From: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx> Subject: 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@xxxxxxxxxx/ Thanks a lot to Andy for suggestions to improve code. Link: http://lkml.kernel.org/r/20190112203816.85534-2-joel@xxxxxxxxxxxxxxxxx Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx> Acked-by: John Stultz <john.stultz@xxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Minchan Kim <minchan@xxxxxxxxxx> Cc: Jann Horn <jannh@xxxxxxxxxx> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx> Cc: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: J. Bruce Fields <bfields@xxxxxxxxxxxx> Cc: Jeff Layton <jlayton@xxxxxxxxxx> Cc: Marc-Andr Lureau <marcandre.lureau@xxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Cc: Shuah Khan <shuah@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- fs/hugetlbfs/inode.c | 2 +- include/uapi/linux/fcntl.h | 1 + mm/memfd.c | 3 ++- mm/shmem.c | 25 ++++++++++++++++++++++--- 4 files changed, 26 insertions(+), 5 deletions(-) --- a/fs/hugetlbfs/inode.c~mm-memfd-add-an-f_seal_future_write-seal-to-memfd +++ a/fs/hugetlbfs/inode.c @@ -530,7 +530,7 @@ static long hugetlbfs_punch_hole(struct 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; } --- a/include/uapi/linux/fcntl.h~mm-memfd-add-an-f_seal_future_write-seal-to-memfd +++ a/include/uapi/linux/fcntl.h @@ -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 */ /* --- a/mm/memfd.c~mm-memfd-add-an-f_seal_future_write-seal-to-memfd +++ a/mm/memfd.c @@ -131,7 +131,8 @@ static unsigned int *memfd_file_seals_pt #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) { --- a/mm/shmem.c~mm-memfd-add-an-f_seal_future_write-seal-to-memfd +++ a/mm/shmem.c @@ -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, str 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 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; } _
