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SubjectRe: [PATCH v4 2/6] shm: add sealing API
On Sun, 20 Jul 2014, David Herrmann wrote:

> If two processes share a common memory region, they usually want some
> guarantees to allow safe access. This often includes:
> - one side cannot overwrite data while the other reads it
> - one side cannot shrink the buffer while the other accesses it
> - one side cannot grow the buffer beyond previously set boundaries
>
> If there is a trust-relationship between both parties, there is no need
> for policy enforcement. However, if there's no trust relationship (eg.,
> for general-purpose IPC) sharing memory-regions is highly fragile and
> often not possible without local copies. Look at the following two
> use-cases:
> 1) A graphics client wants to share its rendering-buffer with a
> graphics-server. The memory-region is allocated by the client for
> read/write access and a second FD is passed to the server. While
> scanning out from the memory region, the server has no guarantee that
> the client doesn't shrink the buffer at any time, requiring rather
> cumbersome SIGBUS handling.
> 2) A process wants to perform an RPC on another process. To avoid huge
> bandwidth consumption, zero-copy is preferred. After a message is
> assembled in-memory and a FD is passed to the remote side, both sides
> want to be sure that neither modifies this shared copy, anymore. The
> source may have put sensible data into the message without a separate
> copy and the target may want to parse the message inline, to avoid a
> local copy.
>
> While SIGBUS handling, POSIX mandatory locking and MAP_DENYWRITE provide
> ways to achieve most of this, the first one is unproportionally ugly to
> use in libraries and the latter two are broken/racy or even disabled due
> to denial of service attacks.
>
> This patch introduces the concept of SEALING. If you seal a file, a
> specific set of operations is blocked on that file forever.
> Unlike locks, seals can only be set, never removed. Hence, once you
> verified a specific set of seals is set, you're guaranteed that no-one can
> perform the blocked operations on this file, anymore.
>
> An initial set of SEALS is introduced by this patch:
> - SHRINK: If SEAL_SHRINK is set, the file in question cannot be reduced
> in size. This affects ftruncate() and open(O_TRUNC).
> - GROW: If SEAL_GROW is set, the file in question cannot be increased
> in size. This affects ftruncate(), fallocate() and write().
> - WRITE: If SEAL_WRITE is set, no write operations (besides resizing)
> are possible. This affects fallocate(PUNCH_HOLE), mmap() and
> write().
> - SEAL: If SEAL_SEAL is set, no further seals can be added to a file.
> This basically prevents the F_ADD_SEAL operation on a file and
> can be set to prevent others from adding further seals that you
> don't want.
>
> The described use-cases can easily use these seals to provide safe use
> without any trust-relationship:
> 1) The graphics server can verify that a passed file-descriptor has
> SEAL_SHRINK set. This allows safe scanout, while the client is
> allowed to increase buffer size for window-resizing on-the-fly.
> Concurrent writes are explicitly allowed.
> 2) For general-purpose IPC, both processes can verify that SEAL_SHRINK,
> SEAL_GROW and SEAL_WRITE are set. This guarantees that neither
> process can modify the data while the other side parses it.
> Furthermore, it guarantees that even with writable FDs passed to the
> peer, it cannot increase the size to hit memory-limits of the source
> process (in case the file-storage is accounted to the source).
>
> The new API is an extension to fcntl(), adding two new commands:
> F_GET_SEALS: Return a bitset describing the seals on the file. This
> can be called on any FD if the underlying file supports
> sealing.
> F_ADD_SEALS: Change the seals of a given file. This requires WRITE
> access to the file and F_SEAL_SEAL may not already be set.
> Furthermore, the underlying file must support sealing and
> there may not be any existing shared mapping of that file.
> Otherwise, EBADF/EPERM is returned.
> The given seals are _added_ to the existing set of seals
> on the file. You cannot remove seals again.
>
> The fcntl() handler is currently specific to shmem and disabled on all
> files. A file needs to explicitly support sealing for this interface to
> work. A separate syscall is added in a follow-up, which creates files that
> support sealing. There is no intention to support this on other
> file-systems. Semantics are unclear for non-volatile files and we lack any
> use-case right now. Therefore, the implementation is specific to shmem.
>
> Signed-off-by: David Herrmann <dh.herrmann@gmail.com>

Acked-by: Hugh Dickins <hughd@google.com>

We've just changed the context lines of your hunk to shmem_fallocate(),
sorry, but the reject is easily fixed up in the obvious way.

> ---
> fs/fcntl.c | 5 ++
> include/linux/shmem_fs.h | 17 ++++++
> include/uapi/linux/fcntl.h | 15 +++++
> mm/shmem.c | 143 +++++++++++++++++++++++++++++++++++++++++++++
> 4 files changed, 180 insertions(+)
>
> diff --git a/fs/fcntl.c b/fs/fcntl.c
> index 72c82f6..22d1c3d 100644
> --- a/fs/fcntl.c
> +++ b/fs/fcntl.c
> @@ -21,6 +21,7 @@
> #include <linux/rcupdate.h>
> #include <linux/pid_namespace.h>
> #include <linux/user_namespace.h>
> +#include <linux/shmem_fs.h>
>
> #include <asm/poll.h>
> #include <asm/siginfo.h>
> @@ -336,6 +337,10 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
> case F_GETPIPE_SZ:
> err = pipe_fcntl(filp, cmd, arg);
> break;
> + case F_ADD_SEALS:
> + case F_GET_SEALS:
> + err = shmem_fcntl(filp, cmd, arg);
> + break;
> default:
> break;
> }
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index 4d1771c..50777b5 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -1,6 +1,7 @@
> #ifndef __SHMEM_FS_H
> #define __SHMEM_FS_H
>
> +#include <linux/file.h>
> #include <linux/swap.h>
> #include <linux/mempolicy.h>
> #include <linux/pagemap.h>
> @@ -11,6 +12,7 @@
>
> struct shmem_inode_info {
> spinlock_t lock;
> + unsigned int seals; /* shmem seals */
> unsigned long flags;
> unsigned long alloced; /* data pages alloced to file */
> union {
> @@ -65,4 +67,19 @@ static inline struct page *shmem_read_mapping_page(
> mapping_gfp_mask(mapping));
> }
>
> +#ifdef CONFIG_TMPFS
> +
> +extern int shmem_add_seals(struct file *file, unsigned int seals);
> +extern int shmem_get_seals(struct file *file);
> +extern long shmem_fcntl(struct file *file, unsigned int cmd, unsigned long arg);
> +
> +#else
> +
> +static inline long shmem_fcntl(struct file *f, unsigned int c, unsigned long a)
> +{
> + return -EINVAL;
> +}
> +
> +#endif
> +
> #endif
> diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
> index 074b886..beed138 100644
> --- a/include/uapi/linux/fcntl.h
> +++ b/include/uapi/linux/fcntl.h
> @@ -28,6 +28,21 @@
> #define F_GETPIPE_SZ (F_LINUX_SPECIFIC_BASE + 8)
>
> /*
> + * Set/Get seals
> + */
> +#define F_ADD_SEALS (F_LINUX_SPECIFIC_BASE + 9)
> +#define F_GET_SEALS (F_LINUX_SPECIFIC_BASE + 10)
> +
> +/*
> + * Types of seals
> + */
> +#define F_SEAL_SEAL 0x0001 /* prevent further seals from being set */
> +#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 */
> +/* (1U << 31) is reserved for signed error codes */
> +
> +/*
> * Types of directory notifications that may be requested.
> */
> #define DN_ACCESS 0x00000001 /* File accessed */
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 1140f49..51dccd0 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -66,6 +66,7 @@ static struct vfsmount *shm_mnt;
> #include <linux/highmem.h>
> #include <linux/seq_file.h>
> #include <linux/magic.h>
> +#include <linux/fcntl.h>
>
> #include <asm/uaccess.h>
> #include <asm/pgtable.h>
> @@ -532,6 +533,7 @@ EXPORT_SYMBOL_GPL(shmem_truncate_range);
> static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
> {
> struct inode *inode = dentry->d_inode;
> + struct shmem_inode_info *info = SHMEM_I(inode);
> int error;
>
> error = inode_change_ok(inode, attr);
> @@ -542,6 +544,11 @@ static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
> loff_t oldsize = inode->i_size;
> loff_t newsize = attr->ia_size;
>
> + /* protected by i_mutex */
> + if ((newsize < oldsize && (info->seals & F_SEAL_SHRINK)) ||
> + (newsize > oldsize && (info->seals & F_SEAL_GROW)))
> + return -EPERM;
> +
> if (newsize != oldsize) {
> i_size_write(inode, newsize);
> inode->i_ctime = inode->i_mtime = CURRENT_TIME;
> @@ -1364,6 +1371,7 @@ static struct inode *shmem_get_inode(struct super_block *sb, const struct inode
> info = SHMEM_I(inode);
> memset(info, 0, (char *)inode - (char *)info);
> spin_lock_init(&info->lock);
> + info->seals = F_SEAL_SEAL;
> info->flags = flags & VM_NORESERVE;
> INIT_LIST_HEAD(&info->swaplist);
> simple_xattrs_init(&info->xattrs);
> @@ -1422,7 +1430,17 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
> struct page **pagep, void **fsdata)
> {
> struct inode *inode = mapping->host;
> + struct shmem_inode_info *info = SHMEM_I(inode);
> pgoff_t index = pos >> PAGE_CACHE_SHIFT;
> +
> + /* i_mutex is held by caller */
> + if (unlikely(info->seals)) {
> + if (info->seals & F_SEAL_WRITE)
> + return -EPERM;
> + if ((info->seals & F_SEAL_GROW) && pos + len > inode->i_size)
> + return -EPERM;
> + }
> +
> return shmem_getpage(inode, index, pagep, SGP_WRITE, NULL);
> }
>
> @@ -1760,11 +1778,125 @@ static loff_t shmem_file_llseek(struct file *file, loff_t offset, int whence)
> return offset;
> }
>
> +static int shmem_wait_for_pins(struct address_space *mapping)
> +{
> + return 0;
> +}
> +
> +#define F_ALL_SEALS (F_SEAL_SEAL | \
> + F_SEAL_SHRINK | \
> + F_SEAL_GROW | \
> + F_SEAL_WRITE)
> +
> +int shmem_add_seals(struct file *file, unsigned int seals)
> +{
> + struct inode *inode = file_inode(file);
> + struct shmem_inode_info *info = SHMEM_I(inode);
> + int error;
> +
> + /*
> + * SEALING
> + * Sealing allows multiple parties to share a shmem-file but restrict
> + * access to a specific subset of file operations. Seals can only be
> + * added, but never removed. This way, mutually untrusted parties can
> + * share common memory regions with a well-defined policy. A malicious
> + * peer can thus never perform unwanted operations on a shared object.
> + *
> + * Seals are only supported on special shmem-files and always affect
> + * the whole underlying inode. Once a seal is set, it may prevent some
> + * kinds of access to the file. Currently, the following seals are
> + * defined:
> + * SEAL_SEAL: Prevent further seals from being set on this file
> + * SEAL_SHRINK: Prevent the file from shrinking
> + * SEAL_GROW: Prevent the file from growing
> + * SEAL_WRITE: Prevent write access to the file
> + *
> + * As we don't require any trust relationship between two parties, we
> + * must prevent seals from being removed. Therefore, sealing a file
> + * only adds a given set of seals to the file, it never touches
> + * existing seals. Furthermore, the "setting seals"-operation can be
> + * sealed itself, which basically prevents any further seal from being
> + * added.
> + *
> + * Semantics of sealing are only defined on volatile files. Only
> + * anonymous shmem files support sealing. More importantly, seals are
> + * never written to disk. Therefore, there's no plan to support it on
> + * other file types.
> + */
> +
> + if (file->f_op != &shmem_file_operations)
> + return -EINVAL;
> + if (!(file->f_mode & FMODE_WRITE))
> + return -EPERM;
> + if (seals & ~(unsigned int)F_ALL_SEALS)
> + return -EINVAL;
> +
> + mutex_lock(&inode->i_mutex);
> +
> + if (info->seals & F_SEAL_SEAL) {
> + error = -EPERM;
> + goto unlock;
> + }
> +
> + if ((seals & F_SEAL_WRITE) && !(info->seals & F_SEAL_WRITE)) {
> + error = mapping_deny_writable(file->f_mapping);
> + if (error)
> + goto unlock;
> +
> + error = shmem_wait_for_pins(file->f_mapping);
> + if (error) {
> + mapping_allow_writable(file->f_mapping);
> + goto unlock;
> + }
> + }
> +
> + info->seals |= seals;
> + error = 0;
> +
> +unlock:
> + mutex_unlock(&inode->i_mutex);
> + return error;
> +}
> +EXPORT_SYMBOL_GPL(shmem_add_seals);
> +
> +int shmem_get_seals(struct file *file)
> +{
> + if (file->f_op != &shmem_file_operations)
> + return -EINVAL;
> +
> + return SHMEM_I(file_inode(file))->seals;
> +}
> +EXPORT_SYMBOL_GPL(shmem_get_seals);
> +
> +long shmem_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
> +{
> + long error;
> +
> + switch (cmd) {
> + case F_ADD_SEALS:
> + /* disallow upper 32bit */
> + if (arg > UINT_MAX)
> + return -EINVAL;
> +
> + error = shmem_add_seals(file, arg);
> + break;
> + case F_GET_SEALS:
> + error = shmem_get_seals(file);
> + break;
> + default:
> + error = -EINVAL;
> + break;
> + }
> +
> + return error;
> +}
> +
> static long shmem_fallocate(struct file *file, int mode, loff_t offset,
> loff_t len)
> {
> struct inode *inode = file_inode(file);
> struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
> + struct shmem_inode_info *info = SHMEM_I(inode);
> struct shmem_falloc shmem_falloc;
> pgoff_t start, index, end;
> int error;
> @@ -1781,6 +1913,12 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
> loff_t unmap_start = round_up(offset, PAGE_SIZE);
> loff_t unmap_end = round_down(offset + len, PAGE_SIZE) - 1;
>
> + /* protected by i_mutex */
> + if (info->seals & F_SEAL_WRITE) {
> + error = -EPERM;
> + goto out;
> + }
> +
> shmem_falloc.start = unmap_start >> PAGE_SHIFT;
> shmem_falloc.next = (unmap_end + 1) >> PAGE_SHIFT;
> spin_lock(&inode->i_lock);
> @@ -1801,6 +1939,11 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
> if (error)
> goto out;
>
> + if ((info->seals & F_SEAL_GROW) && offset + len > inode->i_size) {
> + error = -EPERM;
> + goto out;
> + }
> +
> start = offset >> PAGE_CACHE_SHIFT;
> end = (offset + len + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
> /* Try to avoid a swapstorm if len is impossible to satisfy */
> --
> 2.0.2
>
>


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