[PATCH net] net: ena: fix shift-out-of-bounds in exponential backoff

[Krister Johansen <kjlx@templeofstupid.com>]

The ENA adapters on our instances occasionally reset.  Once recently
logged a UBSAN failure to console in the process:

  UBSAN: shift-out-of-bounds in build/linux/drivers/net/ethernet/amazon/ena/ena_com.c:540:13
  shift exponent 32 is too large for 32-bit type 'unsigned int'
  CPU: 28 PID: 70012 Comm: kworker/u72:2 Kdump: loaded not tainted 5.15.117
  Hardware name: Amazon EC2 c5d.9xlarge/, BIOS 1.0 10/16/2017
  Workqueue: ena ena_fw_reset_device [ena]
  Call Trace:
  <TASK>
  dump_stack_lvl+0x4a/0x63
  dump_stack+0x10/0x16
  ubsan_epilogue+0x9/0x36
  __ubsan_handle_shift_out_of_bounds.cold+0x61/0x10e
  ? __const_udelay+0x43/0x50
  ena_delay_exponential_backoff_us.cold+0x16/0x1e [ena]
  wait_for_reset_state+0x54/0xa0 [ena]
  ena_com_dev_reset+0xc8/0x110 [ena]
  ena_down+0x3fe/0x480 [ena]
  ena_destroy_device+0xeb/0xf0 [ena]
  ena_fw_reset_device+0x30/0x50 [ena]
  process_one_work+0x22b/0x3d0
  worker_thread+0x4d/0x3f0
  ? process_one_work+0x3d0/0x3d0
  kthread+0x12a/0x150
  ? set_kthread_struct+0x50/0x50
  ret_from_fork+0x22/0x30
  </TASK>

Apparently, the reset delays are getting so large they can trigger a
UBSAN panic.

Looking at the code, the current timeout is capped at 5000us.  Using a
base value of 100us, the current code will overflow after (1<<29).  Even
at values before 32, this function wraps around, perhaps
unintentionally.

Cap the value of the exponent used for this backoff at (1<<16) which is
larger than currently necessary, but large enough to support bigger
values in the future.

Cc: stable@vger.kernel.org
Fixes: 4bb7f4cf60e3 ("net: ena: reduce driver load time")
Signed-off-by: Krister Johansen <kjlx@templeofstupid.com>
---
 drivers/net/ethernet/amazon/ena/ena_com.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/net/ethernet/amazon/ena/ena_com.c b/drivers/net/ethernet/amazon/ena/ena_com.c
index 451c3a1b6255..633b321d7fdd 100644
--- a/drivers/net/ethernet/amazon/ena/ena_com.c
+++ b/drivers/net/ethernet/amazon/ena/ena_com.c
@@ -35,6 +35,8 @@
 
 #define ENA_REGS_ADMIN_INTR_MASK 1
 
+#define ENA_MAX_BACKOFF_DELAY_EXP 16U
+
 #define ENA_MIN_ADMIN_POLL_US 100
 
 #define ENA_MAX_ADMIN_POLL_US 5000
@@ -536,6 +538,7 @@ static int ena_com_comp_status_to_errno(struct ena_com_admin_queue *admin_queue,
 
 static void ena_delay_exponential_backoff_us(u32 exp, u32 delay_us)
 {
+	exp = min_t(u32, exp, ENA_MAX_BACKOFF_DELAY_EXP);
 	delay_us = max_t(u32, ENA_MIN_ADMIN_POLL_US, delay_us);
 	delay_us = min_t(u32, delay_us * (1U << exp), ENA_MAX_ADMIN_POLL_US);
 	usleep_range(delay_us, 2 * delay_us);
-- 
2.25.1