[PATCH v3 59/60] mm: add arch hook to validate mmap() prot flags

Ard Biesheuvel
2023-03-07T14:13:11Z

Add a hook to permit architectures to perform validation on the prot
flags passed to mmap(), like arch_validate_prot() does for mprotect().
This will be used by arm64 to reject PROT_WRITE+PROT_EXEC mappings on
configurations that run with WXN enabled.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
include/linux/mman.h | 15 +++++++++++++++
mm/mmap.c | 3 +++
2 files changed, 18 insertions(+)

diff --git a/include/linux/mman.h b/include/linux/mman.h
index cee1e4b566d80095..25ffe6948fa21ff8 100644
--- a/include/linux/mman.h
+++ b/include/linux/mman.h
@@ -120,6 +120,21 @@ static inline bool arch_validate_flags(unsigned long flags)
#define arch_validate_flags arch_validate_flags
#endif

+#ifndef arch_validate_mmap_prot
+/*
+ * This is called from mmap(), which ignores unknown prot bits so the default
+ * is to accept anything.
+ *
+ * Returns true if the prot flags are valid
+ */
+static inline bool arch_validate_mmap_prot(unsigned long prot,
+ unsigned long addr)
+{
+ return true;
+}
+#define arch_validate_mmap_prot arch_validate_mmap_prot
+#endif
+
/*
* Optimisation macro. It is equivalent to:
* (x & bit1) ? bit2 : 0
diff --git a/mm/mmap.c b/mm/mmap.c
index 740b54be3ed4140f..02ccd45df0194796 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1214,6 +1214,9 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
if (!(file && path_noexec(&file->f_path)))
prot |= PROT_EXEC;

+ if (!arch_validate_mmap_prot(prot, addr))
+ return -EACCES;
+
/* force arch specific MAP_FIXED handling in get_unmapped_area */
if (flags & MAP_FIXED_NOREPLACE)
flags |= MAP_FIXED;
--
2.39.2