Re: [PATCH 1/3] cpufreq: fix broken buffer overflow detection in trans_stats

[Rafael J. Wysocki]

On Tue, Oct 24, 2023 at 8:30 PM Christian Marangi <ansuelsmth@xxxxxxxxx> wrote:
>
> Commit 3c0897c180c6 ("cpufreq: Use scnprintf() for avoiding potential
> buffer overflow") switched from snprintf to the more secure scnprintf
> but never updated the exit condition for PAGE_SIZE.
>
> As the commit say and as scnprintf document, what scnprintf returns what
> is actually written not counting the '\0' end char. This results in the
> case of len exceeding the size, len set to PAGE_SIZE - 1, as it can be
> written at max PAGESIZE - 1 (as '\0' is not counted)
>
> Because of len is never set to PAGE_SIZE, the function never break early,
> never print the warning and never return -EFBIG.
>
> Fix this by fixing the condition to PAGE_SIZE -1 to correctly trigger
> the error condition.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 3c0897c180c6 ("cpufreq: Use scnprintf() for avoiding potential buffer overflow")
> Signed-off-by: Christian Marangi <ansuelsmth@xxxxxxxxx>
> ---
>  drivers/cpufreq/cpufreq_stats.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq_stats.c b/drivers/cpufreq/cpufreq_stats.c
> index a33df3c66c88..40a9ff18da06 100644
> --- a/drivers/cpufreq/cpufreq_stats.c
> +++ b/drivers/cpufreq/cpufreq_stats.c
> @@ -131,23 +131,23 @@ static ssize_t show_trans_table(struct cpufreq_policy *policy, char *buf)
>         len += sysfs_emit_at(buf, len, "   From  :    To\n");
>         len += sysfs_emit_at(buf, len, "         : ");
>         for (i = 0; i < stats->state_num; i++) {
> -               if (len >= PAGE_SIZE)
> +               if (len >= PAGE_SIZE - 1)
>                         break;
>                 len += sysfs_emit_at(buf, len, "%9u ", stats->freq_table[i]);
>         }
> -       if (len >= PAGE_SIZE)
> -               return PAGE_SIZE;
> +       if (len >= PAGE_SIZE - 1)
> +               return PAGE_SIZE - 1;
>
>         len += sysfs_emit_at(buf, len, "\n");
>
>         for (i = 0; i < stats->state_num; i++) {
> -               if (len >= PAGE_SIZE)
> +               if (len >= PAGE_SIZE - 1)
>                         break;
>
>                 len += sysfs_emit_at(buf, len, "%9u: ", stats->freq_table[i]);
>
>                 for (j = 0; j < stats->state_num; j++) {
> -                       if (len >= PAGE_SIZE)
> +                       if (len >= PAGE_SIZE - 1)
>                                 break;
>
>                         if (pending)
> @@ -157,12 +157,12 @@ static ssize_t show_trans_table(struct cpufreq_policy *policy, char *buf)
>
>                         len += sysfs_emit_at(buf, len, "%9u ", count);
>                 }
> -               if (len >= PAGE_SIZE)
> +               if (len >= PAGE_SIZE - 1)
>                         break;
>                 len += sysfs_emit_at(buf, len, "\n");
>         }
>
> -       if (len >= PAGE_SIZE) {
> +       if (len >= PAGE_SIZE - 1) {
>                 pr_warn_once("cpufreq transition table exceeds PAGE_SIZE. Disabling\n");
>                 return -EFBIG;
>         }
> --

Applied (with some edits in the subject and changelog) as 6.7 material, thanks!