[PATCH 6.6.y 3/5] mm: refactor map_deny_write_exec()

[Lorenzo Stoakes]

[ Upstream commit 0fb4a7ad270b3b209e510eb9dc5b07bf02b7edaf ]

Refactor the map_deny_write_exec() to not unnecessarily require a VMA
parameter but rather to accept VMA flags parameters, which allows us to
use this function early in mmap_region() in a subsequent commit.

While we're here, we refactor the function to be more readable and add
some additional documentation.

Link: https://lkml.kernel.org/r/6be8bb59cd7c68006ebb006eb9d8dc27104b1f70.1730224667.git.lorenzo.stoakes@xxxxxxxxxx
Fixes: deb0f6562884 ("mm/mmap: undo ->mmap() when arch_validate_flags() fails")
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Reported-by: Jann Horn <jannh@xxxxxxxxxx>
Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
Reviewed-by: Jann Horn <jannh@xxxxxxxxxx>
Cc: Andreas Larsson <andreas@xxxxxxxxxxx>
Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
Cc: David S. Miller <davem@xxxxxxxxxxxxx>
Cc: Helge Deller <deller@xxxxxx>
Cc: James E.J. Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Mark Brown <broonie@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
---
 include/linux/mman.h | 21 ++++++++++++++++++---
 mm/mmap.c            |  2 +-
 mm/mprotect.c        |  2 +-
 3 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/include/linux/mman.h b/include/linux/mman.h
index db4741007bef..651705c2bf47 100644
--- a/include/linux/mman.h
+++ b/include/linux/mman.h
@@ -187,16 +187,31 @@ static inline bool arch_memory_deny_write_exec_supported(void)
  *
  *	d)	mmap(PROT_READ | PROT_EXEC)
  *		mmap(PROT_READ | PROT_EXEC | PROT_BTI)
+ *
+ * This is only applicable if the user has set the Memory-Deny-Write-Execute
+ * (MDWE) protection mask for the current process.
+ *
+ * @old specifies the VMA flags the VMA originally possessed, and @new the ones
+ * we propose to set.
+ *
+ * Return: false if proposed change is OK, true if not ok and should be denied.
  */
-static inline bool map_deny_write_exec(struct vm_area_struct *vma,  unsigned long vm_flags)
+static inline bool map_deny_write_exec(unsigned long old, unsigned long new)
 {
+	/* If MDWE is disabled, we have nothing to deny. */
 	if (!test_bit(MMF_HAS_MDWE, &current->mm->flags))
 		return false;
 
-	if ((vm_flags & VM_EXEC) && (vm_flags & VM_WRITE))
+	/* If the new VMA is not executable, we have nothing to deny. */
+	if (!(new & VM_EXEC))
+		return false;
+
+	/* Under MDWE we do not accept newly writably executable VMAs... */
+	if (new & VM_WRITE)
 		return true;
 
-	if (!(vma->vm_flags & VM_EXEC) && (vm_flags & VM_EXEC))
+	/* ...nor previously non-executable VMAs becoming executable. */
+	if (!(old & VM_EXEC))
 		return true;
 
 	return false;
diff --git a/mm/mmap.c b/mm/mmap.c
index 9fefd13640d1..d71ac65563b2 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2826,7 +2826,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 		vma_set_anonymous(vma);
 	}
 
-	if (map_deny_write_exec(vma, vma->vm_flags)) {
+	if (map_deny_write_exec(vma->vm_flags, vma->vm_flags)) {
 		error = -EACCES;
 		goto close_and_free_vma;
 	}
diff --git a/mm/mprotect.c b/mm/mprotect.c
index b94fbb45d5c7..7e870a8c9402 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -791,7 +791,7 @@ static int do_mprotect_pkey(unsigned long start, size_t len,
 			break;
 		}
 
-		if (map_deny_write_exec(vma, newflags)) {
+		if (map_deny_write_exec(vma->vm_flags, newflags)) {
 			error = -EACCES;
 			break;
 		}
-- 
2.47.0