[PATCH v2 0/5] binder: fix issues with mremap()

[PATCH v2 0/5] binder: fix issues with mremap()

[Carlos Llamas]

This is a follow up series fixing some pre-existing issues found by
sashiko during a review of an unrelated patchset here:
https://sashiko.dev/#/patchset/20260813193433.3318288-1-surenb@google.com

The goal is to safely reject mremap() requests on binder's vma to
prevent pages from temporary leaking and accidental IPC tear-down.

--
v2:
 - Set VM_DONTEXPAND in [1/5] and [2/5] per Sashiko's feedback.
 - Check vma->vm_start to prevent incorrect ->close() in [3/5] per
   Sashiko's feedback.

v1:
https://lore.kernel.org/all/20260831224145.169403-1-cmllamas@google.com/

Carlos Llamas (5):
  binder: set VM_DONTEXPAND
  rust_binder: set VM_DONTEXPAND
  binder: check vma->vm_start in binder_vma_close()
  binder: reject mremap()
  rust_binder: reject mremap()

 drivers/android/binder.c             | 12 +++++++++++-
 drivers/android/binder/page_range.rs | 15 ++++++++++++---
 drivers/android/binder/process.rs    |  1 +
 3 files changed, 24 insertions(+), 4 deletions(-)

-- 
2.55.0.897.gb25b4bd76c-goog

[PATCH v2 1/5] binder: set VM_DONTEXPAND

[Carlos Llamas]

Binder does not support expanding VMAs. It caches the original size
under alloc->buffer_size during mmap() and expanding the VMA would only
result in a wasted virtual range. Set the VM_DONTEXPAND flag to prevent
the vma from being expanded via mremap().

Cc: stable@vger.kernel.org
Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260831224145.169403-1-cmllamas%40google.com?part=1
Signed-off-by: Carlos Llamas <cmllamas@google.com>
---
 drivers/android/binder.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 8f2ef1bd539f..1b492ed48ea6 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -6056,7 +6056,8 @@ static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
 		       proc->pid, vma->vm_start, vma->vm_end, "bad vm_flags", -EPERM);
 		return -EPERM;
 	}
-	vm_flags_mod(vma, VM_DONTCOPY | VM_MIXEDMAP, VM_MAYWRITE);
+	vm_flags_mod(vma, VM_DONTCOPY | VM_MIXEDMAP | VM_DONTEXPAND,
+		     VM_MAYWRITE);
 
 	vma->vm_ops = &binder_vm_ops;
 	vma->vm_private_data = proc;
-- 
2.55.0.897.gb25b4bd76c-goog

[PATCH v2 2/5] rust_binder: set VM_DONTEXPAND

[Carlos Llamas]

Binder does not support expanding VMAs. It uses the original size to
create its RangeAllocator during mmap() and expanding the VMA would only
result in a wasted virtual range. Set the VM_DONTEXPAND flag to prevent
the vma from being expanded via mremap().

Cc: stable@vger.kernel.org
Fixes: eafedbc7c050 ("rust_binder: add Rust Binder driver")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260831224145.169403-1-cmllamas%40google.com?part=1
Signed-off-by: Carlos Llamas <cmllamas@google.com>
---
 drivers/android/binder/process.rs | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/android/binder/process.rs b/drivers/android/binder/process.rs
index 5372bfbd93b3..9038f3d13cfd 100644
--- a/drivers/android/binder/process.rs
+++ b/drivers/android/binder/process.rs
@@ -1786,6 +1786,7 @@ pub(crate) fn mmap(
         vma.try_clear_maywrite().map_err(|_| EPERM)?;
         vma.set_dontcopy();
         vma.set_mixedmap();
+        vma.set_dontexpand();
 
         // TODO: Set ops. We need to learn when the user unmaps so that we can stop using it.
         this.create_mapping(vma)
-- 
2.55.0.897.gb25b4bd76c-goog

[PATCH v2 3/5] binder: check vma->vm_start in binder_vma_close()

[Carlos Llamas]

Certain operations like a failed mremap() might trigger vm_ops->close()
on temporary mappings. To avoid tearing-down the main binder mapping on
these, let's verify that the VMA matches the expected starting address.

Cc: stable@vger.kernel.org
Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260831224145.169403-1-cmllamas%40google.com?part=1
Signed-off-by: Carlos Llamas <cmllamas@google.com>
---
 drivers/android/binder.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 1b492ed48ea6..3ec3a77af106 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -6018,6 +6018,9 @@ static void binder_vma_close(struct vm_area_struct *vma)
 {
 	struct binder_proc *proc = vma->vm_private_data;
 
+	if (vma->vm_start != proc->alloc.vm_start)
+		return;
+
 	binder_debug(BINDER_DEBUG_OPEN_CLOSE,
 		     "%d close vm area %lx-%lx (%ld K) vma %lx pagep %lx\n",
 		     proc->pid, vma->vm_start, vma->vm_end,
-- 
2.55.0.897.gb25b4bd76c-goog

[PATCH v2 4/5] binder: reject mremap()

[Carlos Llamas]

Binder does not support mremap() as it caches the mapping address in
alloc->vm_start. Moving the mapping breaks the IPC communication for the
process and can temporarily leak pages during a shrinker reclaim.

Fix this by explicitly rejecting the .mremap() operation.

Cc: stable@vger.kernel.org
Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260813193433.3318288-1-surenb%40google.com?part=2
Signed-off-by: Carlos Llamas <cmllamas@google.com>
---
 drivers/android/binder.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index 3ec3a77af106..76fc2edc1937 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -6035,10 +6035,16 @@ VISIBLE_IF_KUNIT vm_fault_t binder_vm_fault(struct vm_fault *vmf)
 }
 EXPORT_SYMBOL_IF_KUNIT(binder_vm_fault);
 
+static int binder_mremap(struct vm_area_struct *vma)
+{
+	return -EINVAL;
+}
+
 static const struct vm_operations_struct binder_vm_ops = {
 	.open = binder_vma_open,
 	.close = binder_vma_close,
 	.fault = binder_vm_fault,
+	.mremap = binder_mremap,
 };
 
 static int binder_mmap(struct file *filp, struct vm_area_struct *vma)
-- 
2.55.0.897.gb25b4bd76c-goog

[PATCH v2 5/5] rust_binder: reject mremap()

[Carlos Llamas]

Binder does not support mremap() as it caches the mapping address in
Inner::vma_addr. Moving the mapping breaks the IPC communication for the
process and can temporarily leak pages during a shrinker reclaim.

Fix this by explicitly rejecting the .mremap() operation.

Cc: stable@vger.kernel.org
Fixes: eafedbc7c050 ("rust_binder: add Rust Binder driver")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260813193433.3318288-1-surenb%40google.com?part=2
Signed-off-by: Carlos Llamas <cmllamas@google.com>
---
 drivers/android/binder/page_range.rs | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/android/binder/page_range.rs b/drivers/android/binder/page_range.rs
index 52ffbf3504e7..411efcc2116f 100644
--- a/drivers/android/binder/page_range.rs
+++ b/drivers/android/binder/page_range.rs
@@ -24,7 +24,7 @@
 use kernel::{
     bindings,
     error::Result,
-    ffi::{c_ulong, c_void},
+    ffi::{c_int, c_ulong, c_void},
     mm::{virt, Mm, MmWithUser},
     new_mutex, new_spinlock,
     page::{Page, PAGE_SHIFT, PAGE_SIZE},
@@ -144,8 +144,17 @@ pub(crate) struct ShrinkablePageRange {
     _pin: PhantomPinned,
 }
 
-// We do not define any ops. For now, used only to check identity of vmas.
-static BINDER_VM_OPS: AssertSync<bindings::vm_operations_struct> = AssertSync(pin_init::zeroed());
+unsafe extern "C" fn binder_mremap(_: *mut bindings::vm_area_struct) -> c_int {
+    EINVAL.to_errno()
+}
+
+static BINDER_VM_OPS: AssertSync<bindings::vm_operations_struct> = {
+    let ops = bindings::vm_operations_struct {
+        mremap: Some(binder_mremap),
+        ..pin_init::zeroed()
+    };
+    AssertSync(ops)
+};
 
 // To ensure that we do not accidentally install pages into or zap pages from the wrong vma, we
 // check its vm_ops and private data before using it.
-- 
2.55.0.897.gb25b4bd76c-goog

Re: [PATCH v2 3/5] binder: check vma->vm_start in binder_vma_close()

[Alice Ryhl]

On Tue, Sep 01, 2026 at 03:04:14AM +0000, Carlos Llamas wrote:
> Certain operations like a failed mremap() might trigger vm_ops->close()
> on temporary mappings. To avoid tearing-down the main binder mapping on
> these, let's verify that the VMA matches the expected starting address.
> 
> Cc: stable@vger.kernel.org
> Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://sashiko.dev/#/patchset/20260831224145.169403-1-cmllamas%40google.com?part=1
> Signed-off-by: Carlos Llamas <cmllamas@google.com>

I don't quite understand this. You explicitly made sure mremap() can't
happen to begin with, so when is this called?

Alice

Re: [PATCH v2 3/5] binder: check vma->vm_start in binder_vma_close()

[Carlos Llamas]

On Tue, Sep 01, 2026 at 09:31:27AM +0000, Alice Ryhl wrote:
> On Tue, Sep 01, 2026 at 03:04:14AM +0000, Carlos Llamas wrote:
> > Certain operations like a failed mremap() might trigger vm_ops->close()
> > on temporary mappings. To avoid tearing-down the main binder mapping on
> > these, let's verify that the VMA matches the expected starting address.
> > 
> > Cc: stable@vger.kernel.org
> > Fixes: 457b9a6f09f0 ("Staging: android: add binder driver")
> > Reported-by: Sashiko <sashiko-bot@kernel.org>
> > Closes: https://sashiko.dev/#/patchset/20260831224145.169403-1-cmllamas%40google.com?part=1
> > Signed-off-by: Carlos Llamas <cmllamas@google.com>
> 
> I don't quite understand this. You explicitly made sure mremap() can't
> happen to begin with, so when is this called?

The issue is mremap() first clones the old vma and then calls ->mremap()
but we return -EINVAL. So it now starts to tear-down the cloned vma and
it calls ->close() as part of it.

The idea was to avoid shutting down our IPC when this happens.

However, sashiko pointed out a second issue with a tail split. I need to
expand this check to allow the ->close() on anything within the range of
the original VMA.

I'll have to send a v3.