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authorDavid Herrmann <dh.herrmann@gmail.com>2014-08-08 14:25:27 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2014-08-08 15:57:31 -0700
commit40e041a2c858b3caefc757e26cb85bfceae5062b (patch)
treeb55bfe2e25e490c94dba630b6299c60342d49cd8 /mm
parent4bb5f5d9395bc112d93a134d8f5b05611eddc9c0 (diff)
shm: add sealing API
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> Cc: Michael Kerrisk <mtk.manpages@gmail.com> Cc: Ryan Lortie <desrt@desrt.ca> Cc: Lennart Poettering <lennart@poettering.net> Cc: Daniel Mack <zonque@gmail.com> Cc: Andy Lutomirski <luto@amacapital.net> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm')
-rw-r--r--mm/shmem.c143
1 files changed, 143 insertions, 0 deletions
diff --git a/mm/shmem.c b/mm/shmem.c
index 6dc80d298f9d..8b43bb7a4efe 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>
@@ -547,6 +548,7 @@ 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);
@@ -557,6 +559,11 @@ 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) {
error = shmem_reacct_size(SHMEM_I(inode)->flags,
oldsize, newsize);
@@ -1412,6 +1419,7 @@ 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);
@@ -1470,7 +1478,17 @@ 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);
}
@@ -1808,11 +1826,125 @@ 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;
@@ -1828,6 +1960,12 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
loff_t unmap_end = round_down(offset + len, PAGE_SIZE) - 1;
DECLARE_WAIT_QUEUE_HEAD_ONSTACK(shmem_falloc_waitq);
+ /* protected by i_mutex */
+ if (info->seals & F_SEAL_WRITE) {
+ error = -EPERM;
+ goto out;
+ }
+
shmem_falloc.waitq = &shmem_falloc_waitq;
shmem_falloc.start = unmap_start >> PAGE_SHIFT;
shmem_falloc.next = (unmap_end + 1) >> PAGE_SHIFT;
@@ -1854,6 +1992,11 @@ 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 */