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SubjectKASAN: null-ptr-deref Read in rds_ib_get_mr
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
==================================================================


= 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.

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