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From:   Joey Gouly <joey.gouly@arm.com>
To:     Catalin Marinas <catalin.marinas@arm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Lennart Poettering <lennart@poettering.net>,
        =?UTF-8?q?Zbigniew=20J=C4=99drzejewski-Szmek?= <zbyszek@in.waw.pl>
CC:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Kees Cook <keescook@chromium.org>,
        Szabolcs Nagy <szabolcs.nagy@arm.com>,
        Mark Brown <broonie@kernel.org>,
        Jeremy Linton <jeremy.linton@arm.com>,
        Topi Miettinen <toiwoton@gmail.com>, <linux-mm@kvack.org>,
        <linux-arm-kernel@lists.infradead.org>,
        <linux-kernel@vger.kernel.org>,
        <linux-abi-devel@lists.sourceforge.net>, <nd@arm.com>,
        <joey.gouly@arm.com>, <shuah@kernel.org>
Subject: [PATCH v2 1/2] mm: Implement memory-deny-write-execute as a prctl
Date:   Thu, 19 Jan 2023 16:03:43 +0000
Message-ID: <20230119160344.54358-2-joey.gouly@arm.com>
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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           | 34 ++++++++++++++++++++++++++++++++++
 include/linux/sched/coredump.h |  6 +++++-
 include/uapi/linux/prctl.h     |  6 ++++++
 kernel/sys.c                   | 33 +++++++++++++++++++++++++++++++++
 mm/mmap.c                      | 10 ++++++++++
 mm/mprotect.c                  |  5 +++++
 6 files changed, 93 insertions(+), 1 deletion(-)

diff --git a/include/linux/mman.h b/include/linux/mman.h
index 58b3abd457a3..cee1e4b566d8 100644
--- a/include/linux/mman.h
+++ b/include/linux/mman.h
@@ -156,4 +156,38 @@ calc_vm_flag_bits(unsigned long flags)
 }
 
 unsigned long vm_commit_limit(void);
+
+/*
+ * Denies creating a writable executable mapping or gaining executable permissions.
+ *
+ * This denies the following:
+ *
+ * 	a)	mmap(PROT_WRITE | PROT_EXEC)
+ *
+ *	b)	mmap(PROT_WRITE)
+ *		mprotect(PROT_EXEC)
+ *
+ *	c)	mmap(PROT_WRITE)
+ *		mprotect(PROT_READ)
+ *		mprotect(PROT_EXEC)
+ *
+ * But allows the following:
+ *
+ *	d)	mmap(PROT_READ | PROT_EXEC)
+ *		mmap(PROT_READ | PROT_EXEC | PROT_BTI)
+ */
+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->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..1312a137f7fb 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_REFUSE_EXEC_GAIN	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..b3cab94545ed 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2348,6 +2348,33 @@ static int prctl_set_vma(unsigned long opt, unsigned long start,
 }
 #endif /* CONFIG_ANON_VMA_NAME */
 
+static inline int prctl_set_mdwe(unsigned long bits, unsigned long arg3,
+				 unsigned long arg4, unsigned long arg5)
+{
+	if (arg3 || arg4 || arg5)
+		return -EINVAL;
+
+	if (bits & ~(PR_MDWE_REFUSE_EXEC_GAIN))
+		return -EINVAL;
+
+	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;
+}
+
+static inline int prctl_get_mdwe(unsigned long arg2, unsigned long arg3,
+				 unsigned long arg4, unsigned long arg5)
+{
+	if (arg2 || arg3 || arg4 || arg5)
+		return -EINVAL;
+
+	return test_bit(MMF_HAS_MDWE, &current->mm->flags) ?
+		PR_MDWE_REFUSE_EXEC_GAIN : 0;
+}
+
 SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 		unsigned long, arg4, unsigned long, arg5)
 {
@@ -2623,6 +2650,12 @@ 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(arg2, arg3, arg4, arg5);
+		break;
+	case PR_GET_MDWE:
+		error = prctl_get_mdwe(arg2, arg3, arg4, arg5);
+		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 87d929316d57..99a4d9e2b0d8 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2665,6 +2665,16 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 		vma_set_anonymous(vma);
 	}
 
+	if (map_deny_write_exec(vma, vma->vm_flags)) {
+		error = -EACCES;
+		if (file)
+			goto close_and_free_vma;
+		else if (vma->vm_file)
+			goto unmap_and_free_vma;
+		else
+			goto free_vma;
+	}
+
 	/* Allow architectures to sanity-check the vm_flags */
 	if (!arch_validate_flags(vma->vm_flags)) {
 		error = -EINVAL;
diff --git a/mm/mprotect.c b/mm/mprotect.c
index 908df12caa26..bc0587df042f 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -762,6 +762,11 @@ 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