bcachefs/btree_io.c b/fs/bcachefs/btree_io.c index cb48a9477514..2a0420605bd7 100644 --- a/fs/bcachefs/btree_io.c +++ b/fs/bcachefs/btree_io.c @@ -1195,6 +1195,10 @@ int bch2_btree_node_read_done(struct bch_fs *c, struct bch_dev *ca, set_btree_bset(b, b->set, &b->data->keys); b->nr = bch2_key_sort_fix_overlapping(c, &sorted->keys, iter); + memset((uint8_t *)(sorted + 1) + b->nr.live_u64s * sizeof(u64), 0, + btree_buf_bytes(b) - + sizeof(struct btree_node) - + b->nr.live_u64s * sizeof(u64)); u64s = le16_to_cpu(sorted->keys.u64s); *sorted = *b->data; -- 2.46.0
