AFAIK no Intel CPU has ever had that behavior, and always cleared the segments; I don't Intel has any plans of supporting such a CPUID bit (although I'd certainly be willing to take such a request back to the CPU teams on request.)
That being said, this sounds like an ideal use for the hypervisor CPU feature flag. Maybe we should consider a migration hypervisor flag too to explicitly tell the kernel not to rely on hardware probing that breaks migration in general.
Now, with a CPUID but being introduced, the right thing would be to use the CPUID bit as a feature instead of using a bug flag, and add whitelisting in the vendor-specific code as applicable.
On October 18, 2021 11:17:30 AM PDT, Borislav Petkov bp@alien8.de wrote:
On Wed, Oct 13, 2021 at 03:22:30PM +0100, Jane Malalane wrote:
@@ -650,6 +651,27 @@ static void early_init_amd(struct cpuinfo_x86 *c) if (c->x86_power & BIT(14)) set_cpu_cap(c, X86_FEATURE_RAPL);
- /*
* Zen1 and earlier CPUs don't clear segment base/limits when* loading a NULL selector. This has been designated* X86_BUG_NULL_SEG.** Zen3 CPUs advertise Null Selector Clears Base in CPUID.* Zen2 CPUs also have this behaviour, but no CPUID bit.** A hypervisor may sythesize the bit, but may also hide it* for migration safety, so we must not probe for model* specific behaviour when virtualised.*/- if (c->extended_cpuid_level >= 0x80000021 && cpuid_eax(0x80000021) & BIT(6))
nscb = true;- if (!cpu_has(c, X86_FEATURE_HYPERVISOR) && !nscb && c->x86 == 0x17)
nscb = check_null_seg_clears_base(c);- if (!nscb)
set_cpu_bug(c, X86_BUG_NULL_SEG);#ifdef CONFIG_X86_64 set_cpu_cap(c, X86_FEATURE_SYSCALL32); #else
Can we do something like this?
It is carved out into a separate function which you can simply call from early_init_amd() and early_init_hygon().
I guess you can put that function in arch/x86/kernel/cpu/common.c or so.
Then, you should put the comments right over the code like I've done below so that one can follow what's going on with each particular check.
I've also flipped the logic a bit and it is simpler this way.
Totally untested of course.
static void early_probe_null_seg_clearing_base(struct cpuinfo_x86 *c) { /* * A hypervisor may sythesize the bit, but may also hide it * for migration safety, so do not probe for model-specific * behaviour when virtualised. */ if (cpu_has(c, X86_FEATURE_HYPERVISOR)) return;
/* Zen3 CPUs advertise Null Selector Clears Base in CPUID. */ if (c->extended_cpuid_level >= 0x80000021 && cpuid_eax(0x80000021) & BIT(6)) return;
/* Zen2 CPUs also have this behaviour, but no CPUID bit. */ if (c->x86 == 0x17 && check_null_seg_clears_base(c)) return;
/* All the remaining ones are affected */ set_cpu_bug(c, X86_BUG_NULL_SEG); }
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c index 0f8885949e8c..2ca4afb97247 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -1395,7 +1395,7 @@ void __init early_cpu_init(void) early_identify_cpu(&boot_cpu_data); } -static void detect_null_seg_behavior(struct cpuinfo_x86 *c) +bool check_null_seg_clears_base(struct cpuinfo_x86 *c) { #ifdef CONFIG_X86_64 /* @@ -1418,10 +1418,10 @@ static void detect_null_seg_behavior(struct cpuinfo_x86 *c) wrmsrl(MSR_FS_BASE, 1); loadsegment(fs, 0); rdmsrl(MSR_FS_BASE, tmp);
- if (tmp != 0)
set_cpu_bug(c, X86_BUG_NULL_SEG);
- return tmp == 0;
#endif
- return true;
} static void generic_identify(struct cpuinfo_x86 *c) @@ -1457,8 +1457,6 @@ static void generic_identify(struct cpuinfo_x86 *c) get_model_name(c); /* Default name */
- detect_null_seg_behavior(c);
- /*
- ESPFIX is a strange bug. All real CPUs have it. Paravirt
- systems that run Linux at CPL > 0 may or may not have the
So this function is called on all x86 CPUs. Are you sure others besides AMD and Hygon do not have the same issue?
IOW, I wouldn't remove that call here.
But then this is the identify() phase in the boot process and you've moved it to early_identify() by putting it in the ->c_early_init() function pointer on AMD and Hygon.
Is there any particular reasoning for this or can that detection remain in ->c_identify()?
Because if this null seg behavior detection should happen on all CPUs - and I think it should, because, well, it has been that way until now - then the vendor specific identification minus what detect_null_seg_behavior() does should run first and then after ->c_identify() is done, you should do something like:
if (!cpu_has_bug(c, X86_BUG_NULL_SEG)) { if (!check_null_seg_clears_base(c)) set_cpu_bug(c, X86_BUG_NULL_SEG); }
so that it still takes place on all CPUs.
I.e., you should split the detection.
I hope I'm making sense ...
Ah, btw, that @c parameter to detect_null_seg_behavior() is unused - you should remove it in a pre-patch.
Thx.