linux-kernel - Re: [PATCH 2/3] x86/boot: Fix another __read

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Message-ID: <CALCETrVF_VEVQCyGjoFCGOhvamL_m9hnZKy6pp+KCAPXoB6bqg@mail.gmail.com>
Date:   Thu, 29 Sep 2016 12:09:00 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        david@...giorato.net, Brian Gerst <brgerst@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] x86/boot: Fix another __read_cr4() case on 486

On Thu, Sep 29, 2016 at 4:48 AM, Borislav Petkov <bp@...en8.de> wrote:
> On Wed, Sep 28, 2016 at 12:34:15PM -0700, Andy Lutomirski wrote:
>> The condition for reading CR4 was wrong: there are some CPUs with
>> CPUID but not CR4.  Rather than trying to make the condition exact,
>> using __read_cr4_safe().
>>
>> Reported-by: david@...giorato.net
>> Fixes: 18bc7bd523e0 ("x86/boot: Synchronize trampoline_cr4_features and mmu_cr4_features directly")
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> ---
>>  arch/x86/kernel/setup.c | 10 +++++++---
>>  1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
>> index 0fa60f5f5a16..5930a4d191b4 100644
>> --- a/arch/x86/kernel/setup.c
>> +++ b/arch/x86/kernel/setup.c
>> @@ -1137,9 +1137,13 @@ void __init setup_arch(char **cmdline_p)
>>        * auditing all the early-boot CR4 manipulation would be needed to
>>        * rule it out.
>>        */
>> -     if (boot_cpu_data.cpuid_level >= 0)
>> -             /* A CPU has %cr4 if and only if it has CPUID. */
>> -             mmu_cr4_features = __read_cr4();
>> +     if (boot_cpu_data.cpuid_level >= 0) {
>> +             /*
>> +              * CPUs without CPUID don't have CR4.  CPUs with CPUID
>> +              * usually have CR4.
>> +              */
>> +             mmu_cr4_features = __read_cr4_safe();
>> +     }
>
> Why are we even doing the CPUID check instead of unconditionally doing
> __read_cr4_safe()?
>
> The safe variant will give 0 on !CR4 machines.

Will do.