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

[Ricardo Neri]

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");
+
 	if (emulate_umip_insn(&insn, umip_inst, dummy_data, &dummy_data_size))
 		return false;
 
-- 
2.7.4