x86/process: Don't mix user/kernel regs in 64bit __show_regs()

When the kernel.print-fatal-signals sysctl has been enabled, a simple
userspace crash will cause the kernel to write a crash dump that contains,
among other things, the kernel gsbase into dmesg.

As suggested by Andy, limit output to pt_regs, FS_BASE and KERNEL_GS_BASE
in this case.

This also moves the bitness-specific logic from show_regs() into
process_{32,64}.c.

Fixes: 45807a1df9f5 ("vdso: print fatal signals")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180831194151.123586-1-jannh@google.com

arch/x86/include/asm/kdebug.h

@@ -22,10 +22,20 @@ enum die_val {
 	DIE_NMIUNKNOWN,
 };
 
+enum show_regs_mode {
+	SHOW_REGS_SHORT,
+	/*
+	 * For when userspace crashed, but we don't think it's our fault, and
+	 * therefore don't print kernel registers.
+	 */
+	SHOW_REGS_USER,
+	SHOW_REGS_ALL
+};
+
 extern void die(const char *, struct pt_regs *,long);
 extern int __must_check __die(const char *, struct pt_regs *, long);
 extern void show_stack_regs(struct pt_regs *regs);
-extern void __show_regs(struct pt_regs *regs, int all);
+extern void __show_regs(struct pt_regs *regs, enum show_regs_mode);
 extern void show_iret_regs(struct pt_regs *regs);
 extern unsigned long oops_begin(void);
 extern void oops_end(unsigned long, struct pt_regs *, int signr);

arch/x86/kernel/dumpstack.c

@@ -146,7 +146,7 @@ static void show_regs_if_on_stack(struct stack_info *info, struct pt_regs *regs,
 	 * they can be printed in the right context.
 	 */
 	if (!partial && on_stack(info, regs, sizeof(*regs))) {
-		__show_regs(regs, 0);
+		__show_regs(regs, SHOW_REGS_SHORT);
 
 	} else if (partial && on_stack(info, (void *)regs + IRET_FRAME_OFFSET,
 				       IRET_FRAME_SIZE)) {
@@ -344,7 +344,7 @@ void oops_end(unsigned long flags, struct pt_regs *regs, int signr)
 	oops_exit();
 
 	/* Executive summary in case the oops scrolled away */
-	__show_regs(&exec_summary_regs, true);
+	__show_regs(&exec_summary_regs, SHOW_REGS_ALL);
 
 	if (!signr)
 		return;
@@ -407,14 +407,9 @@ void die(const char *str, struct pt_regs *regs, long err)
 
 void show_regs(struct pt_regs *regs)
 {
-	bool all = true;
-
 	show_regs_print_info(KERN_DEFAULT);
 
-	if (IS_ENABLED(CONFIG_X86_32))
-		all = !user_mode(regs);
-
-	__show_regs(regs, all);
+	__show_regs(regs, user_mode(regs) ? SHOW_REGS_USER : SHOW_REGS_ALL);
 
 	/*
 	 * When in-kernel, we also print out the stack at the time of the fault..

arch/x86/kernel/process_32.c

@@ -59,7 +59,7 @@
 #include <asm/intel_rdt_sched.h>
 #include <asm/proto.h>
 
-void __show_regs(struct pt_regs *regs, int all)
+void __show_regs(struct pt_regs *regs, enum show_regs_mode mode)
 {
 	unsigned long cr0 = 0L, cr2 = 0L, cr3 = 0L, cr4 = 0L;
 	unsigned long d0, d1, d2, d3, d6, d7;
@@ -85,7 +85,7 @@ void __show_regs(struct pt_regs *regs, int all)
 	printk(KERN_DEFAULT "DS: %04x ES: %04x FS: %04x GS: %04x SS: %04x EFLAGS: %08lx\n",
 	       (u16)regs->ds, (u16)regs->es, (u16)regs->fs, gs, ss, regs->flags);
 
-	if (!all)
+	if (mode != SHOW_REGS_ALL)
 		return;
 
 	cr0 = read_cr0();

arch/x86/kernel/process_64.c

@@ -62,7 +62,7 @@
 __visible DEFINE_PER_CPU(unsigned long, rsp_scratch);
 
 /* Prints also some state that isn't saved in the pt_regs */
-void __show_regs(struct pt_regs *regs, int all)
+void __show_regs(struct pt_regs *regs, enum show_regs_mode mode)
 {
 	unsigned long cr0 = 0L, cr2 = 0L, cr3 = 0L, cr4 = 0L, fs, gs, shadowgs;
 	unsigned long d0, d1, d2, d3, d6, d7;
@@ -87,9 +87,17 @@ void __show_regs(struct pt_regs *regs, int all)
 	printk(KERN_DEFAULT "R13: %016lx R14: %016lx R15: %016lx\n",
 	       regs->r13, regs->r14, regs->r15);
 
-	if (!all)
+	if (mode == SHOW_REGS_SHORT)
 		return;
 
+	if (mode == SHOW_REGS_USER) {
+		rdmsrl(MSR_FS_BASE, fs);
+		rdmsrl(MSR_KERNEL_GS_BASE, shadowgs);
+		printk(KERN_DEFAULT "FS:  %016lx GS:  %016lx\n",
+		       fs, shadowgs);
+		return;
+	}
+
 	asm("movl %%ds,%0" : "=r" (ds));
 	asm("movl %%cs,%0" : "=r" (cs));
 	asm("movl %%es,%0" : "=r" (es));