UPSTREAM: seccomp: cap SECCOMP_RET_ERRNO data to MAX_ERRNO

The value resulting from the SECCOMP_RET_DATA mask could exceed MAX_ERRNO
when setting errno during a SECCOMP_RET_ERRNO filter action.  This makes
sure we have a reliable value being set, so that an invalid errno will not
be ignored by userspace.

Signed-off-by: Kees Cook <keescook@chromium.org>
Reported-by: Dmitry V. Levin <ldv@altlinux.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

(cherry picked from commit 580c57f1076872ebc2427f898b927944ce170f2d)
Signed-off-by: Kees Cook <keescook@google.com>
Change-Id: I069b2e858c626283819b9f0fc308c35c3c847b55

kernel/seccomp.c

@@ -600,7 +600,9 @@ int __secure_computing(int this_syscall)
 		ret &= SECCOMP_RET_ACTION;
 		switch (ret) {
 		case SECCOMP_RET_ERRNO:
-			/* Set the low-order 16-bits as a errno. */
+			/* Set low-order bits as an errno, capped at MAX_ERRNO. */
+			if (data > MAX_ERRNO)
+				data = MAX_ERRNO;
 			syscall_set_return_value(current, regs,
 						 -data, 0);
 			goto skip;