From 24e41bf8a6b424c76c5902fb999e9eca61bdf83d Mon Sep 17 00:00:00 2001
From: Florent Revest <revest@chromium.org>
Date: Mon, 28 Aug 2023 17:08:57 +0200
Subject: mm: add a NO_INHERIT flag to the PR_SET_MDWE prctl

This extends the current PR_SET_MDWE prctl arg with a bit to indicate that
the process doesn't want MDWE protection to propagate to children.

To implement this no-inherit mode, the tag in current->mm->flags must be
absent from MMF_INIT_MASK.  This means that the encoding for "MDWE but
without inherit" is different in the prctl than in the mm flags.  This
leads to a bit of bit-mangling in the prctl implementation.

Link: https://lkml.kernel.org/r/20230828150858.393570-6-revest@chromium.org
Signed-off-by: Florent Revest <revest@chromium.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Alexey Izbyshev <izbyshev@ispras.ru>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Ayush Jain <ayush.jain3@amd.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: KP Singh <kpsingh@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Szabolcs Nagy <Szabolcs.Nagy@arm.com>
Cc: Topi Miettinen <toiwoton@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 kernel/sys.c | 32 ++++++++++++++++++++++++++------
 1 file changed, 26 insertions(+), 6 deletions(-)

(limited to 'kernel/sys.c')

diff --git a/kernel/sys.c b/kernel/sys.c
index 2410e3999ebe..4a8073c1b255 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2368,19 +2368,41 @@ static int prctl_set_vma(unsigned long opt, unsigned long start,
 }
 #endif /* CONFIG_ANON_VMA_NAME */
 
+static inline unsigned long get_current_mdwe(void)
+{
+	unsigned long ret = 0;
+
+	if (test_bit(MMF_HAS_MDWE, &current->mm->flags))
+		ret |= PR_MDWE_REFUSE_EXEC_GAIN;
+	if (test_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags))
+		ret |= PR_MDWE_NO_INHERIT;
+
+	return ret;
+}
+
 static inline int prctl_set_mdwe(unsigned long bits, unsigned long arg3,
 				 unsigned long arg4, unsigned long arg5)
 {
+	unsigned long current_bits;
+
 	if (arg3 || arg4 || arg5)
 		return -EINVAL;
 
-	if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN))
+	if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN | PR_MDWE_NO_INHERIT))
+		return -EINVAL;
+
+	/* NO_INHERIT only makes sense with REFUSE_EXEC_GAIN */
+	if (bits & PR_MDWE_NO_INHERIT && !(bits & PR_MDWE_REFUSE_EXEC_GAIN))
 		return -EINVAL;
 
+	current_bits = get_current_mdwe();
+	if (current_bits && current_bits != bits)
+		return -EPERM; /* Cannot unset the flags */
+
+	if (bits & PR_MDWE_NO_INHERIT)
+		set_bit(MMF_HAS_MDWE_NO_INHERIT, &current->mm->flags);
 	if (bits & PR_MDWE_REFUSE_EXEC_GAIN)
 		set_bit(MMF_HAS_MDWE, &current->mm->flags);
-	else if (test_bit(MMF_HAS_MDWE, &current->mm->flags))
-		return -EPERM; /* Cannot unset the flag */
 
 	return 0;
 }
@@ -2390,9 +2412,7 @@ static inline int prctl_get_mdwe(unsigned long arg2, unsigned long arg3,
 {
 	if (arg2 || arg3 || arg4 || arg5)
 		return -EINVAL;
-
-	return test_bit(MMF_HAS_MDWE, &current->mm->flags) ?
-		PR_MDWE_REFUSE_EXEC_GAIN : 0;
+	return get_current_mdwe();
 }
 
 static int prctl_get_auxv(void __user *addr, unsigned long len)
-- 
cgit