powerpc: Don't print kernel instructions in show_user_instructions()

mpe · mpe · commit a932ed3b7181 · 2018-10-05T23:18:12.000+10:00
Recently we implemented show_user_instructions() which dumps the code around the NIP when a user space process dies with an unhandled signal. This was modelled on the x86 code, and we even went so far as to implement the exact same bug, namely that if the user process crashed with its NIP pointing into the kernel we will dump kernel text to dmesg. eg: bad-bctr[2996]: segfault (11) at c000000000010000 nip c000000000010000 lr 12d0b0894 code 1 bad-bctr[2996]: code: fbe10068 7cbe2b78 7c7f1b78 fb610048 38a10028 38810020 fb810050 7f8802a6 bad-bctr[2996]: code: 3860001c f8010080 48242371 60000000 <7c7b1b79> 4082002c e8010080 eb610048 This was discovered on x86 by Jann Horn and fixed in commit 342db04 ("x86/dumpstack: Don't dump kernel memory based on usermode RIP"). Fix it by checking the adjusted NIP value (pc) and number of instructions against USER_DS, and bail if we fail the check, eg: bad-bctr[2969]: segfault (11) at c000000000010000 nip c000000000010000 lr 107930894 code 1 bad-bctr[2969]: Bad NIP, not dumping instructions. Fixes: 88b0fe1 ("powerpc: Add show_user_instructions()") Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
@@ -1306,6 +1306,16 @@ void show_user_instructions(struct pt_regs *regs)
 
 	pc = regs->nip - (instructions_to_print * 3 / 4 * sizeof(int));
 
+	/*
+	 * Make sure the NIP points at userspace, not kernel text/data or
+	 * elsewhere.
+	 */
+	if (!__access_ok(pc, instructions_to_print * sizeof(int), USER_DS)) {
+		pr_info("%s[%d]: Bad NIP, not dumping instructions.\n",
+			current->comm, current->pid);
+		return;
+	}
+
 	pr_info("%s[%d]: code: ", current->comm, current->pid);
 
 	for (i = 0; i < instructions_to_print; i++) {
