Re: [PATCH v3] x86/umip: Warn if UMIP-protected instructions are used

[Ingo Molnar]

* Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx> wrote:

> Print a rate-limited warning when a user space program attempts to execute
> any of the instructions that UMIP protects (i.e., SGDT, SIDT, SLDT, STR
> and SMSW).
> 
> This is useful because, when CONFIG_X86_INTEL_UMIP is selected and
> supported by the hardware, user space programs that try to execute such
> instructions will receive a SIGSEGV signal that they might not expect.
> In the specific cases for which emulation is provided (instructions SGDT,
> SIDT and SMSW in protected and virtual-8086 modes), no signal is
> generated. However, a warning is helpful to encourage updates in such
> programs to avoid the use of such instructions.
> 
> Warnings are printed via a customized printk() function that also provides
> information about the program that attempted to use the affected
> instructions.
> 
> Utility macros are defined to wrap umip_printk() for the error and warning
> kernel log levels.
> 
> While here, replace an existing call to the generic rate-limited pr_err()
> with the new umip_pr_err().
> 
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxx>
> Cc: Tony Luck <tony.luck@xxxxxxxxx>
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Ravi V. Shankar <ravi.v.shankar@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> ---
> Changes since V2:
> * Dropped patches 1, 2, and 3 as they were applied on the tip repository.
> * Renamed umip_pr_warn() to umip_printk(). Instead, added utility macros
>   for the error and warning kernel log levels.
> * Implemented umip_printk() as a variadic function. This also includes
>   using the __printf() macro to type-check arguments of the format
>   string and arguments.
> * Capitalized all the instructions' mnemonics in strings used for printing
>   warnings.
> 
> Changes since V1:
> * Capitalized all the instructions' mnemonics in both code and patch
>   descriptions.
> * Corrected documentation of umip_pr_warn() to correctly reflect the function
>   name.
> * Updated description of patch #4 to describe the update to the existing
>   rate-limited pr_err().
> ---
>  arch/x86/kernel/umip.c | 62 ++++++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 58 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
> index 1f1f2d5..dabbac3 100644
> --- a/arch/x86/kernel/umip.c
> +++ b/arch/x86/kernel/umip.c
> @@ -82,6 +82,57 @@
>  #define	UMIP_INST_SLDT  3       /* 0F 00 /0 */
>  #define	UMIP_INST_STR   4       /* 0F 00 /1 */
>  
> +const char * const umip_insns[5] = {
> +	[UMIP_INST_SGDT] = "SGDT",
> +	[UMIP_INST_SIDT] = "SIDT",
> +	[UMIP_INST_SMSW] = "SMSW",
> +	[UMIP_INST_SLDT] = "SLDT",
> +	[UMIP_INST_STR] = "STR",
> +};
> +
> +#define umip_pr_err(regs, fmt, ...) \
> +	umip_printk(regs, KERN_ERR, fmt, ##__VA_ARGS__)
> +#define umip_pr_warning(regs, fmt, ...) \
> +	umip_printk(regs, KERN_WARNING, fmt,  ##__VA_ARGS__)
> +
> +/**
> + * umip_printk() - Print a rate-limited message
> + * @regs:	Register set with the context in which the warning is printed
> + * @log_level:	Kernel log level to print the message
> + * @fmt:	The text string to print
> + *
> + * Print the text contained in @fmt. The print rate is limited to bursts of 5
> + * messages every two minutes. The purpose of this customized version of
> + * printk() is to print messages when user space processes use any of the
> + * UMIP-protected instructions. Thus, the printed text is prepended with the
> + * task name and process ID number of the current task as well as the
> + * instruction and stack pointers in @regs as seen when entering kernel mode.
> + *
> + * Returns:
> + *
> + * None.
> + */
> +static __printf(3, 4)
> +void umip_printk(const struct pt_regs *regs, const char *log_level,
> +		 const char *fmt, ...)
> +{
> +	/* Bursts of 5 messages every two minutes */
> +	static DEFINE_RATELIMIT_STATE(ratelimit, 2 * 60 * HZ, 5);
> +	struct task_struct *tsk = current;
> +	struct va_format vaf;
> +	va_list args;
> +
> +	if (!__ratelimit(&ratelimit))
> +		return;
> +
> +	va_start(args, fmt);
> +	vaf.fmt = fmt;
> +	vaf.va = &args;
> +	printk("%s" pr_fmt("%s[%d] ip:%lx sp:%lx: %pV"), log_level, tsk->comm,
> +	       task_pid_nr(tsk), regs->ip, regs->sp, &vaf);
> +	va_end(args);
> +}
> +
>  /**
>   * identify_insn() - Identify a UMIP-protected instruction
>   * @insn:	Instruction structure with opcode and ModRM byte.
> @@ -236,10 +287,8 @@ static void force_sig_info_umip_fault(void __user *addr, struct pt_regs *regs)
>  	if (!(show_unhandled_signals && unhandled_signal(tsk, SIGSEGV)))
>  		return;
>  
> -	pr_err_ratelimited("%s[%d] umip emulation segfault ip:%lx sp:%lx error:%x in %lx\n",
> -			   tsk->comm, task_pid_nr(tsk), regs->ip,
> -			   regs->sp, X86_PF_USER | X86_PF_WRITE,
> -			   regs->ip);
> +	umip_pr_err(regs, "segfault in emulation. error%x\n",
> +		    X86_PF_USER | X86_PF_WRITE);
>  }
>  
>  /**
> @@ -326,10 +375,15 @@ bool fixup_umip_exception(struct pt_regs *regs)
>  	if (umip_inst < 0)
>  		return false;
>  
> +	umip_pr_warning(regs, "%s instruction cannot be used by applications.\n",
> +			umip_insns[umip_inst]);
> +
>  	/* Do not emulate SLDT, STR or user long mode processes. */
>  	if (umip_inst == UMIP_INST_STR || umip_inst == UMIP_INST_SLDT || user_64bit_mode(regs))
>  		return false;
>  
> +	umip_pr_warning(regs, "For now, expensive software emulation returns the result.\n");

Ok, this looks much nicer all around. I'll test it and push it out if all is good.

Thanks,

	Ingo