signals, pkeys: Notify userspace about protection key faults

A protection key fault is very similar to any other access error.
There must be a VMA, etc... We even want to take the same action
(SIGSEGV) that we do with a normal access fault.

However, we do need to let userspace know that something is
different. We do this the same way what we did with SEGV_BNDERR
with Memory Protection eXtensions (MPX): define a new SEGV code:
SEGV_PKUERR.

We add a siginfo field: si_pkey that reveals to userspace which
protection key was set on the PTE that we faulted on. There is
no other easy way for userspace to figure this out. They could
parse smaps but that would be a bit cruel.

We share space with in siginfo with _addr_bnd. #BR faults from
MPX are completely separate from page faults (#PF) that trigger
from protection key violations, so we never need both at the same
time.

Note that _pkey is a 64-bit value. The current hardware only
supports 4-bit protection keys. We do this because there is
_plenty_ of space in _sigfault and it is possible that future
processors would support more than 4 bits of protection keys.

The x86 code to actually fill in the siginfo is in the next
patch.

Signed-off-by: Dave Hansen <[e-mail]>
Reviewed-by: Thomas Gleixner <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Amanieu d'Antras <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Denys Vlasenko <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Palmer Dabbelt <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Richard Weinberger <[e-mail]>
Cc: Rik van Riel <[e-mail]>
Cc: Sasha Levin <[e-mail]>
Cc: Vegard Nossum <[e-mail]>
Cc: Vladimir Davydov <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

include/uapi/asm-generic/siginfo.h

···
 			int _trapno;	/* TRAP # which caused the signal */
 #endif
 			short _addr_lsb; /* LSB of the reported address */
-			struct {
-				void __user *_lower;
-				void __user *_upper;
-			} _addr_bnd;
+			union {
+				/* used when si_code=SEGV_BNDERR */
+				struct {
+					void __user *_lower;
+					void __user *_upper;
+				} _addr_bnd;
+				/* used when si_code=SEGV_PKUERR */
+				u64 _pkey;
+			};
 		} _sigfault;
 
 		/* SIGPOLL */
···
 #define si_addr_lsb	_sifields._sigfault._addr_lsb
 #define si_lower	_sifields._sigfault._addr_bnd._lower
 #define si_upper	_sifields._sigfault._addr_bnd._upper
+#define si_pkey		_sifields._sigfault._pkey
 #define si_band		_sifields._sigpoll._band
 #define si_fd		_sifields._sigpoll._fd
 #ifdef __ARCH_SIGSYS
···
 #define SEGV_MAPERR	(__SI_FAULT|1)	/* address not mapped to object */
 #define SEGV_ACCERR	(__SI_FAULT|2)	/* invalid permissions for mapped object */
 #define SEGV_BNDERR	(__SI_FAULT|3)  /* failed address bound checks */
-#define NSIGSEGV	3
+#define SEGV_PKUERR	(__SI_FAULT|4)  /* failed protection key checks */
+#define NSIGSEGV	4
 
 /*
  * SIGBUS si_codes

kernel/signal.c

···
 			err |= __put_user(from->si_upper, &to->si_upper);
 		}
 #endif
+#ifdef SEGV_PKUERR
+		if (from->si_signo == SIGSEGV && from->si_code == SEGV_PKUERR)
+			err |= __put_user(from->si_pkey, &to->si_pkey);
+#endif
 		break;
 	case __SI_CHLD:
 		err |= __put_user(from->si_pid, &to->si_pid);