Commit ab3948f5 authored by Joel Fernandes (Google)'s avatar Joel Fernandes (Google) Committed by Linus Torvalds
Browse files

mm/memfd: add an F_SEAL_FUTURE_WRITE seal to memfd 

Android uses ashmem for sharing memory regions.  We are looking forward
to migrating all usecases of ashmem to memfd so that we can possibly
remove the ashmem driver in the future from staging while also
benefiting from using memfd and contributing to it.  Note staging
drivers are also not ABI and generally can be removed at anytime.

One of the main usecases Android has is the ability to create a region
and mmap it as writeable, then add protection against making any
"future" writes while keeping the existing already mmap'ed
writeable-region active.  This allows us to implement a usecase where
receivers of the shared memory buffer can get a read-only view, while
the sender continues to write to the buffer.  See CursorWindow
documentation in Android for more details:

  https://developer.android.com/reference/android/database/CursorWindow

This usecase cannot be implemented with the existing F_SEAL_WRITE seal.
To support the usecase, this patch adds a new F_SEAL_FUTURE_WRITE seal
which prevents any future mmap and write syscalls from succeeding while
keeping the existing mmap active.

A better way to do F_SEAL_FUTURE_WRITE seal was discussed [1] last week
where we don't need to modify core VFS structures to get the same
behavior of the seal.  This solves several side-effects pointed by Andy.
self-tests are provided in later patch to verify the expected semantics.

[1] https://lore.kernel.org/lkml/20181111173650.GA256781@google.com/

Thanks a lot to Andy for suggestions to improve code.

Link: http://lkml.kernel.org/r/20190112203816.85534-2-joel@joelfernandes.org


Signed-off-by: default avatarJoel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: default avatarJohn Stultz <john.stultz@linaro.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: J. Bruce Fields <bfields@fieldses.org>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Marc-Andr Lureau <marcandre.lureau@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 7f188251

fs/hugetlbfs/inode.c

+1 −1
Original line number Diff line number Diff line
@@ -530,7 +530,7 @@ static long hugetlbfs_punch_hole(struct inode *inode, loff_t offset, loff_t len) 
		inode_lock(inode);



		/* protected by i_mutex */

		if (info->seals & F_SEAL_WRITE) {

		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {

			inode_unlock(inode);

			return -EPERM;

		}

include/uapi/linux/fcntl.h

+1 −0
Original line number Diff line number Diff line
@@ -41,6 +41,7 @@ 
#define F_SEAL_SHRINK	0x0002	/* prevent file from shrinking */

#define F_SEAL_GROW	0x0004	/* prevent file from growing */

#define F_SEAL_WRITE	0x0008	/* prevent writes */

#define F_SEAL_FUTURE_WRITE	0x0010  /* prevent future writes while mapped */

/* (1U << 31) is reserved for signed error codes */



/*

mm/memfd.c

+2 −1
Original line number Diff line number Diff line
@@ -131,7 +131,8 @@ static unsigned int *memfd_file_seals_ptr(struct file *file) 
#define F_ALL_SEALS (F_SEAL_SEAL | \

		     F_SEAL_SHRINK | \

		     F_SEAL_GROW | \

		     F_SEAL_WRITE)

		     F_SEAL_WRITE | \

		     F_SEAL_FUTURE_WRITE)



static int memfd_add_seals(struct file *file, unsigned int seals)

{

mm/shmem.c

+22 −3
Original line number Diff line number Diff line
@@ -2190,6 +2190,24 @@ out_nomem: 


static int shmem_mmap(struct file *file, struct vm_area_struct *vma)

{

	struct shmem_inode_info *info = SHMEM_I(file_inode(file));



	if (info->seals & F_SEAL_FUTURE_WRITE) {

		/*

		 * New PROT_WRITE and MAP_SHARED mmaps are not allowed when

		 * "future write" seal active.

		 */

		if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_WRITE))

			return -EPERM;



		/*

		 * Since the F_SEAL_FUTURE_WRITE seals allow for a MAP_SHARED

		 * read-only mapping, take care to not allow mprotect to revert

		 * protections.

		 */

		vma->vm_flags &= ~(VM_MAYWRITE);

	}



	file_accessed(file);

	vma->vm_ops = &shmem_vm_ops;

	if (IS_ENABLED(CONFIG_TRANSPARENT_HUGE_PAGECACHE) &&
@@ -2440,8 +2458,9 @@ shmem_write_begin(struct file *file, struct address_space *mapping, 
	pgoff_t index = pos >> PAGE_SHIFT;



	/* i_mutex is held by caller */

	if (unlikely(info->seals & (F_SEAL_WRITE | F_SEAL_GROW))) {

		if (info->seals & F_SEAL_WRITE)

	if (unlikely(info->seals & (F_SEAL_GROW |

				   F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))) {

		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE))

			return -EPERM;

		if ((info->seals & F_SEAL_GROW) && pos + len > inode->i_size)

			return -EPERM;
@@ -2704,7 +2723,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset, 
		DECLARE_WAIT_QUEUE_HEAD_ONSTACK(shmem_falloc_waitq);



		/* protected by i_mutex */

		if (info->seals & F_SEAL_WRITE) {

		if (info->seals & (F_SEAL_WRITE | F_SEAL_FUTURE_WRITE)) {

			error = -EPERM;

			goto out;

		}

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