[PATCH v2 2/3] x86/traps: Print non-canonical 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 whether the #GP
looks like it was caused by a non-canonical address, and if so, print
that address.

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.

Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
---

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)

 arch/x86/kernel/traps.c | 45 +++++++++++++++++++++++++++++++++++++++--
 1 file changed, 43 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index c90312146da0..12d42697a18e 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>
@@ -509,6 +511,38 @@ dotraplinkage void do_bounds(struct pt_regs *regs, long error_code)
 	do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, error_code, 0, NULL);
 }
 
+/*
+ * On 64-bit, if an uncaught #GP occurs while dereferencing a non-canonical
+ * address, print that address.
+ */
+static void print_kernel_gp_address(struct pt_regs *regs)
+{
+#ifdef CONFIG_X86_64
+	u8 insn_bytes[MAX_INSN_SIZE];
+	struct insn insn;
+	unsigned long addr_ref;
+
+	if (probe_kernel_read(insn_bytes, (void *)regs->ip, MAX_INSN_SIZE))
+		return;
+
+	kernel_insn_init(&insn, insn_bytes, MAX_INSN_SIZE);
+	insn_get_modrm(&insn);
+	insn_get_sib(&insn);
+	addr_ref = (unsigned long)insn_get_addr_ref(&insn, regs);
+
+	/* Bail out if insn_get_addr_ref() failed or we got a kernel address. */
+	if (addr_ref >= ~__VIRTUAL_MASK)
+		return;
+
+	/* Bail out if the entire operand is in the canonical user half. */
+	if (addr_ref + insn.opnd_bytes - 1 <= __VIRTUAL_MASK)
+		return;
+
+	pr_alert("probably dereferencing non-canonical address 0x%016lx\n",
+		 addr_ref);
+#endif
+}
+
 dotraplinkage void
 do_general_protection(struct pt_regs *regs, long error_code)
 {
@@ -547,8 +581,15 @@ 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)
+			pr_alert("GPF is segment-related (see error code)\n");
+		else
+			print_kernel_gp_address(regs);
+
+		die(desc, regs, error_code);
 		return;
 	}
 
-- 
2.24.0.432.g9d3f5f5b63-goog