[PATCHv1 3/4] RDMA/cxgb4: add missing padding at end of struct c4iw_create_cq_resp

Yann Droneaud
2014-05-05T17:33:23Z

i386 ABI disagree with most other ABIs regarding alignment
of data type larger than 4 bytes: on most ABIs a padding must
be added at end of the structures, while it is not required on
i386.

So for most ABI struct c4iw_create_cq_resp get implicitly padded
to be aligned on a 8 bytes multiple, while for i386, such padding
is not added.

Tool pahole could be used to find such implicit padding:

$ pahole --anon_include \
--nested_anon_include \
--recursive \
--class_name c4iw_create_cq_resp \
drivers/infiniband/hw/cxgb4/iw_cxgb4.o

Then, structure layout can be compared between i386 and x86_64:

+++ obj-i386/drivers/infiniband/hw/cxgb4/iw_cxgb4.o.pahole.txt 2014-03-28 11:43:05.547432195 +0100
--- obj-x86_64/drivers/infiniband/hw/cxgb4/iw_cxgb4.o.pahole.txt 2014-03-28 10:55:10.990133017 +0100
@@ -14,9 +13,8 @@ struct c4iw_create_cq_resp {
__u32 size; /* 28 4 */
__u32 qid_mask; /* 32 4 */

- /* size: 36, cachelines: 1, members: 6 */
- /* last cacheline: 36 bytes */
+ /* size: 40, cachelines: 1, members: 6 */
+ /* padding: 4 */
+ /* last cacheline: 40 bytes */
};

This ABI disagreement will make an x86_64 kernel try to write past
the buffer provided by an i386 binary.

When boundary check will be implemented, the x86_64 kernel will
refuse to write past the i386 userspace provided buffer
and the uverbs will fail.

If the structure lay in memory on a page boundary and next page
is not mapped, ib_copy_to_udata() will fail and the uverbs
will fail.

This patch adds an explicit padding at end of structure
c4iw_create_cq_resp, and, like 92b0ca7cb149 ('IB/mlx5: Fix stack
info leak in mlx5_ib_alloc_ucontext()'), makes function c4iw_create_cq()
not writting this padding field to userspace. This way, x86_64 kernel
will be able to write struct c4iw_create_cq_resp as expected by unpatched
and patched i386 libcxgb4.

Link: http://marc.info/?i=cover.1399309513.git.ydroneaud@opteya.com
Cc: stable@vger.kernel.org
Fixes: cfdda9d764362 ('RDMA/cxgb4: Add driver for Chelsio T4 RNIC')
Fixes: e24a72a3302a6 ('RDMA/cxgb4: Fix four byte info leak in c4iw_create_cq()')
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Yann Droneaud <ydroneaud@opteya.com>
---
drivers/infiniband/hw/cxgb4/cq.c | 4 ++--
drivers/infiniband/hw/cxgb4/user.h | 1 +
2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/infiniband/hw/cxgb4/cq.c b/drivers/infiniband/hw/cxgb4/cq.c
index cfaa56ada189..7151a02b4ebb 100644
--- a/drivers/infiniband/hw/cxgb4/cq.c
+++ b/drivers/infiniband/hw/cxgb4/cq.c
@@ -940,7 +940,6 @@ struct ib_cq *c4iw_create_cq(struct ib_device *ibdev, int entries,
if (!mm2)
goto err4;

- memset(&uresp, 0, sizeof(uresp));
uresp.qid_mask = rhp->rdev.cqmask;
uresp.cqid = chp->cq.cqid;
uresp.size = chp->cq.size;
@@ -951,7 +950,8 @@ struct ib_cq *c4iw_create_cq(struct ib_device *ibdev, int entries,
uresp.gts_key = ucontext->key;
ucontext->key += PAGE_SIZE;
spin_unlock(&ucontext->mmap_lock);
- ret = ib_copy_to_udata(udata, &uresp, sizeof uresp);
+ ret = ib_copy_to_udata(udata, &uresp,
+ sizeof(uresp) - sizeof(uresp.reserved));
if (ret)
goto err5;

diff --git a/drivers/infiniband/hw/cxgb4/user.h b/drivers/infiniband/hw/cxgb4/user.h
index 11ccd276e5d9..9b7534b5f07d 100644
--- a/drivers/infiniband/hw/cxgb4/user.h
+++ b/drivers/infiniband/hw/cxgb4/user.h
@@ -48,6 +48,7 @@ struct c4iw_create_cq_resp {
__u32 cqid;
__u32 size;
__u32 qid_mask;
+ __u32 reserved; /* explicit padding (optional for i386) */
};

--
1.9.0