Re: [PATCH][next] hwmon: (aspeed-pwm-tacho) Fix -Wstringop-overflow warning in aspeed_create_fan_tach_channel()

On 11/14/23 08:52, Guenter Roeck wrote:

On Mon, Nov 13, 2023 at 01:38:12PM -0600, Gustavo A. R. Silva wrote:

quoted

Based on the documentation below, the maximum number of Fan tach
channels is 16:

Documentation/devicetree/bindings/hwmon/aspeed-pwm-tacho.txt:45:
  45 - aspeed,fan-tach-ch : should specify the Fan tach input channel.
  46                 integer value in the range 0 through 15, with 0 indicating
  47                 Fan tach channel 0 and 15 indicating Fan tach channel 15.
  48                 At least one Fan tach input channel is required.

However, the compiler doesn't know that, and legitimaly warns about a potential
overwrite in array `u8 fan_tach_ch_source[16]` in `struct aspeed_pwm_tacho_data`,
in case `index` takes a value outside the boundaries of the array:

All that doesn't warrant introducing checkpatch warnings.

Do you mean this?

WARNING: Prefer a maximum 75 chars per line (possible unwrapped commit description?)
#17:
  46                 integer value in the range 0 through 15, with 0 indicating

I honestly didn't consider that relevant, and I didn't want to alter the format of
the Doc text.

However, if you want me to split any offending line, that's not a problem. Just let
me know. :)

quoted

drivers/hwmon/aspeed-pwm-tacho.c:
179 struct aspeed_pwm_tacho_data {
...
184         bool fan_tach_present[16];
...
193         u8 fan_tach_ch_source[16];
...
196 };

In function ‘aspeed_create_fan_tach_channel’,
     inlined from ‘aspeed_create_fan’ at drivers/hwmon/aspeed-pwm-tacho.c:877:2,
     inlined from ‘aspeed_pwm_tacho_probe’ at drivers/hwmon/aspeed-pwm-tacho.c:936:9:
drivers/hwmon/aspeed-pwm-tacho.c:751:49: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
   751 |                 priv->fan_tach_ch_source[index] = pwm_source;
       |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
drivers/hwmon/aspeed-pwm-tacho.c: In function ‘aspeed_pwm_tacho_probe’:
drivers/hwmon/aspeed-pwm-tacho.c:193:12: note: at offset [48, 255] into destination object ‘fan_tach_ch_source’ of size 16
   193 |         u8 fan_tach_ch_source[16];
       |            ^~~~~~~~~~~~~~~~~~

Fix this by sanity checking `index` before using it to index arrays of
size 16 elements in `struct aspeed_pwm_tacho_data`. Also, and just for
completeness, add a `pr_err()` message to display in the unlikely case
`0 > index >= 16`.

This is probably the last remaining -Wstringop-overflow issue in the
kernel, and this patch helps with the ongoing efforts to enable such
compiler option globally.

I am sorry, but this description almost completely misses the point.
The real issue is that the values in aspeed,fan-tach-ch are not range
checked, which can cause real problems if is elements are set to values
larger than 15. This is a real problem and has nothing to do with string
overflows.

Yeah, the above paragraph was extra, and I removed it in v2[1]. The rest
of the changelog text describes the issue in the code.

This should use dev_err() (and, yes, that means dev needs to be passed
as argument), and the function should return -EINVAL if this is
encountered. Also, error handling should come first.

		if (index >= MAX_ASPEED_FAN_TACH_CHANNELS) {
			dev_err(dev, "Invalid Fan Tach input channel %u\n.", index);
			return -EINVAL;
		}

Done in v2.

Thanks a lot for the feedback.
--
Gustavo

[1] https://lore.kernel.org/linux-hardening/ZVPQJIP26dIzRAr6@work/ (local)

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