Re: [PATCH v2 1/2] mm: Implement memory-deny-write-execute as a prctl

From: Alexey Izbyshev
Date: Tue Mar 07 2023 - 08:02:41 EST

On 2023-01-19 19:03, Joey Gouly wrote:
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@xxxxxxx>
Co-developed-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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;
+ }
+

Why is the cleanup dispatch logic duplicated here, instead of simply doing "goto close_and_free_vma" (where basically the same dispatch is done)?

/* 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;
+ }
+

Why does this check use "goto out", thereby skipping post-loop cleanup, instead of "break" like all other checks? This looks like a bug to me.

Alexey
/* Allow architectures to sanity-check the new flags */
if (!arch_validate_flags(newflags)) {
error = -EINVAL;