Re: [Devel] [PATCH RK VZ10] x86/traps: emulate CPUID faults for user tasks in exc_general_protection()

[Pavel Tikhomirov <ptikhomirov@virtuozzo.com>]

Reviewed-by: Pavel Tikhomirov <ptikhomirov@virtuozzo.com>

On 11/2/26 11:04, Vladimir Riabchun wrote:
> A non-empty /proc/vz/cpuid_override table makes ve_setup_task() set
> TIF_CPUID_OVERRIDE on every task entering a VE, so the CPU raises #GP
> on each CPUID the task executes and check_cpuid_fault() is supposed
> to emulate the instruction with the table values.
> 
> The call sits after the user-mode branch of exc_general_protection(),
> which ends in gp_user_force_sig_segv() and goto exit, so it is reached
> only for a kernel-mode #GP, where CPUID never faults. Every Container
> task dies with SIGSEGV on the first CPUID of glibc init_cpu_features()
> instead of getting the override:
> 
> traps: bash[179661] general protection fault ip:7fa8b470357f ... in
> ld-linux-x86-64.so.2[1857f,...]
> 
> The original vz8 patch was written for 4.18, where do_general_protection()
> handled kernel mode first and the call landed on the user path. The vz9
> port kept the relative position after upstream had reversed the order of
> the two branches.
> 
> Move the call into the user-mode branch, before the signal is sent.
> check_cpuid_fault() still declines tasks with TIF_NOCPUID, so
> arch_prctl(ARCH_SET_CPUID, 0) keeps its SIGSEGV semantics.
> 
> Fixes: 1033fe455434 ("arch/x86: introduce cpuid override")
> https://virtuozzo.atlassian.net/browse/VSTOR-148656
> 
> Feature: x86: cpu pools (cpuid override)
> Reviewed-by: Vasileios Almpanis vasileios.almpanis@virtuozzo.com
> Link: http://lore.virtuozzo.com/20261006-b4-prep-khorenko-vstor-148656-just-cpuid-v1-1-6f2a2a6deef3@virtuozzo.com
> Signed-off-by: Konstantin Khorenko khorenko@virtuozzo.com
> 
> ReadyKernel-specific changes:
> We can't patch exc_general_protection directly, so we need another place.
> fixup_vdso_exception is not inlined and is the last check on user-mode path.
> We can perform CPUID fault check inside fixup_vdso_exception on false path,
> that will be the closes reproduction of fix behavior.
> 
> RK_BUG: VSTOR-152668
> RK_CHANGELOG: Fix GP handler to support CPUID emulation and allow container start.
> 
> Signed-off-by: Vladimir Riabchun vladimir.riabchun@virtuozzo.com
> ---
>  arch/x86/entry/vdso/extable.c | 31 ++++++++++++++++++++++++++++++-
>  1 file changed, 30 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/entry/vdso/extable.c b/arch/x86/entry/vdso/extable.c
> index afcf5b65beef..ca8c184925ec 100644
> --- a/arch/x86/entry/vdso/extable.c
> +++ b/arch/x86/entry/vdso/extable.c
> @@ -9,6 +9,30 @@ struct vdso_exception_table_entry {
>  	int insn, fixup;
>  };
>  
> +static int check_cpuid_fault(struct pt_regs *regs, long error_code)
> +{
> +	unsigned long addr;
> +	unsigned short opcode;
> +
> +	if (error_code != 0)
> +		return 0;
> +
> +	if (test_thread_flag(TIF_NOCPUID))
> +		return 0;
> +
> +	addr = convert_ip_to_linear(current, regs);
> +	if (get_user(opcode, (unsigned short __user *)addr))
> +		return 0;
> +
> +	if (opcode != 0xa20f)
> +		return 0;
> +
> +	do_cpuid_fault(regs);
> +
> +	regs->ip += 2;
> +	return 1;
> +}
> +
>  bool fixup_vdso_exception(struct pt_regs *regs, int trapnr,
>  			  unsigned long error_code, unsigned long fault_addr)
>  {
> @@ -26,7 +50,7 @@ bool fixup_vdso_exception(struct pt_regs *regs, int trapnr,
>  		return false;
>  
>  	if (!current->mm->context.vdso)
> -		return false;
> +		goto try_fixup_cupid;
>  
>  	base =  (unsigned long)current->mm->context.vdso + image->extable_base;
>  	nr_entries = image->extable_len / (sizeof(*extable));
> @@ -42,5 +66,10 @@ bool fixup_vdso_exception(struct pt_regs *regs, int trapnr,
>  		}
>  	}
>  
> +try_fixup_cupid:
> +	/* Only exc_general_protection can go this way */
> +	if (trapnr == X86_TRAP_GP && check_cpuid_fault(regs, error_code))
> +		return true;
> +
>  	return false;
>  }

-- 
Best regards, Pavel Tikhomirov
Senior Software Developer, Virtuozzo.

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