[PATCH RFC 2/4] mm, personality: Implement memory-deny-write-execute as a personality flag

[Catalin Marinas <catalin.marinas@xxxxxxx>]

The aim of such policy is to prevent a user task from inadvertently
creating an executable mapping that is or was writeable (and
subsequently made read-only).

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);

With the past vma writeable permission tracking, mprotect() below would
also fail with -EACCESS:

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

While the above could be achieved by checking PROT_WRITE & PROT_EXEC on
mmap/mprotect and denying mprotect(PROT_EXEC) altogether (current
systemd MDWE approach via SECCOMP BPF filters), we want the following
scenario to succeed:

	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.

The choice for a DENY_WRITE_EXEC personality flag, inherited on fork()
and execve(), was made by analogy to READ_IMPLIES_EXEC.

Note that it is sufficient to check for VM_WAS_WRITE in
map_deny_write_exec() as this flag is always set on VM_WRITE mappings.

Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
 include/linux/mman.h             | 10 ++++++++++
 include/uapi/linux/personality.h |  1 +
 mm/mmap.c                        |  3 +++
 mm/mprotect.c                    |  5 +++++
 4 files changed, 19 insertions(+)

diff --git a/include/linux/mman.h b/include/linux/mman.h
index 2d841ddae2aa..17e91a1bdfb3 100644
--- a/include/linux/mman.h
+++ b/include/linux/mman.h
@@ -166,4 +166,14 @@ calc_vm_flag_bits(unsigned long flags)
 }
 
 unsigned long vm_commit_limit(void);
+
+static inline bool map_deny_write_exec(unsigned long vm_flags)
+{
+	if (IS_ENABLED(CONFIG_ARCH_ENABLE_DENY_WRITE_EXEC) &&
+	    (current->personality & DENY_WRITE_EXEC) &&
+	    (vm_flags & VM_EXEC) && (vm_flags & VM_WAS_WRITE))
+		return true;
+	return false;
+}
+
 #endif /* _LINUX_MMAN_H */
diff --git a/include/uapi/linux/personality.h b/include/uapi/linux/personality.h
index 49796b7756af..c8d924be3dcd 100644
--- a/include/uapi/linux/personality.h
+++ b/include/uapi/linux/personality.h
@@ -22,6 +22,7 @@ enum {
 	WHOLE_SECONDS =		0x2000000,
 	STICKY_TIMEOUTS	=	0x4000000,
 	ADDR_LIMIT_3GB = 	0x8000000,
+	DENY_WRITE_EXEC =	0x10000000,
 };
 
 /*
diff --git a/mm/mmap.c b/mm/mmap.c
index 3aa839f81e63..8e894270a80e 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1579,6 +1579,9 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
 			vm_flags |= VM_NORESERVE;
 	}
 
+	if (map_deny_write_exec(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 b69ce7a7b2b7..ff0d13a4c1ed 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -627,6 +627,11 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
 			goto out;
 		}
 
+		if (map_deny_write_exec(newflags)) {
+			error = -EACCES;
+			goto out;
+		}
+
 		/* Allow architectures to sanity-check the new flags */
 		if (!arch_validate_flags(newflags)) {
 			error = -EINVAL;