x86/shstk: Don't retry vm_munmap() on -EINTR

The existing comment around handling vm_munmap() failure when freeing a
shadow stack is wrong. It asserts that vm_munmap() returns -EINTR when
the mmap lock is only being held for a short time, and so the caller
should retry. Based on this wrong understanding, unmap_shadow_stack() will
loop retrying vm_munmap().

What -EINTR actually means in this case is that the process is going
away (see ae79878), and the whole MM will be torn down soon. In order
to facilitate this, the task should not linger in the kernel, but
actually do the opposite. So don't loop in this scenario, just abandon
the operation and let exit_mmap() clean it up. Also, update the comment
to reflect the actual meaning of the error code.

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20230706233858.446232-1-rick.p.edgecombe%40intel.com

Author: rpedgeco

arch/x86/kernel/shstk.c

@@ -134,28 +134,24 @@ static unsigned long adjust_shstk_size(unsigned long size)
 
 static void unmap_shadow_stack(u64 base, u64 size)
 {
-	while (1) {
-		int r;
-
-		r = vm_munmap(base, size);
-
-		/*
-		 * vm_munmap() returns -EINTR when mmap_lock is held by
-		 * something else, and that lock should not be held for a
-		 * long time.  Retry it for the case.
-		 */
-		if (r == -EINTR) {
-			cond_resched();
-			continue;
-		}
+	int r;
 
-		/*
-		 * For all other types of vm_munmap() failure, either the
-		 * system is out of memory or there is bug.
-		 */
-		WARN_ON_ONCE(r);
-		break;
-	}
+	r = vm_munmap(base, size);
+
+	/*
+	 * mmap_write_lock_killable() failed with -EINTR. This means
+	 * the process is about to die and have it's MM cleaned up.
+	 * This task shouldn't ever make it back to userspace. In this
+	 * case it is ok to leak a shadow stack, so just exit out.
+	 */
+	if (r == -EINTR)
+		return;
+
+	/*
+	 * For all other types of vm_munmap() failure, either the
+	 * system is out of memory or there is bug.
+	 */
+	WARN_ON_ONCE(r);
 }
 
 static int shstk_setup(void)