[PATCH v4 2/4] 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)
    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)
    
    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 | 64 +++++++++++++++++++++++++++++++++++++++--
 1 file changed, 61 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index c90312146da0..b90635f29b9f 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>
@@ -509,11 +511,50 @@ 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, return that address.
+ */
+static bool get_kernel_gp_address(struct pt_regs *regs, unsigned long *addr,
+					   bool *non_canonical)
+{
+#ifdef CONFIG_X86_64
+	u8 insn_buf[MAX_INSN_SIZE];
+	struct insn insn;
+
+	if (probe_kernel_read(insn_buf, (void *)regs->ip, MAX_INSN_SIZE))
+		return false;
+
+	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 == (unsigned long)-1L)
+		return false;
+
+	/*
+	 * Check that:
+	 *  - the address is not in the kernel half or -1 (which means the
+	 *    decoder failed to decode it)
+	 *  - the last byte of the address is not in the user canonical half
+	 */
+	*non_canonical = *addr < ~__VIRTUAL_MASK &&
+			 *addr + insn.opnd_bytes - 1 > __VIRTUAL_MASK;
+
+	return true;
+#else
+	return false;
+#endif
+}
+
+#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);
@@ -531,6 +572,10 @@ do_general_protection(struct pt_regs *regs, long error_code)
 
 	tsk = current;
 	if (!user_mode(regs)) {
+		bool addr_resolved = false;
+		unsigned long gp_addr;
+		bool non_canonical;
+
 		if (fixup_exception(regs, X86_TRAP_GP, error_code, 0))
 			return;
 
@@ -547,8 +592,21 @@ 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
+			addr_resolved = get_kernel_gp_address(regs, &gp_addr,
+							      &non_canonical);
+
+		if (addr_resolved)
+			snprintf(desc, sizeof(desc),
+			    GPFSTR " probably for %saddress 0x%lx",
+			    non_canonical ? "non-canonical " : "", gp_addr);
+
+		die(desc, regs, error_code);
 		return;
 	}
 
-- 
2.24.0.432.g9d3f5f5b63-goog