>-----Original Message----- >From: Andrew Morton [mailto:akpm@xxxxxxxxxxxxxxxxxxxx] >Sent: Thursday, September 08, 2011 6:53 PM >To: Chumbalkar, Nagananda >Cc: davej@xxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; >cpufreq@xxxxxxxxxxxxxxx; stable@xxxxxxxxxx >Subject: Re: [PATCH] cpufreq: pcc-cpufreq: sanity check to prevent a >NULL pointer dereference > >On Wed, 31 Aug 2011 20:08:45 +0000 (UTC) >Naga Chumbalkar <nagananda.chumbalkar@xxxxxx> wrote: > >> If, for whatever reason, "pr" ends up being NULL we would end up in a >PANIC >> as seen below: >> >> Loading CPUFreq modules[ 437.661360] BUG: unable to handle kernel >NULL pointer >> dereference at (null) >> IP: [<ffffffffa0434314>] pcc_cpufreq_cpu_init+0x74/0x220 [pcc_cpufreq] >> >> It's better to prevent the PANIC by failing the driver, and allowing >the system to boot. >> >> Signed-off-by: Naga Chumbalkar <nagananda.chumbalkar@xxxxxx> >> Cc: stable@xxxxxxxxxx >> >> diff --git a/drivers/cpufreq/pcc-cpufreq.c b/drivers/cpufreq/pcc- >cpufreq.c >> index 7b0603e..cdc02ac 100644 >> --- a/drivers/cpufreq/pcc-cpufreq.c >> +++ b/drivers/cpufreq/pcc-cpufreq.c >> @@ -261,6 +261,9 @@ static int pcc_get_offset(int cpu) >> pr = per_cpu(processors, cpu); >> pcc_cpu_data = per_cpu_ptr(pcc_cpu_info, cpu); >> >> + if (!pr) >> + return -ENODEV; >> + >> status = acpi_evaluate_object(pr->handle, "PCCP", NULL, &buffer); >> if (ACPI_FAILURE(status)) >> return -ENODEV; > >hm, from reading drivers/acpi/processor_driver.c it appears that >per_cpu(processors, n)==NULL is an expected and valid state. Heaven >knows what that state actually *means* - apparently this is a secret. > >I assume that you've hit this crash in real live code, hence your >suggestion of a -stable backport? Yes - the above oops can be reproduced on a shipping Linux distro when, for x2APIC implementations, the BIOS does not use the ACPI "Device" declaration for processors with Local APIC ID >= 255. Such a declaration is necessitated by Sec 8.4 of ACPI 4.0a. - naga - -- To unsubscribe from this list: send the line "unsubscribe cpufreq" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
