Re: [PATCH] x86/dumpstack: fix address space casting in show_opcodes()

[Borislav Petkov <bp@suse.de>]

On Thu, Aug 30, 2018 at 09:47:36PM +0200, Jann Horn wrote:
> I sloppily passed a kernel-typed pointer to __range_not_ok(), and sparse
> doesn't like that.
> Make `prologue` a __user pointer (to protect against accidental
> dereferences) and force-cast it to a kernel pointer when calling
> probe_kernel_read(), which will then immediately force-cast it back to a
> user pointer.

Yeah, that's some crazy casting.

Can we define a local __user pointer only for the check instead? It is
less casting and looks simpler and actually even easier to understand
what we're doing...

---
diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 605c60b1624f..9c5a15491108 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -97,14 +97,17 @@ void show_opcodes(struct pt_regs *regs, const char *loglvl)
 #define OPCODE_BUFSIZE (PROLOGUE_SIZE + 1 + EPILOGUE_SIZE)
 	u8 opcodes[OPCODE_BUFSIZE];
 	u8 *prologue = (u8 *)(regs->ip - PROLOGUE_SIZE);
-	bool bad_ip;
+	bool bad_ip = false;
 
 	/*
 	 * Make sure userspace isn't trying to trick us into dumping kernel
 	 * memory by pointing the userspace instruction pointer at it.
 	 */
-	bad_ip = user_mode(regs) &&
-		__range_not_ok(prologue, OPCODE_BUFSIZE, TASK_SIZE_MAX);
+	if (user_mode(regs)) {
+		u8 __user *up = (u8 __user *)prologue;
+
+		bad_ip = __range_not_ok(up, OPCODE_BUFSIZE, TASK_SIZE_MAX);
+	}
 
 	if (bad_ip || probe_kernel_read(opcodes, prologue, OPCODE_BUFSIZE)) {
 		printk("%sCode: Bad RIP value.\n", loglvl);

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
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