The state->xmit.buf can be freed, and set to NULL, by uart_shutdown while
uart_put_char and uart_write is waiting to acquire the uart_port_lock
causing a NULL pointer dereference.
Only observed through uart_write, but similar issue could happen if
uart_put_char was called. The issue was triggered on an older kernel,
version 4.4, but is present in current code as well.
Crash report:
Unable to handle kernel NULL pointer dereference at virtual address 00000196
...
Workqueue: events_unbound flush_to_ldisc
task: ffffffc1eb1cdb00 ti: ffffffc1eac6c000 task.ti: ffffffc1eac6c000
PC is at __memcpy+0xa0/0x180
LR is at uart_write+0x158/0x240
...
Call trace:
[<ffffffc0004104a0>] __memcpy+0xa0/0x180
[<ffffffc00057f1c0>] do_output_char+0xa8/0x1d8
[<ffffffc00057f4bc>] __process_echoes+0x1cc/0x280
[<ffffffc00057f66c>] commit_echoes+0x6c/0xb0
[<ffffffc000581818>] n_tty_receive_char_special+0x7b8/0xa98
[<ffffffc000582388>] n_tty_receive_buf_common+0x520/0xa38
[<ffffffc0005828e0>] n_tty_receive_buf2+0x40/0x50
[<ffffffc000585ae0>] flush_to_ldisc+0xd0/0x168
[<ffffffc0000c1784>] process_one_work+0x2b4/0x4d8
[<ffffffc0000c1c70>] worker_thread+0x2c8/0x4d8
[<ffffffc0000c8944>] kthread+0x104/0x118
[<ffffffc000084fa0>] ret_from_fork+0x10/0x30
---[ end trace 8fd3b56af7a64339 ]---
Signed-off-by: Ole Bjørn Midtbø <omidtbo@xxxxxxxxx>
---
drivers/tty/serial/serial_core.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 80bb56facfb6..39a016ffdd17 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -544,7 +544,7 @@ static int uart_put_char(struct tty_struct *tty, unsigned char c)
return 0;
port = uart_port_lock(state, flags);
- if (port && uart_circ_chars_free(circ) != 0) {
+ if (port && circ->buf && uart_circ_chars_free(circ) != 0) {
circ->buf[circ->head] = c;
circ->head = (circ->head + 1) & (UART_XMIT_SIZE - 1);
ret = 1;
@@ -581,6 +581,11 @@ static int uart_write(struct tty_struct *tty,
return 0;
port = uart_port_lock(state, flags);
+ if (!circ->buf) {
+ uart_port_unlock(port, flags);
+ return 0;
+ }
+
while (port) {
c = CIRC_SPACE_TO_END(circ->head, circ->tail, UART_XMIT_SIZE);
if (count < c)
--
2.10.2
