Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 29 Sep 2016 12:09:00 -0700 | Subject | Re: [PATCH 2/3] x86/boot: Fix another __read_cr4() case on 486 |
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On Thu, Sep 29, 2016 at 4:48 AM, Borislav Petkov <bp@alien8.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@saggiorato.net >> Fixes: 18bc7bd523e0 ("x86/boot: Synchronize trampoline_cr4_features and mmu_cr4_features directly") >> Signed-off-by: Andy Lutomirski <luto@kernel.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.
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