x86/kprobes: Stop calling fixup_exception() from kprobe_fault_handler()

This removes the call into exception fixup that was added in commit
c28f896 ("[PATCH] kprobes: fix broken fault handling for x86_64").

On X86, kprobe_fault_handler() is called from two places:
do_general_protection() (for #GP) and kprobes_fault() (for #PF).  In both
paths, the fixup_exception() call in the kprobe fault handler is redundant.

In case of #GP, fixup_exception() is called immediately before
kprobe_fault_handler() is invoked, so no need to try that again. This
assumes that the kprobe's fault handler isn't going to do something crazy
like changing RIP so that it suddenly points to an instruction that does
userspace access.

For #PF on a kernel address from kernel space, after the kprobe fault
handler has run, no_context() is invoked, which calls fixup_exception().

Signed-off-by: Jann Horn <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Tested-by: Kees Cook <[email protected]>
Acked-by: Masami Hiramatsu <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: "Naveen N. Rao" <[email protected]>
Cc: Anil S Keshavamurthy <[email protected]>
Cc: "David S. Miller" <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: [email protected]
Cc: Borislav Petkov <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

Author: thejh

arch/x86/kernel/kprobes/core.c

@@ -1020,15 +1020,6 @@ int kprobe_fault_handler(struct pt_regs *regs, int trapnr)
 		 */
 		if (cur->fault_handler && cur->fault_handler(cur, regs, trapnr))
 			return 1;
-
-		/*
-		 * In case the user-specified fault handler returned
-		 * zero, try to fix up.
-		 */
-		if (fixup_exception(regs, trapnr))
-			return 1;
-
-		/* fixup routine could not handle it. */
 	}
 
 	return 0;