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Message-ID: <a2e08779-e0ba-2711-9e0d-444d812c0182@roeck-us.net>
Date: Mon, 30 Sep 2019 17:01:28 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Nick Desaulniers <ndesaulniers@...gle.com>
Cc: clang-built-linux@...glegroups.com, jdelvare@...e.com,
Tomasz Paweł Gajc <tpgxyz@...il.com>,
Nathan Chancellor <natechancellor@...il.com>,
Henrik Rydberg <rydberg@...math.org>,
linux-hwmon@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] hwmon: (applesmc) fix UB and udelay overflow
On 9/24/19 10:47 AM, Nick Desaulniers wrote:
> Fixes the following 2 issues in the driver:
> 1. Left shifting a signed integer is undefined behavior. Unsigned
> integral types should be used for bitwise operations.
> 2. The delay scales from 0x0010 to 0x20000 by powers of 2, but udelay
> will result in a linkage failure when given a constant that's greater
> than 20000 (0x4E20). Agressive loop unrolling can fully unroll the
> loop, resulting in later iterations overflowing the call to udelay.
>
> 2 is fixed via splitting the loop in two, iterating the first up to the
> point where udelay would overflow, then switching to mdelay, as
> suggested in Documentation/timers/timers-howto.rst.
>
> Reported-by: Tomasz Paweł Gajc <tpgxyz@...il.com>
> Link: https://github.com/ClangBuiltLinux/linux/issues/678
> Debugged-by: Nathan Chancellor <natechancellor@...il.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
> Changes V1 -> V2:
> * The first loop in send_byte() needs to break out on the same condition
> now. Technically, the loop condition could even be removed. The diff
> looks funny because of the duplicated logic between existing and newly
> added for loops.
>
That is a delay()-internal dependency, and completely undocumented. This code
will fall apart if the implementation of udelay() is ever changed. This
also depends on the architecture - in some cases, mdelay() is implemented
as udelay(n * 1000).
> drivers/hwmon/applesmc.c | 35 +++++++++++++++++++++++++++++++----
> 1 file changed, 31 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/hwmon/applesmc.c b/drivers/hwmon/applesmc.c
> index 183ff3d25129..c76adb504dff 100644
> --- a/drivers/hwmon/applesmc.c
> +++ b/drivers/hwmon/applesmc.c
> @@ -46,6 +46,7 @@
> #define APPLESMC_MIN_WAIT 0x0010
> #define APPLESMC_RETRY_WAIT 0x0100
> #define APPLESMC_MAX_WAIT 0x20000
> +#define APPLESMC_UDELAY_MAX 20000
>
This is not really a problem in this driver; it is a system problem.
Anyone can call udelay() with a parameter longer than 20,000 us.
We can't add code like this all over the place because the implementation
of delay() is broken.
Besides, calling delay() with a parameter of 20,000 or more is a strong
indication that something is really wrong with the code. More on that
see below.
> #define APPLESMC_READ_CMD 0x10
> #define APPLESMC_WRITE_CMD 0x11
> @@ -157,14 +158,23 @@ static struct workqueue_struct *applesmc_led_wq;
> static int wait_read(void)
> {
> u8 status;
> - int us;
> - for (us = APPLESMC_MIN_WAIT; us < APPLESMC_MAX_WAIT; us <<= 1) {
> + unsigned int us;
> +
> + for (us = APPLESMC_MIN_WAIT; us < APPLESMC_UDELAY_MAX; us <<= 1) {
> udelay(us);
> status = inb(APPLESMC_CMD_PORT);
> /* read: wait for smc to settle */
> if (status & 0x01)
> return 0;
> }
> + /* switch to mdelay for longer sleeps */
> + for (; us < APPLESMC_MAX_WAIT; us <<= 1) {
> + mdelay(us);
Shouldn't that be us / 1000 ? Seems to me the above will wait for
at least 20000 ms, which is a a tiny bit long.
Also, mdelay(n) is by default implemented as udelay(n * 1000).
Also, at the very least, this should be something like
if (us < limit)
delay(us);
else
mdelay(us / 1000);
instead of introducing a second loop. But more on that below.
> + status = inb(APPLESMC_CMD_PORT);
> + /* read: wait for smc to settle */
> + if (status & 0x01)
> + return 0;
> + }
>
> pr_warn("wait_read() fail: 0x%02x\n", status);
> return -EIO;
> @@ -177,10 +187,10 @@ static int wait_read(void)
> static int send_byte(u8 cmd, u16 port)
> {
> u8 status;
> - int us;
> + unsigned int us;
>
> outb(cmd, port);
> - for (us = APPLESMC_MIN_WAIT; us < APPLESMC_MAX_WAIT; us <<= 1) {
> + for (us = APPLESMC_MIN_WAIT; us < APPLESMC_UDELAY_MAX; us <<= 1) {
> udelay(us);
> status = inb(APPLESMC_CMD_PORT);
> /* write: wait for smc to settle */
> @@ -190,6 +200,23 @@ static int send_byte(u8 cmd, u16 port)
> if (status & 0x04)
> return 0;
> /* timeout: give up */
> + if (us << 1 == APPLESMC_UDELAY_MAX)
> + break;
> + /* busy: long wait and resend */
> + udelay(APPLESMC_RETRY_WAIT);
> + outb(cmd, port);
> + }
> + /* switch to mdelay for longer sleeps */
> + for (; us < APPLESMC_MAX_WAIT; us <<= 1) {
> + mdelay(us);
Again, I fail to understand why waiting for a multiple of 20 seconds
under any circumstances would make any sense. Maybe the idea was
to divide us by 1000 before entering the second loop ?
Looking into the code, there is no need to use udelay() in the first
place. It should be possible to replace the longer waits with
usleep_range(). Something like
if (us < some_low_value) // eg. 0x80
delay(us)
else
usleep_range(us, us * 2);
should do, and at the same time prevent the system from turning
into a space heater.
Thanks,
Guenter
> + status = inb(APPLESMC_CMD_PORT);
> + /* write: wait for smc to settle */
> + if (status & 0x02)
> + continue;
> + /* ready: cmd accepted, return */
> + if (status & 0x04)
> + return 0;
> + /* timeout: give up */
> if (us << 1 == APPLESMC_MAX_WAIT)
> break;
> /* busy: long wait and resend */
>
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