x86/umip: Print a warning into the syslog if UMIP-protected instructions are used

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=y 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().

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi V. Shankar <ravi.v.shankar@intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: ricardo.neri@intel.com
Link: http://lkml.kernel.org/r/1511233476-17088-1-git-send-email-ricardo.neri-calderon@linux.intel.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/umip.c b/arch/x86/kernel/umip.c
index 1f1f2d54dab5aae4d76d9592038dea5128a42826..dabbac30acdff563c74ca245cac6ed9a8a07216b 100644 (file)
--- 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");
+
        if (emulate_umip_insn(&insn, umip_inst, dummy_data, &dummy_data_size))
                return false;