[PATCH v1 1/2] mm: Implement memory-deny-write-execute as a prctl

[Joey Gouly <joey.gouly@arm.com>]

The aim of such policy is to prevent a user task from creating an
executable mapping that is also writeable.

An example of mmap() returning -EACCESS if the policy is enabled:

	mmap(0, size, PROT_READ | PROT_WRITE | PROT_EXEC, flags, 0, 0);

Similarly, mprotect() would return -EACCESS below:

	addr = mmap(0, size, PROT_READ | PROT_EXEC, flags, 0, 0);
	mprotect(addr, size, PROT_READ | PROT_WRITE | PROT_EXEC);

The BPF filter that systemd MDWE uses is stateless, and disallows
mprotect() with PROT_EXEC completely. This new prctl allows PROT_EXEC to
be enabled if it was already PROT_EXEC, which allows the following case:

	addr = mmap(0, size, PROT_READ | PROT_EXEC, flags, 0, 0);
	mprotect(addr, size, PROT_READ | PROT_EXEC | PROT_BTI);

where PROT_BTI enables branch tracking identification on arm64.

Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Co-developed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 include/linux/mman.h           | 15 +++++++++++++++
 include/linux/sched/coredump.h |  6 +++++-
 include/uapi/linux/prctl.h     |  6 ++++++
 kernel/sys.c                   | 18 ++++++++++++++++++
 mm/mmap.c                      |  3 +++
 mm/mprotect.c                  |  5 +++++
 6 files changed, 52 insertions(+), 1 deletion(-)

diff --git a/include/linux/mman.h b/include/linux/mman.h
index 58b3abd457a3..d84fdeab6b5e 100644
--- a/include/linux/mman.h
+++ b/include/linux/mman.h
@@ -156,4 +156,19 @@ calc_vm_flag_bits(unsigned long flags)
 }
 
 unsigned long vm_commit_limit(void);
+
+static inline bool map_deny_write_exec(struct vm_area_struct *vma,  unsigned long vm_flags)
+{
+	if (!test_bit(MMF_HAS_MDWE, &current->mm->flags))
+		return false;
+
+	if ((vm_flags & VM_EXEC) && (vm_flags & VM_WRITE))
+		return true;
+
+	if (vma && !(vma->vm_flags & VM_EXEC) && (vm_flags & VM_EXEC))
+		return true;
+
+	return false;
+}
+
 #endif /* _LINUX_MMAN_H */
diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
index 8270ad7ae14c..0e17ae7fbfd3 100644
--- a/include/linux/sched/coredump.h
+++ b/include/linux/sched/coredump.h
@@ -81,9 +81,13 @@ static inline int get_dumpable(struct mm_struct *mm)
  * lifecycle of this mm, just for simplicity.
  */
 #define MMF_HAS_PINNED		27	/* FOLL_PIN has run, never cleared */
+
+#define MMF_HAS_MDWE		28
+#define MMF_HAS_MDWE_MASK	(1 << MMF_HAS_MDWE)
+
 #define MMF_DISABLE_THP_MASK	(1 << MMF_DISABLE_THP)
 
 #define MMF_INIT_MASK		(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\
-				 MMF_DISABLE_THP_MASK)
+				 MMF_DISABLE_THP_MASK | MMF_HAS_MDWE_MASK)
 
 #endif /* _LINUX_SCHED_COREDUMP_H */
diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index a5e06dcbba13..ab9db1e86230 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -281,6 +281,12 @@ struct prctl_mm_map {
 # define PR_SME_VL_LEN_MASK		0xffff
 # define PR_SME_VL_INHERIT		(1 << 17) /* inherit across exec */
 
+/* Memory deny write / execute */
+#define PR_SET_MDWE			65
+# define PR_MDWE_FLAG_MMAP		1
+
+#define PR_GET_MDWE			66
+
 #define PR_SET_VMA		0x53564d41
 # define PR_SET_VMA_ANON_NAME		0
 
diff --git a/kernel/sys.c b/kernel/sys.c
index 5fd54bf0e886..08e1dd6d2533 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2348,6 +2348,18 @@ static int prctl_set_vma(unsigned long opt, unsigned long start,
 }
 #endif /* CONFIG_ANON_VMA_NAME */
 
+static inline int prctl_set_mdwe(void)
+{
+	set_bit(MMF_HAS_MDWE, &current->mm->flags);
+
+	return 0;
+}
+
+static inline int prctl_get_mdwe(void)
+{
+	return test_bit(MMF_HAS_MDWE, &current->mm->flags);
+}
+
 SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		unsigned long, arg4, unsigned long, arg5)
 {
@@ -2623,6 +2635,12 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		error = sched_core_share_pid(arg2, arg3, arg4, arg5);
 		break;
 #endif
+	case PR_SET_MDWE:
+		error = prctl_set_mdwe();
+		break;
+	case PR_GET_MDWE:
+		error = prctl_get_mdwe();
+		break;
 	case PR_SET_VMA:
 		error = prctl_set_vma(arg2, arg3, arg4, arg5);
 		break;
diff --git a/mm/mmap.c b/mm/mmap.c
index 099468aee4d8..42eaf6683216 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1409,6 +1409,9 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
 			vm_flags |= VM_NORESERVE;
 	}
 
+	if (map_deny_write_exec(NULL, vm_flags))
+		return -EACCES;
+
 	addr = mmap_region(file, addr, len, vm_flags, pgoff, uf);
 	if (!IS_ERR_VALUE(addr) &&
 	    ((vm_flags & VM_LOCKED) ||
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 8d770855b591..af71ef0788fd 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -766,6 +766,11 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
 			break;
 		}
 
+		if (map_deny_write_exec(vma, newflags)) {
+			error = -EACCES;
+			goto out;
+		}
+
 		/* Allow architectures to sanity-check the new flags */
 		if (!arch_validate_flags(newflags)) {
 			error = -EINVAL;
-- 
2.17.1