[PATCH] nvdimm: fix potential double-fetch bug

[Meng Xu]

From: Meng Xu <mengxu.gatech@xxxxxxxxx>

While examining the kernel source code, I found a dangerous operation that
could turn into a double-fetch situation (a race condition bug) where
the same userspace memory region are fetched twice into kernel with sanity
checks after the first fetch while missing checks after the second fetch.

In the case of _IOC_NR(ioctl_cmd) == ND_CMD_CALL:

1. The first fetch happens in line 935 copy_from_user(&pkg, p, sizeof(pkg)

2. subsequently `pkg.nd_reserved2` is asserted to be all zeroes
(line 984 to 986).

3. The second fetch happens in line 1022 copy_from_user(buf, p, buf_len)

4. Given that `p` can be fully controlled in userspace, an attacker can
race condition to override the header part of `p`, say,
`((struct nd_cmd_pkg *)p)->nd_reserved2` to arbitrary value
(say nine 0xFFFFFFFF for `nd_reserved2`) after the first fetch but before the
second fetch. The changed value will be copied to `buf`.

5. There is no checks on the second fetches until the use of it in
line 1034: nd_cmd_clear_to_send(nvdimm_bus, nvdimm, cmd, buf) and
line 1038: nd_desc->ndctl(nd_desc, nvdimm, cmd, buf, buf_len, &cmd_rc)
which means that the assumed relation, `p->nd_reserved2` are all zeroes might
not hold after the second fetch. And once the control goes to these functions
we lose the context to assert the assumed relation.

6. Based on my manual analysis, `p->nd_reserved2` is not used in function
`nd_cmd_clear_to_send` and potential implementations of `nd_desc->ndctl`
so there is no working exploit against it right now. However, this could
easily turns to an exploitable one if careless developers start to use
`p->nd_reserved2` later and assume that they are all zeroes.

Proposed patch:

The patch explicitly overrides `buf->nd_reserved2` after the second fetch with
the value `pkg.nd_reserved2` from the first fetch. In this way, it is assured
that the relation, `buf->nd_reserved2` are all zeroes, holds after the second
fetch.

Signed-off-by: Meng Xu <mengxu.gatech@xxxxxxxxx>
---
 drivers/nvdimm/bus.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/nvdimm/bus.c b/drivers/nvdimm/bus.c
index 937fafa..20c4d0f 100644
--- a/drivers/nvdimm/bus.c
+++ b/drivers/nvdimm/bus.c
@@ -1024,6 +1024,12 @@ static int __nd_ioctl(struct nvdimm_bus *nvdimm_bus, struct nvdimm *nvdimm,
 		goto out;
 	}
 
+	if (cmd == ND_CMD_CALL) {
+		struct nd_cmd_pkg *hdr = (struct nd_cmd_pkg *)buf;
+		memcpy(hdr->nd_reserved2, pkg.nd_reserved2,
+				sizeof(pkg.nd_reserved2));
+	}
+
 	nvdimm_bus_lock(&nvdimm_bus->dev);
 	rc = nd_cmd_clear_to_send(nvdimm_bus, nvdimm, func, buf);
 	if (rc)
-- 
2.7.4