[RFC PATCH v3 5/8] exec: take care of disabling PTI upon execve()

[Willy Tarreau <w@1wt.eu>]

Here's what we do here :
  - TIF_DISABLE_PTI_NOW is always cleared as we don't want an unprotected
    process to pass its lack of protection to any possible other program
    it could exec.

  - TIF_DISABLE_PTI_NEXT is copied into TIF_DISABLE_PTI_NOW and cleared,
    this is used by wrappers to disable PTI for a single exec call.

Thanks to this, PTI-aware programs can adjust TIF_DISABLE_PTI_NOW for
themselves, and a simple wrapper can be implemented by setting
TIF_DISABLE_PTI_NEXT to manage those unable to set TIF_DISABLE_PTI_NOW
themselves.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
---
 fs/exec.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/exec.c b/fs/exec.c
index 7eb8d21..cf42ddc 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1814,6 +1814,16 @@ static int do_execveat_common(int fd, struct filename *filename,
 	putname(filename);
 	if (displaced)
 		put_files_struct(displaced);
+
+#ifdef CONFIG_PER_PROCESS_PTI
+	/*
+	 * TIF_DISABLE_PTI_NOW doesn't pass execve(). TIF_DISABLE_PTI_NEXT
+	 * turns into TIF_DISABLE_PTI_NOW and disappears.
+	 */
+	clear_thread_flag(TIF_DISABLE_PTI_NOW);
+	if (test_and_clear_thread_flag(TIF_DISABLE_PTI_NEXT))
+		set_thread_flag(TIF_DISABLE_PTI_NOW);
+#endif
 	return retval;
 
 out:
-- 
1.7.12.1