[PATCH 1/2] i2c: imx-lpi2c: clean rx/tx buffers upon new message

[Alexander Stein <alexander.stein@ew.tq-group.com>]

When start sending a new message clear the Rx & Tx buffer pointers in
order to avoid using stale pointers.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
---
I noticed an ambigous stack corruption once my rtc-pcf85063 driver probes.

[    2.695684] Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: pcf85063_rtc_read_time+0x10/0x100
[    2.706669] CPU: 1 PID: 63 Comm: kworker/u8:2 Not tainted 6.2.0-rc6-next-20230130+ #1185 ca067559321ae817c063baccdba80d328f10f73
[    2.718331] Hardware name: TQ-Systems i.MX8QXP TQMa8XQP on MBa8Xx (DT)
[    2.724866] Workqueue: events_unbound deferred_probe_work_func
[    2.730712] Call trace:
[    2.733161]  dump_backtrace+0x9c/0x11c
[    2.736914]  show_stack+0x14/0x1c
[    2.740232]  dump_stack_lvl+0x5c/0x78
[    2.743907]  dump_stack+0x14/0x1c
[    2.747225]  panic+0x34c/0x39c
[    2.750283]  __ktime_get_real_seconds+0x0/0xc
[    2.754653]  pcf85063_ioctl+0x0/0xf0
[    2.758232]  __rtc_read_time+0x44/0x114
[    2.762081]  __rtc_read_alarm+0x258/0x460
[    2.766095]  __devm_rtc_register_device+0x174/0x2b4
[    2.770986]  pcf85063_probe+0x258/0x4d4
[    2.774825]  i2c_device_probe+0x100/0x33c

The backtrace did not indicate the actual cause of it. Checking the code the
RTC driver seemed to be ok, so it has to be in the i2c bus driver.
At some point I noticed that I see both Rx and Tx interrupts at the same time,
which is odd. Also both rx_buf and tx_buf was set simultaneously. Clearly
a bug to me.
Clearing the buffer pointers upon each new i2c message triggered a NULL
pointer dereference:

[    2.694923] Unable to handle kernel NULL pointer dereference at virtual address 0000000000000001
[    2.703730] Mem abort info:
[    2.706525]   ESR = 0x0000000096000004
[    2.710278]   EC = 0x25: DABT (current EL), IL = 32 bits
[    2.715595]   SET = 0, FnV = 0
[    2.718653]   EA = 0, S1PTW = 0
[    2.721798]   FSC = 0x04: level 0 translation fault
[    2.726680] Data abort info:
[    2.729556]   ISV = 0, ISS = 0x00000004
[    2.733387]   CM = 0, WnR = 0
[    2.736358] [0000000000000001] user address but active_mm is swapper
[    2.742719] Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
[    2.748990] Modules linked in:
[    2.752051] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.2.0-rc6-next-20230130+ #1184 44a8abebca6bfabc93e20ac52bce
47da7f92cec1
[    2.763368] Hardware name: TQ-Systems i.MX8QXP TQMa8XQP on MBa8Xx (DT)
[    2.769902] pstate: 800000c5 (Nzcv daIF -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[    2.776868] pc : lpi2c_imx_write_txfifo+0x44/0xb0
[    2.781585] lr : lpi2c_imx_isr+0x60/0x8c
[    2.785512] sp : ffff800008003ef0
[    2.788831] x29: ffff800008003ef0 x28: ffff8000099c1ec0 x27: 00000000bfe632c8
[    2.795980] x26: 0000000000000000 x25: ffff800009b935ed x24: ffff800009a4d4c0
[    2.803130] x23: ffff00000365e800 x22: 0000000000000128 x21: 0000000000000000
[    2.810280] x20: ffff0000033f4080 x19: 0000000003000103 x18: 0000000000000000
[    2.817430] x17: ffff80003688a000 x16: ffff800008000000 x15: 0000000000000000
[    2.824579] x14: 0000000000000000 x13: ffff8000099d1db8 x12: 0000000000000000
[    2.831729] x11: ffff800009503180 x10: 0000000000000a80 x9 : ffff8000099b3d20
[    2.838879] x8 : ffff8000099c29a0 x7 : 00000000000000c0 x6 : ffff000002838028
[    2.846029] x5 : 0000000000000002 x4 : 0000000000000000 x3 : 0000000000000000
[    2.849626] imx-scu system-controller: RPC send msg timeout
[    2.853178] x2 : ffff800009c88060 x1 : 0000000000000001 x0 : ffff0000033f4080
[    2.858764]  enet1: failed to power off resource 252 ret -110
[    2.865897] Call trace:
[    2.865901]  lpi2c_imx_write_txfifo+0x44/0xb0
[    2.878443]  __handle_irq_event_percpu+0x5c/0x188
[    2.883151]  handle_irq_event+0x48/0xb0

$ ./scripts/faddr2line build_arm64/vmlinux lpi2c_imx_write_txfifo+0x44/0xb0
lpi2c_imx_write_txfifo+0x44/0xb0:
lpi2c_imx_write_txfifo at drivers/i2c/busses/i2c-imx-lpi2c.c:364

This now clearly pinpoints the wrong access which previously corrupted the
stack. The error leading to this wrong access is addressed in the
following patch.

 drivers/i2c/busses/i2c-imx-lpi2c.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/i2c/busses/i2c-imx-lpi2c.c b/drivers/i2c/busses/i2c-imx-lpi2c.c
index 188f2a36d2fd..c6d0225246e6 100644
--- a/drivers/i2c/busses/i2c-imx-lpi2c.c
+++ b/drivers/i2c/busses/i2c-imx-lpi2c.c
@@ -463,6 +463,8 @@ static int lpi2c_imx_xfer(struct i2c_adapter *adapter,
 		if (num == 1 && msgs[0].len == 0)
 			goto stop;
 
+		lpi2c_imx->rx_buf = NULL;
+		lpi2c_imx->tx_buf = NULL;
 		lpi2c_imx->delivered = 0;
 		lpi2c_imx->msglen = msgs[i].len;
 		init_completion(&lpi2c_imx->complete);
-- 
2.34.1