Re: KASAN: null-ptr-deref Read in rds_ib_get_mr

[Yanjun Zhu]

On 2018/5/11 13:20, DaeRyong Jeong wrote:
> We report the crash: KASAN: null-ptr-deref Read in rds_ib_get_mr
>
> Note that this bug is previously reported by syzkaller.
> https://syzkaller.appspot.com/bug?id=0bb56a5a48b000b52aa2b0d8dd20b1f545214d91
> Nonetheless, this bug has not fixed yet, and we hope that this report and our
> analysis, which gets help by the RaceFuzzer's feature, will helpful to fix the
> crash.
>
> This crash has been found in v4.17-rc1 using RaceFuzzer (a modified
> version of Syzkaller), which we describe more at the end of this
> report. Our analysis shows that the race occurs when invoking two
> syscalls concurrently, bind$rds and setsockopt$RDS_GET_MR.
>
>
> Analysis:
> We think the concurrent execution of __rds_rdma_map() and rds_bind()
> causes the problem. __rds_rdma_map() checks whether rs->rs_bound_addr is 0
> or not. But the concurrent execution with rds_bind() can by-pass this
> check. Therefore, __rds_rdmap_map() calls rs->rs_transport->get_mr() and
> rds_ib_get_mr() causes the null deref at ib_rdma.c:544 in v4.17-rc1, when
> dereferencing rs_conn.
>
>
> Thread interleaving:
> CPU0 (__rds_rdma_map)					CPU1 (rds_bind)
> 							// rds_add_bound() sets rs->bound_addr as none 0
> 							ret = rds_add_bound(rs, sin->sin_addr.s_addr, &sin->sin_port);
> if (rs->rs_bound_addr == 0 || !rs->rs_transport) {
> 	ret = -ENOTCONN; /* XXX not a great errno */
> 	goto out;
> }
> 							if (rs->rs_transport) { /* previously bound */
> 								trans = rs->rs_transport;
> 								if (trans->laddr_check(sock_net(sock->sk),
> 										       sin->sin_addr.s_addr) != 0) {
> 									ret = -ENOPROTOOPT;
> 									// rds_remove_bound() sets rs->bound_addr as 0
> 									rds_remove_bound(rs);
> ...
> trans_private = rs->rs_transport->get_mr(sg, nents, rs,
> 					 &mr->r_key);
> (in rds_ib_get_mr())
> struct rds_ib_connection *ic = rs->rs_conn->c_transport_data;
>
>
> Call sequence (v4.17-rc1):
> CPU0
> rds_setsockopt
> 	rds_get_mr
> 		__rds_rdma_map
> 			rds_ib_get_mr
>
>
> CPU1
> rds_bind
> 	rds_add_bound
> 	...
> 	rds_remove_bound
>
>
> Crash log:
> ==================================================================
> BUG: KASAN: null-ptr-deref in rds_ib_get_mr+0x3a/0x150 net/rds/ib_rdma.c:544
> Read of size 8 at addr 0000000000000068 by task syz-executor0/32067
>
> CPU: 0 PID: 32067 Comm: syz-executor0 Not tainted 4.17.0-rc1 #1
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.8.2-0-g33fbe13 by qemu-project.org 04/01/2014
> Call Trace:
>   __dump_stack lib/dump_stack.c:77 [inline]
>   dump_stack+0x166/0x21c lib/dump_stack.c:113
>   kasan_report_error mm/kasan/report.c:352 [inline]
>   kasan_report+0x140/0x360 mm/kasan/report.c:412
>   check_memory_region_inline mm/kasan/kasan.c:260 [inline]
>   __asan_load8+0x54/0x90 mm/kasan/kasan.c:699
>   rds_ib_get_mr+0x3a/0x150 net/rds/ib_rdma.c:544
>   __rds_rdma_map+0x521/0x9d0 net/rds/rdma.c:271
>   rds_get_mr+0xad/0xf0 net/rds/rdma.c:333
>   rds_setsockopt+0x57f/0x720 net/rds/af_rds.c:347
>   __sys_setsockopt+0x147/0x230 net/socket.c:1903
>   __do_sys_setsockopt net/socket.c:1914 [inline]
>   __se_sys_setsockopt net/socket.c:1911 [inline]
>   __x64_sys_setsockopt+0x67/0x80 net/socket.c:1911
>   do_syscall_64+0x15f/0x4a0 arch/x86/entry/common.c:287
>   entry_SYSCALL_64_after_hwframe+0x49/0xbe
> RIP: 0033:0x4563f9
> RSP: 002b:00007f6a2b3c2b28 EFLAGS: 00000246 ORIG_RAX: 0000000000000036
> RAX: ffffffffffffffda RBX: 000000000072bee0 RCX: 00000000004563f9
> RDX: 0000000000000002 RSI: 0000000000000114 RDI: 0000000000000015
> RBP: 0000000000000575 R08: 0000000000000020 R09: 0000000000000000
> R10: 0000000020000140 R11: 0000000000000246 R12: 00007f6a2b3c36d4
> R13: 00000000ffffffff R14: 00000000006fd398 R15: 0000000000000000
> ==================================================================
diff --git a/net/rds/ib_rdma.c b/net/rds/ib_rdma.c
index e678699..2228b50 100644
--- a/net/rds/ib_rdma.c
+++ b/net/rds/ib_rdma.c
@@ -539,11 +539,17 @@ void rds_ib_flush_mrs(void)
Âvoid *rds_ib_get_mr(struct scatterlist *sg, unsigned long nents,
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ struct rds_sock *rs, u32 *key_ret)
Â{
-ÂÂÂÂÂÂ struct rds_ib_device *rds_ibdev;
+ÂÂÂÂÂÂ struct rds_ib_device *rds_ibdev = NULL;
ÂÂÂÂÂÂÂ struct rds_ib_mr *ibmr = NULL;
-ÂÂÂÂÂÂ struct rds_ib_connection *ic = rs->rs_conn->c_transport_data;
+ÂÂÂÂÂÂ struct rds_ib_connection *ic = NULL;
ÂÂÂÂÂÂÂ int ret;

+ÂÂÂÂÂÂ if (rs->rs_bound_addr == 0) {
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ret = -EPERM;
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ goto out;
+ÂÂÂÂÂÂ }
+
+ÂÂÂÂÂÂ ic = rs->rs_conn->c_transport_data;
ÂÂÂÂÂÂÂ rds_ibdev = rds_ib_get_device(rs->rs_bound_addr);
ÂÂÂÂÂÂÂ if (!rds_ibdev) {
ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ret = -ENODEV;

I made this raw patch. If you can reproduce this bug, please make tests 
with it.

Thanks a lot.
> = About RaceFuzzer
>
> RaceFuzzer is a customized version of Syzkaller, specifically tailored
> to find race condition bugs in the Linux kernel. While we leverage
> many different technique, the notable feature of RaceFuzzer is in
> leveraging a custom hypervisor (QEMU/KVM) to interleave the
> scheduling. In particular, we modified the hypervisor to intentionally
> stall a per-core execution, which is similar to supporting per-core
> breakpoint functionality. This allows RaceFuzzer to force the kernel
> to deterministically trigger racy condition (which may rarely happen
> in practice due to randomness in scheduling).
>
> RaceFuzzer's C repro always pinpoints two racy syscalls. Since C
> repro's scheduling synchronization should be performed at the user
> space, its reproducibility is limited (reproduction may take from 1
> second to 10 minutes (or even more), depending on a bug). This is
> because, while RaceFuzzer precisely interleaves the scheduling at the
> kernel's instruction level when finding this bug, C repro cannot fully
> utilize such a feature. Please disregard all code related to
> "should_hypercall" in the C repro, as this is only for our debugging
> purposes using our own hypervisor.