[PATCH v5 1/4] x86/insn-eval: Add support for 64-bit kernel mode

[PATCH v5 1/4] x86/insn-eval: Add support for 64-bit kernel mode

[Jann Horn]

To support evaluating 64-bit kernel mode instructions:

Replace existing checks for user_64bit_mode() with a new helper that
checks whether code is being executed in either 64-bit kernel mode or
64-bit user mode.

Select the GS base depending on whether the instruction is being
evaluated in kernel mode.

Signed-off-by: Jann Horn <jannh@google.com>
---

Notes:
    v2-v5:
      no changes

 arch/x86/include/asm/ptrace.h | 13 +++++++++++++
 arch/x86/lib/insn-eval.c      | 26 +++++++++++++++-----------
 2 files changed, 28 insertions(+), 11 deletions(-)

diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
index 5057a8ed100b..ac45b06941a5 100644
--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h
@@ -159,6 +159,19 @@ static inline bool user_64bit_mode(struct pt_regs *regs)
 #endif
 }
 
+/*
+ * Determine whether the register set came from any context that is running in
+ * 64-bit mode.
+ */
+static inline bool any_64bit_mode(struct pt_regs *regs)
+{
+#ifdef CONFIG_X86_64
+	return !user_mode(regs) || user_64bit_mode(regs);
+#else
+	return false;
+#endif
+}
+
 #ifdef CONFIG_X86_64
 #define current_user_stack_pointer()	current_pt_regs()->sp
 #define compat_user_stack_pointer()	current_pt_regs()->sp
diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
index 306c3a0902ba..31600d851fd8 100644
--- a/arch/x86/lib/insn-eval.c
+++ b/arch/x86/lib/insn-eval.c
@@ -155,7 +155,7 @@ static bool check_seg_overrides(struct insn *insn, int regoff)
  */
 static int resolve_default_seg(struct insn *insn, struct pt_regs *regs, int off)
 {
-	if (user_64bit_mode(regs))
+	if (any_64bit_mode(regs))
 		return INAT_SEG_REG_IGNORE;
 	/*
 	 * Resolve the default segment register as described in Section 3.7.4
@@ -266,7 +266,7 @@ static int resolve_seg_reg(struct insn *insn, struct pt_regs *regs, int regoff)
 	 * which may be invalid at this point.
 	 */
 	if (regoff == offsetof(struct pt_regs, ip)) {
-		if (user_64bit_mode(regs))
+		if (any_64bit_mode(regs))
 			return INAT_SEG_REG_IGNORE;
 		else
 			return INAT_SEG_REG_CS;
@@ -289,7 +289,7 @@ static int resolve_seg_reg(struct insn *insn, struct pt_regs *regs, int regoff)
 	 * In long mode, segment override prefixes are ignored, except for
 	 * overrides for FS and GS.
 	 */
-	if (user_64bit_mode(regs)) {
+	if (any_64bit_mode(regs)) {
 		if (idx != INAT_SEG_REG_FS &&
 		    idx != INAT_SEG_REG_GS)
 			idx = INAT_SEG_REG_IGNORE;
@@ -646,23 +646,27 @@ unsigned long insn_get_seg_base(struct pt_regs *regs, int seg_reg_idx)
 		 */
 		return (unsigned long)(sel << 4);
 
-	if (user_64bit_mode(regs)) {
+	if (any_64bit_mode(regs)) {
 		/*
 		 * Only FS or GS will have a base address, the rest of
 		 * the segments' bases are forced to 0.
 		 */
 		unsigned long base;
 
-		if (seg_reg_idx == INAT_SEG_REG_FS)
+		if (seg_reg_idx == INAT_SEG_REG_FS) {
 			rdmsrl(MSR_FS_BASE, base);
-		else if (seg_reg_idx == INAT_SEG_REG_GS)
+		} else if (seg_reg_idx == INAT_SEG_REG_GS) {
 			/*
 			 * swapgs was called at the kernel entry point. Thus,
 			 * MSR_KERNEL_GS_BASE will have the user-space GS base.
 			 */
-			rdmsrl(MSR_KERNEL_GS_BASE, base);
-		else
+			if (user_mode(regs))
+				rdmsrl(MSR_KERNEL_GS_BASE, base);
+			else
+				rdmsrl(MSR_GS_BASE, base);
+		} else {
 			base = 0;
+		}
 		return base;
 	}
 
@@ -703,7 +707,7 @@ static unsigned long get_seg_limit(struct pt_regs *regs, int seg_reg_idx)
 	if (sel < 0)
 		return 0;
 
-	if (user_64bit_mode(regs) || v8086_mode(regs))
+	if (any_64bit_mode(regs) || v8086_mode(regs))
 		return -1L;
 
 	if (!sel)
@@ -948,7 +952,7 @@ static int get_eff_addr_modrm(struct insn *insn, struct pt_regs *regs,
 	 * following instruction.
 	 */
 	if (*regoff == -EDOM) {
-		if (user_64bit_mode(regs))
+		if (any_64bit_mode(regs))
 			tmp = regs->ip + insn->length;
 		else
 			tmp = 0;
@@ -1250,7 +1254,7 @@ static void __user *get_addr_ref_32(struct insn *insn, struct pt_regs *regs)
 	 * After computed, the effective address is treated as an unsigned
 	 * quantity.
 	 */
-	if (!user_64bit_mode(regs) && ((unsigned int)eff_addr > seg_limit))
+	if (!any_64bit_mode(regs) && ((unsigned int)eff_addr > seg_limit))
 		goto out;
 
 	/*
-- 
2.24.0.432.g9d3f5f5b63-goog

[PATCH v5 2/4] x86/traps: Print address on #GP

[Jann Horn]

A frequent cause of #GP exceptions are memory accesses to non-canonical
addresses. Unlike #PF, #GP doesn't come with a fault address in CR2, so
the kernel doesn't currently print the fault address for #GP.
Luckily, we already have the necessary infrastructure for decoding X86
instructions and computing the memory address that is being accessed;
hook it up to the #GP handler so that we can figure out the address
operand of the faulting instruction and print it.

Distinguish two cases:
  a) (Part of) the memory range being accessed lies in the non-canonical
     address range; in this case, is is likely that the address we
     decoded is actually the one that caused the #GP.
  b) The entire memory range of the operand we decoded lies in canonical
     address space; the #GP may or may not be related in some way to the
     address we computed. We'll still print it, but with hedging
     language in the message.

While it is already possible to compute the faulting address manually by
disassembling the opcode dump and evaluating the instruction against the
register dump, this should make it slightly easier to identify crashes
at a glance.

Note that the operand length, which we get from the instruction decoder
and use to determine whether the access straddles into non-canonical
address space, is currently somewhat unreliable; but it should be good
enough, considering that Linux on x86-64 never maps the page directly
before the start of the non-canonical range anyway, and therefore the
case where a memory range begins in that page and potentially straddles
into the non-canonical range should be fairly uncommon.
And if we do get this wrong, it only influences whether the error
message claims that the access is canonical.

Signed-off-by: Jann Horn <jannh@google.com>
---

Notes:
    v2:
     - print different message for segment-related GP (Borislav)
     - rewrite check for non-canonical address (Sean)
     - make it clear we don't know for sure why the GP happened (Andy)
    v3:
     - change message format to one line (Borislav)
    v4:
     - rename insn_bytes to insn_buf (Ingo)
     - add space after GPFSTR (Ingo)
     - make sizeof(desc) clearer (Ingo, Borislav)
     - also print the address (with a different message) if it's canonical (Ingo)
    v5:
     - reword comment on get_kernel_gp_address() (Sean)
     - make get_kernel_gp_address() also work on 32-bit (Sean)
     - minor nits (Sean)
     - more hedging for canonical GP (Sean)
     - let get_kernel_gp_address() return an enum (Sean)
     - rewrite commit message
    
    I have already sent a patch to syzkaller that relaxes their parsing of GPF
    messages (https://github.com/google/syzkaller/commit/432c7650) such that
    changes like the one in this patch don't break it.
    That patch has already made its way into syzbot's syzkaller instances
    according to <https://syzkaller.appspot.com/upstream>.

 arch/x86/kernel/traps.c | 70 +++++++++++++++++++++++++++++++++++++++--
 1 file changed, 67 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index f19de6f45d48..9b6e4d04112a 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -56,6 +56,8 @@
 #include <asm/mpx.h>
 #include <asm/vm86.h>
 #include <asm/umip.h>
+#include <asm/insn.h>
+#include <asm/insn-eval.h>
 
 #ifdef CONFIG_X86_64
 #include <asm/x86_init.h>
@@ -518,11 +520,56 @@ dotraplinkage void do_bounds(struct pt_regs *regs, long error_code)
 	do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, error_code, 0, NULL);
 }
 
+enum kernel_gp_hint {
+	GP_NO_HINT,
+	GP_NON_CANONICAL,
+	GP_CANONICAL
+};
+
+/*
+ * When an uncaught #GP occurs, try to determine a memory address accessed by
+ * the instruction and return that address to the caller.
+ * Also try to figure out whether any part of the access to that address was
+ * non-canonical.
+ */
+static enum kernel_gp_hint get_kernel_gp_address(struct pt_regs *regs,
+						 unsigned long *addr)
+{
+	u8 insn_buf[MAX_INSN_SIZE];
+	struct insn insn;
+
+	if (probe_kernel_read(insn_buf, (void *)regs->ip, MAX_INSN_SIZE))
+		return GP_NO_HINT;
+
+	kernel_insn_init(&insn, insn_buf, MAX_INSN_SIZE);
+	insn_get_modrm(&insn);
+	insn_get_sib(&insn);
+	*addr = (unsigned long)insn_get_addr_ref(&insn, regs);
+
+	if (*addr == -1UL)
+		return GP_NO_HINT;
+
+#ifdef CONFIG_X86_64
+	/*
+	 * Check that:
+	 *  - the operand is not in the kernel half
+	 *  - the last byte of the operand is not in the user canonical half
+	 */
+	if (*addr < ~__VIRTUAL_MASK &&
+	    *addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK)
+		return GP_NON_CANONICAL;
+#endif
+
+	return GP_CANONICAL;
+}
+
+#define GPFSTR "general protection fault"
+
 dotraplinkage void
 do_general_protection(struct pt_regs *regs, long error_code)
 {
-	const char *desc = "general protection fault";
 	struct task_struct *tsk;
+	char desc[sizeof(GPFSTR) + 50 + 2*sizeof(unsigned long) + 1] = GPFSTR;
 
 	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
 	cond_local_irq_enable(regs);
@@ -540,6 +587,9 @@ do_general_protection(struct pt_regs *regs, long error_code)
 
 	tsk = current;
 	if (!user_mode(regs)) {
+		enum kernel_gp_hint hint = GP_NO_HINT;
+		unsigned long gp_addr;
+
 		if (fixup_exception(regs, X86_TRAP_GP, error_code, 0))
 			return;
 
@@ -556,8 +606,22 @@ do_general_protection(struct pt_regs *regs, long error_code)
 			return;
 
 		if (notify_die(DIE_GPF, desc, regs, error_code,
-			       X86_TRAP_GP, SIGSEGV) != NOTIFY_STOP)
-			die(desc, regs, error_code);
+			       X86_TRAP_GP, SIGSEGV) == NOTIFY_STOP)
+			return;
+
+		if (error_code)
+			snprintf(desc, sizeof(desc), "segment-related " GPFSTR);
+		else
+			hint = get_kernel_gp_address(regs, &gp_addr);
+
+		if (hint != GP_NO_HINT)
+			snprintf(desc, sizeof(desc), GPFSTR " %s 0x%lx",
+				 (hint == GP_NON_CANONICAL) ?
+				 "probably for non-canonical address" :
+				 "maybe for address",
+				 gp_addr);
+
+		die(desc, regs, error_code);
 		return;
 	}
 
-- 
2.24.0.432.g9d3f5f5b63-goog

[PATCH v5 3/4] x86/dumpstack: Split out header line printing from __die()

[Jann Horn]

Split __die() into __die_header() and __die_body(). This allows callers to
insert extra information below the header line that initiates the bug
report.

This can e.g. be used by __die() callers to allow KASAN to print additional
information below the header line of the bug report.

Signed-off-by: Jann Horn <jannh@google.com>
---

Notes:
    I think that it's nicer to have KASAN's notes about the
    bug below the first oops line from the kernel.
    This also means that tools that work with kernel oops
    reports can just trigger on the "general protection fault"
    line with the die counter and so on, and just include the
    text from on there, and the KASAN message will automatically
    be included.
    But if you think that the code looks too ugly, I'd be
    happy to change that back and drop this patch from the
    series.
    
    v3:
      new patch
    v4-v5:
      no changes

 arch/x86/include/asm/kdebug.h |  3 +++
 arch/x86/kernel/dumpstack.c   | 13 ++++++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/kdebug.h b/arch/x86/include/asm/kdebug.h
index 75f1e35e7c15..a0050fabce42 100644
--- a/arch/x86/include/asm/kdebug.h
+++ b/arch/x86/include/asm/kdebug.h
@@ -33,6 +33,9 @@ enum show_regs_mode {
 };
 
 extern void die(const char *, struct pt_regs *,long);
+extern void __die_header(const char *str, struct pt_regs *regs, long err);
+extern int __must_check __die_body(const char *str, struct pt_regs *regs,
+				   long err);
 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, enum show_regs_mode);
diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index e07424e19274..6436f3f5f803 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -365,7 +365,7 @@ void oops_end(unsigned long flags, struct pt_regs *regs, int signr)
 }
 NOKPROBE_SYMBOL(oops_end);
 
-int __die(const char *str, struct pt_regs *regs, long err)
+void __die_header(const char *str, struct pt_regs *regs, long err)
 {
 	const char *pr = "";
 
@@ -384,7 +384,11 @@ int __die(const char *str, struct pt_regs *regs, long err)
 	       IS_ENABLED(CONFIG_KASAN)   ? " KASAN"           : "",
 	       IS_ENABLED(CONFIG_PAGE_TABLE_ISOLATION) ?
 	       (boot_cpu_has(X86_FEATURE_PTI) ? " PTI" : " NOPTI") : "");
+}
+NOKPROBE_SYMBOL(__die_header);
 
+int __die_body(const char *str, struct pt_regs *regs, long err)
+{
 	show_regs(regs);
 	print_modules();
 
@@ -394,6 +398,13 @@ int __die(const char *str, struct pt_regs *regs, long err)
 
 	return 0;
 }
+NOKPROBE_SYMBOL(__die_body);
+
+int __die(const char *str, struct pt_regs *regs, long err)
+{
+	__die_header(str, regs, err);
+	return __die_body(str, regs, err);
+}
 NOKPROBE_SYMBOL(__die);
 
 /*
-- 
2.24.0.432.g9d3f5f5b63-goog

[PATCH v5 4/4] x86/kasan: Print original address on #GP

[Jann Horn]

Make #GP exceptions caused by out-of-bounds KASAN shadow accesses easier
to understand by computing the address of the original access and
printing that. More details are in the comments in the patch.

This turns an error like this:

    kasan: CONFIG_KASAN_INLINE enabled
    kasan: GPF could be caused by NULL-ptr deref or user memory access
    general protection fault probably for non-canonical address
        0xe017577ddf75b7dd: 0000 [#1] PREEMPT SMP KASAN PTI

into this:

    general protection fault probably for non-canonical address
        0xe017577ddf75b7dd: 0000 [#1] PREEMPT SMP KASAN PTI
    KASAN: maybe wild-memory-access in range
        [0x00badbeefbadbee8-0x00badbeefbadbeef]

The hook is placed in architecture-independent code, but is currently
only wired up to the X86 exception handler because I'm not sufficiently
familiar with the address space layout and exception handling mechanisms
on other architectures.

Signed-off-by: Jann Horn <jannh@google.com>
---

Notes:
    v2:
     - move to mm/kasan/report.c (Dmitry)
     - change hook name to be more generic
     - use TASK_SIZE instead of TASK_SIZE_MAX for compiling on non-x86
     - don't open-code KASAN_SHADOW_MASK (Dmitry)
     - add "KASAN: " prefix, but not "BUG: " (Andrey, Dmitry)
     - use same naming scheme as get_wild_bug_type (Andrey)
     - this version was "Reviewed-by: Dmitry Vyukov <dvyukov@google.com>"
    v3:
     - adjusted example output in commit message based on
       changes in preceding patch
     - ensure that KASAN output happens after bust_spinlocks(1)
     - moved hook in arch/x86/kernel/traps.c such that output
       appears after the first line of KASAN-independent error report
    v4:
     - adjust patch to changes in x86/traps patch
    v5:
     - adjust patch to changes in x86/traps patch
     - fix bug introduced in v3: remove die() call after oops_end()

 arch/x86/kernel/traps.c     | 12 ++++++++++-
 arch/x86/mm/kasan_init_64.c | 21 -------------------
 include/linux/kasan.h       |  6 ++++++
 mm/kasan/report.c           | 40 +++++++++++++++++++++++++++++++++++++
 4 files changed, 57 insertions(+), 22 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 9b6e4d04112a..a7dade19783a 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -37,6 +37,7 @@
 #include <linux/mm.h>
 #include <linux/smp.h>
 #include <linux/io.h>
+#include <linux/kasan.h>
 #include <asm/stacktrace.h>
 #include <asm/processor.h>
 #include <asm/debugreg.h>
@@ -589,6 +590,8 @@ do_general_protection(struct pt_regs *regs, long error_code)
 	if (!user_mode(regs)) {
 		enum kernel_gp_hint hint = GP_NO_HINT;
 		unsigned long gp_addr;
+		unsigned long flags;
+		int sig;
 
 		if (fixup_exception(regs, X86_TRAP_GP, error_code, 0))
 			return;
@@ -621,7 +624,14 @@ do_general_protection(struct pt_regs *regs, long error_code)
 				 "maybe for address",
 				 gp_addr);
 
-		die(desc, regs, error_code);
+		flags = oops_begin();
+		sig = SIGSEGV;
+		__die_header(desc, regs, error_code);
+		if (hint == GP_NON_CANONICAL)
+			kasan_non_canonical_hook(gp_addr);
+		if (__die_body(desc, regs, error_code))
+			sig = 0;
+		oops_end(flags, regs, sig);
 		return;
 	}
 
diff --git a/arch/x86/mm/kasan_init_64.c b/arch/x86/mm/kasan_init_64.c
index 296da58f3013..69c437fb21cc 100644
--- a/arch/x86/mm/kasan_init_64.c
+++ b/arch/x86/mm/kasan_init_64.c
@@ -245,23 +245,6 @@ static void __init kasan_map_early_shadow(pgd_t *pgd)
 	} while (pgd++, addr = next, addr != end);
 }
 
-#ifdef CONFIG_KASAN_INLINE
-static int kasan_die_handler(struct notifier_block *self,
-			     unsigned long val,
-			     void *data)
-{
-	if (val == DIE_GPF) {
-		pr_emerg("CONFIG_KASAN_INLINE enabled\n");
-		pr_emerg("GPF could be caused by NULL-ptr deref or user memory access\n");
-	}
-	return NOTIFY_OK;
-}
-
-static struct notifier_block kasan_die_notifier = {
-	.notifier_call = kasan_die_handler,
-};
-#endif
-
 void __init kasan_early_init(void)
 {
 	int i;
@@ -298,10 +281,6 @@ void __init kasan_init(void)
 	int i;
 	void *shadow_cpu_entry_begin, *shadow_cpu_entry_end;
 
-#ifdef CONFIG_KASAN_INLINE
-	register_die_notifier(&kasan_die_notifier);
-#endif
-
 	memcpy(early_top_pgt, init_top_pgt, sizeof(early_top_pgt));
 
 	/*
diff --git a/include/linux/kasan.h b/include/linux/kasan.h
index cc8a03cc9674..7305024b44e3 100644
--- a/include/linux/kasan.h
+++ b/include/linux/kasan.h
@@ -194,4 +194,10 @@ static inline void *kasan_reset_tag(const void *addr)
 
 #endif /* CONFIG_KASAN_SW_TAGS */
 
+#ifdef CONFIG_KASAN_INLINE
+void kasan_non_canonical_hook(unsigned long addr);
+#else /* CONFIG_KASAN_INLINE */
+static inline void kasan_non_canonical_hook(unsigned long addr) { }
+#endif /* CONFIG_KASAN_INLINE */
+
 #endif /* LINUX_KASAN_H */
diff --git a/mm/kasan/report.c b/mm/kasan/report.c
index 621782100eaa..5ef9f24f566b 100644
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c
@@ -512,3 +512,43 @@ void __kasan_report(unsigned long addr, size_t size, bool is_write, unsigned lon
 
 	end_report(&flags);
 }
+
+#ifdef CONFIG_KASAN_INLINE
+/*
+ * With CONFIG_KASAN_INLINE, accesses to bogus pointers (outside the high
+ * canonical half of the address space) cause out-of-bounds shadow memory reads
+ * before the actual access. For addresses in the low canonical half of the
+ * address space, as well as most non-canonical addresses, that out-of-bounds
+ * shadow memory access lands in the non-canonical part of the address space.
+ * Help the user figure out what the original bogus pointer was.
+ */
+void kasan_non_canonical_hook(unsigned long addr)
+{
+	unsigned long orig_addr;
+	const char *bug_type;
+
+	if (addr < KASAN_SHADOW_OFFSET)
+		return;
+
+	orig_addr = (addr - KASAN_SHADOW_OFFSET) << KASAN_SHADOW_SCALE_SHIFT;
+	/*
+	 * For faults near the shadow address for NULL, we can be fairly certain
+	 * that this is a KASAN shadow memory access.
+	 * For faults that correspond to shadow for low canonical addresses, we
+	 * can still be pretty sure - that shadow region is a fairly narrow
+	 * chunk of the non-canonical address space.
+	 * But faults that look like shadow for non-canonical addresses are a
+	 * really large chunk of the address space. In that case, we still
+	 * print the decoded address, but make it clear that this is not
+	 * necessarily what's actually going on.
+	 */
+	if (orig_addr < PAGE_SIZE)
+		bug_type = "null-ptr-deref";
+	else if (orig_addr < TASK_SIZE)
+		bug_type = "probably user-memory-access";
+	else
+		bug_type = "maybe wild-memory-access";
+	pr_alert("KASAN: %s in range [0x%016lx-0x%016lx]\n", bug_type,
+		 orig_addr, orig_addr + KASAN_SHADOW_MASK);
+}
+#endif
-- 
2.24.0.432.g9d3f5f5b63-goog

Re: [PATCH v5 2/4] x86/traps: Print address on #GP

[Sean Christopherson]

On Thu, Nov 28, 2019 at 12:49:14AM +0100, Jann Horn wrote:

With a few nits below,

Reviewed-and-tested-by: Sean Christopherson <sean.j.christopherson@intel.com>

> +#define GPFSTR "general protection fault"
> +
>  dotraplinkage void
>  do_general_protection(struct pt_regs *regs, long error_code)
>  {
> -	const char *desc = "general protection fault";
>  	struct task_struct *tsk;
> +	char desc[sizeof(GPFSTR) + 50 + 2*sizeof(unsigned long) + 1] = GPFSTR;

Nit, x86 maintainers prefer inverse fir tree for variable declarations.

>  
>  	RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
>  	cond_local_irq_enable(regs);
> @@ -540,6 +587,9 @@ do_general_protection(struct pt_regs *regs, long error_code)
>  
>  	tsk = current;
>  	if (!user_mode(regs)) {
> +		enum kernel_gp_hint hint = GP_NO_HINT;
> +		unsigned long gp_addr;
> +
>  		if (fixup_exception(regs, X86_TRAP_GP, error_code, 0))
>  			return;
>  
> @@ -556,8 +606,22 @@ do_general_protection(struct pt_regs *regs, long error_code)
>  			return;
>  
>  		if (notify_die(DIE_GPF, desc, regs, error_code,
> -			       X86_TRAP_GP, SIGSEGV) != NOTIFY_STOP)
> -			die(desc, regs, error_code);
> +			       X86_TRAP_GP, SIGSEGV) == NOTIFY_STOP)
> +			return;
> +
> +		if (error_code)
> +			snprintf(desc, sizeof(desc), "segment-related " GPFSTR);
> +		else
> +			hint = get_kernel_gp_address(regs, &gp_addr);
> +
> +		if (hint != GP_NO_HINT)
> +			snprintf(desc, sizeof(desc), GPFSTR " %s 0x%lx",

Nit, probably should have a comma before the hint, i.e. GPFSTR ", %s...".

    general protection fault maybe for address 0xffffc9000017cf58: 0000 [#1] SMP
    general protection fault probably for non-canonical address 0xdead000000000000: 0000 [#1] SMP

  vs. 

    general protection fault, maybe for address 0xffffc9000017cf58: 0000 [#1] SMP
    general protection fault, probably for non-canonical address 0xdead000000000000: 0000 [#1] SMP

> +				 (hint == GP_NON_CANONICAL) ?
> +				 "probably for non-canonical address" :
> +				 "maybe for address",
> +				 gp_addr);
> +
> +		die(desc, regs, error_code);
>  		return;
>  	}
>  
> -- 
> 2.24.0.432.g9d3f5f5b63-goog
>