ANDROID: ashmem_rust: add toggle for PROT_READ

The memfd type simulates SET_PROT_MASK using file seals, but there's no
file seal to prevent mapping readable memory. Thus, add a toggle that
removes the ability to unset the PROT_READ bit on an ashmem file.

Bug: 370906207
Change-Id: I2d0347ce124d7d6d543cff765f77b28202cf327f
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

drivers/staging/android/ashmem_rust.rs

@@ -13,7 +13,7 @@
 use core::{
     ffi::c_int,
     pin::Pin,
-    sync::atomic::{AtomicUsize, Ordering},
+    sync::atomic::{AtomicBool, AtomicUsize, Ordering},
 };
 use kernel::{
     bindings::{self, ASHMEM_GET_PIN_STATUS, ASHMEM_PIN, ASHMEM_UNPIN},
@@ -50,7 +50,7 @@ mod shmem;
 use shmem::ShmemFile;
 
 mod ashmem_toggle;
-use ashmem_toggle::{AshmemToggleMisc, AshmemToggleShrinker};
+use ashmem_toggle::{AshmemToggleMisc, AshmemToggleRead, AshmemToggleShrinker};
 
 /// Does PROT_READ imply PROT_EXEC for this task?
 fn read_implies_exec(task: &Task) -> bool {
@@ -66,6 +66,7 @@ fn has_cap_sys_admin() -> bool {
 }
 
 static NUM_PIN_IOCTLS_WAITING: AtomicUsize = AtomicUsize::new(0);
+static IGNORE_UNSET_PROT_READ: AtomicBool = AtomicBool::new(false);
 
 fn shrinker_should_stop() -> bool {
     NUM_PIN_IOCTLS_WAITING.load(Ordering::Relaxed) > 0
@@ -82,6 +83,7 @@ module! {
 struct AshmemModule {
     _misc: Pin<Box<MiscDeviceRegistration<Ashmem>>>,
     _toggle_unpin: Pin<Box<MiscDeviceRegistration<AshmemToggleMisc<AshmemToggleShrinker>>>>,
+    _toggle_read: Pin<Box<MiscDeviceRegistration<AshmemToggleMisc<AshmemToggleRead>>>>,
 }
 
 impl kernel::Module for AshmemModule {
@@ -105,6 +107,7 @@ impl kernel::Module for AshmemModule {
                 GFP_KERNEL,
             )?,
             _toggle_unpin: AshmemToggleMisc::<AshmemToggleShrinker>::new()?,
+            _toggle_read: AshmemToggleMisc::<AshmemToggleRead>::new()?,
         })
     }
 }
@@ -340,6 +343,11 @@ impl Ashmem {
             prot |= PROT_EXEC;
         }
 
+        if IGNORE_UNSET_PROT_READ.load(Ordering::Relaxed) {
+            // Add back PROT_READ if asma.prot_mask has it.
+            prot |= asma.prot_mask & PROT_READ;
+        }
+
         // The user can only remove, not add, protection bits.
         if (asma.prot_mask & prot) != prot {
             return Err(EINVAL);

drivers/staging/android/ashmem_toggle.rs

@@ -4,8 +4,8 @@
 
 //! Provides knobs for Rust ashmem.
 
-use crate::ashmem_range;
-use core::marker::PhantomData;
+use crate::{ashmem_range, IGNORE_UNSET_PROT_READ};
+use core::{marker::PhantomData, sync::atomic::Ordering};
 use kernel::{
     c_str,
     error::to_result,
@@ -79,3 +79,16 @@ impl AshmemToggle for AshmemToggleShrinker {
         ashmem_range::get_shrinker_enabled()
     }
 }
+
+pub(crate) struct AshmemToggleRead;
+
+impl AshmemToggle for AshmemToggleRead {
+    const NAME: &'static CStr = c_str!("ashmem_ignore_unset_prot_read");
+    fn set(enabled: bool) -> Result<()> {
+        IGNORE_UNSET_PROT_READ.store(enabled, Ordering::Relaxed);
+        Ok(())
+    }
+    fn get() -> bool {
+        IGNORE_UNSET_PROT_READ.load(Ordering::Relaxed)
+    }
+}