[PATCH v4 5/6] mm: Add a NO_INHERIT flag to the PR_SET_MDWE prctl

Florent Revest
2023-08-28T15:10:05Z

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.

Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Florent Revest <revest@chromium.org>
---
include/linux/sched/coredump.h | 10 ++++++++++
include/uapi/linux/prctl.h | 1 +
kernel/fork.c | 2 +-
kernel/sys.c | 32 ++++++++++++++++++++++++++------
tools/include/uapi/linux/prctl.h | 1 +
5 files changed, 39 insertions(+), 7 deletions(-)

diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
index 0ee96ea7a0e9..1b37fa8fc723 100644
--- a/include/linux/sched/coredump.h
+++ b/include/linux/sched/coredump.h
@@ -91,4 +91,14 @@ static inline int get_dumpable(struct mm_struct *mm)
MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK)

#define MMF_VM_MERGE_ANY 29
+#define MMF_HAS_MDWE_NO_INHERIT 30
+
+static inline unsigned long mmf_init_flags(unsigned long flags)
+{
+ if (flags & (1UL << MMF_HAS_MDWE_NO_INHERIT))
+ flags &= ~((1UL << MMF_HAS_MDWE) |
+ (1UL << MMF_HAS_MDWE_NO_INHERIT));
+ return flags & MMF_INIT_MASK;
+}
+
#endif /* _LINUX_SCHED_COREDUMP_H */
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 9a85c69782bd..370ed14b1ae0 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -284,6 +284,7 @@ struct prctl_mm_map {
/* Memory deny write / execute */
#define PR_SET_MDWE 65
# define PR_MDWE_REFUSE_EXEC_GAIN (1UL << 0)
+# define PR_MDWE_NO_INHERIT (1UL << 1)

#define PR_GET_MDWE 66

diff --git a/kernel/fork.c b/kernel/fork.c
index 7b8b63fb0438..9da5a1192c98 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1285,7 +1285,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
hugetlb_count_init(mm);

if (current->mm) {
- mm->flags = current->mm->flags & MMF_INIT_MASK;
+ mm->flags = mmf_init_flags(current->mm->flags);
mm->def_flags = current->mm->def_flags & VM_INIT_DEF_MASK;
} else {
mm->flags = default_dump_filter;
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)
diff --git a/tools/include/uapi/linux/prctl.h b/tools/include/uapi/linux/prctl.h
index 9a85c69782bd..370ed14b1ae0 100644
--- a/tools/include/uapi/linux/prctl.h
+++ b/tools/include/uapi/linux/prctl.h
@@ -284,6 +284,7 @@ struct prctl_mm_map {
/* Memory deny write / execute */
#define PR_SET_MDWE 65
# define PR_MDWE_REFUSE_EXEC_GAIN (1UL << 0)
+# define PR_MDWE_NO_INHERIT (1UL << 1)

#define PR_GET_MDWE 66

--
2.42.0.rc2.253.gd59a3bf2b4-goog