bcache: fix stack corruption by PRECEDING_KEY()

commit 31b90956b124240aa8c63250243ae1a53585c5e2 upstream.

Recently people report bcache code compiled with gcc9 is broken, one of
the buggy behavior I observe is that two adjacent 4KB I/Os should merge
into one but they don't. Finally it turns out to be a stack corruption
caused by macro PRECEDING_KEY().

See how PRECEDING_KEY() is defined in bset.h,
437 #define PRECEDING_KEY(_k)                                       \
438 ({                                                              \
439         struct bkey *_ret = NULL;                               \
440                                                                 \
441         if (KEY_INODE(_k) || KEY_OFFSET(_k)) {                  \
442                 _ret = &KEY(KEY_INODE(_k), KEY_OFFSET(_k), 0);  \
443                                                                 \
444                 if (!_ret->low)                                 \
445                         _ret->high--;                           \
446                 _ret->low--;                                    \
447         }                                                       \
448                                                                 \
449         _ret;                                                   \
450 })

At line 442, _ret points to address of a on-stack variable combined by
KEY(), the life range of this on-stack variable is in line 442-446,
once _ret is returned to bch_btree_insert_key(), the returned address
points to an invalid stack address and this address is overwritten in
the following called bch_btree_iter_init(). Then argument 'search' of
bch_btree_iter_init() points to some address inside stackframe of
bch_btree_iter_init(), exact address depends on how the compiler
allocates stack space. Now the stack is corrupted.

Fixes: 0eacac22034c ("bcache: PRECEDING_KEY()")
Signed-off-by: Coly Li <colyli@suse.de>
Reviewed-by: Rolf Fokkens <rolf@rolffokkens.nl>
Reviewed-by: Pierre JUHEN <pierre.juhen@orange.fr>
Tested-by: Shenghui Wang <shhuiw@foxmail.com>
Tested-by: Pierre JUHEN <pierre.juhen@orange.fr>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Cc: Nix <nix@esperi.org.uk>
Cc: stable@vger.kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 13 insertions, 3 deletions

diff --git a/drivers/md/bcache/bset.c b/drivers/md/bcache/bset.c
index e56d3ecdbfcb..1edf9515345e 100644
--- a/drivers/md/bcache/bset.c
+++ b/drivers/md/bcache/bset.c
@@ -825,12 +825,22 @@ unsigned bch_btree_insert_key(struct btree_keys *b, struct bkey *k,
 	struct bset *i = bset_tree_last(b)->data;
 	struct bkey *m, *prev = NULL;
 	struct btree_iter iter;
+	struct bkey preceding_key_on_stack = ZERO_KEY;
+	struct bkey *preceding_key_p = &preceding_key_on_stack;
 
 	BUG_ON(b->ops->is_extents && !KEY_SIZE(k));
 
-	m = bch_btree_iter_init(b, &iter, b->ops->is_extents
-				? PRECEDING_KEY(&START_KEY(k))
-				: PRECEDING_KEY(k));
+	/*
+	 * If k has preceding key, preceding_key_p will be set to address
+	 *  of k's preceding key; otherwise preceding_key_p will be set
+	 * to NULL inside preceding_key().
+	 */
+	if (b->ops->is_extents)
+		preceding_key(&START_KEY(k), &preceding_key_p);
+	else
+		preceding_key(k, &preceding_key_p);
+
+	m = bch_btree_iter_init(b, &iter, preceding_key_p);
 
 	if (b->ops->insert_fixup(b, k, &iter, replace_key))
 		return status;