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329 lines
7.9 KiB
329 lines
7.9 KiB
/* |
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* memfd_create system call and file sealing support |
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* |
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* Code was originally included in shmem.c, and broken out to facilitate |
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* use by hugetlbfs as well as tmpfs. |
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* |
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* This file is released under the GPL. |
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*/ |
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#include <linux/fs.h> |
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#include <linux/vfs.h> |
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#include <linux/pagemap.h> |
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#include <linux/file.h> |
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#include <linux/mm.h> |
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#include <linux/sched/signal.h> |
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#include <linux/khugepaged.h> |
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#include <linux/syscalls.h> |
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#include <linux/hugetlb.h> |
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#include <linux/shmem_fs.h> |
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#include <linux/memfd.h> |
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#include <uapi/linux/memfd.h> |
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/* |
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* We need a tag: a new tag would expand every xa_node by 8 bytes, |
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* so reuse a tag which we firmly believe is never set or cleared on tmpfs |
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* or hugetlbfs because they are memory only filesystems. |
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*/ |
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#define MEMFD_TAG_PINNED PAGECACHE_TAG_TOWRITE |
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#define LAST_SCAN 4 /* about 150ms max */ |
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static void memfd_tag_pins(struct xa_state *xas) |
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{ |
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struct page *page; |
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unsigned int tagged = 0; |
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lru_add_drain(); |
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xas_lock_irq(xas); |
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xas_for_each(xas, page, ULONG_MAX) { |
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if (xa_is_value(page)) |
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continue; |
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page = find_subpage(page, xas->xa_index); |
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if (page_count(page) - page_mapcount(page) > 1) |
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xas_set_mark(xas, MEMFD_TAG_PINNED); |
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if (++tagged % XA_CHECK_SCHED) |
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continue; |
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xas_pause(xas); |
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xas_unlock_irq(xas); |
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cond_resched(); |
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xas_lock_irq(xas); |
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} |
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xas_unlock_irq(xas); |
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} |
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/* |
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* Setting SEAL_WRITE requires us to verify there's no pending writer. However, |
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* via get_user_pages(), drivers might have some pending I/O without any active |
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* user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages |
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* and see whether it has an elevated ref-count. If so, we tag them and wait for |
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* them to be dropped. |
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* The caller must guarantee that no new user will acquire writable references |
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* to those pages to avoid races. |
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*/ |
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static int memfd_wait_for_pins(struct address_space *mapping) |
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{ |
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XA_STATE(xas, &mapping->i_pages, 0); |
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struct page *page; |
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int error, scan; |
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memfd_tag_pins(&xas); |
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error = 0; |
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for (scan = 0; scan <= LAST_SCAN; scan++) { |
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unsigned int tagged = 0; |
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if (!xas_marked(&xas, MEMFD_TAG_PINNED)) |
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break; |
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if (!scan) |
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lru_add_drain_all(); |
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else if (schedule_timeout_killable((HZ << scan) / 200)) |
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scan = LAST_SCAN; |
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xas_set(&xas, 0); |
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xas_lock_irq(&xas); |
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xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) { |
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bool clear = true; |
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if (xa_is_value(page)) |
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continue; |
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page = find_subpage(page, xas.xa_index); |
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if (page_count(page) - page_mapcount(page) != 1) { |
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/* |
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* On the last scan, we clean up all those tags |
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* we inserted; but make a note that we still |
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* found pages pinned. |
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*/ |
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if (scan == LAST_SCAN) |
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error = -EBUSY; |
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else |
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clear = false; |
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} |
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if (clear) |
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xas_clear_mark(&xas, MEMFD_TAG_PINNED); |
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if (++tagged % XA_CHECK_SCHED) |
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continue; |
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xas_pause(&xas); |
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xas_unlock_irq(&xas); |
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cond_resched(); |
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xas_lock_irq(&xas); |
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} |
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xas_unlock_irq(&xas); |
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} |
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return error; |
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} |
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static unsigned int *memfd_file_seals_ptr(struct file *file) |
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{ |
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if (shmem_file(file)) |
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return &SHMEM_I(file_inode(file))->seals; |
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#ifdef CONFIG_HUGETLBFS |
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if (is_file_hugepages(file)) |
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return &HUGETLBFS_I(file_inode(file))->seals; |
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#endif |
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return NULL; |
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} |
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#define F_ALL_SEALS (F_SEAL_SEAL | \ |
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F_SEAL_SHRINK | \ |
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F_SEAL_GROW | \ |
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F_SEAL_WRITE | \ |
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F_SEAL_FUTURE_WRITE) |
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static int memfd_add_seals(struct file *file, unsigned int seals) |
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{ |
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struct inode *inode = file_inode(file); |
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unsigned int *file_seals; |
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int error; |
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/* |
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* SEALING |
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* Sealing allows multiple parties to share a tmpfs or hugetlbfs file |
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* but restrict access to a specific subset of file operations. Seals |
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* can only be added, but never removed. This way, mutually untrusted |
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* parties can share common memory regions with a well-defined policy. |
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* A malicious peer can thus never perform unwanted operations on a |
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* shared object. |
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* |
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* Seals are only supported on special tmpfs or hugetlbfs files and |
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* always affect the whole underlying inode. Once a seal is set, it |
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* may prevent some kinds of access to the file. Currently, the |
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* following seals are defined: |
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* SEAL_SEAL: Prevent further seals from being set on this file |
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* SEAL_SHRINK: Prevent the file from shrinking |
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* SEAL_GROW: Prevent the file from growing |
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* SEAL_WRITE: Prevent write access to the file |
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* |
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* As we don't require any trust relationship between two parties, we |
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* must prevent seals from being removed. Therefore, sealing a file |
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* only adds a given set of seals to the file, it never touches |
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* existing seals. Furthermore, the "setting seals"-operation can be |
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* sealed itself, which basically prevents any further seal from being |
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* added. |
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* |
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* Semantics of sealing are only defined on volatile files. Only |
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* anonymous tmpfs and hugetlbfs files support sealing. More |
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* importantly, seals are never written to disk. Therefore, there's |
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* no plan to support it on other file types. |
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*/ |
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if (!(file->f_mode & FMODE_WRITE)) |
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return -EPERM; |
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if (seals & ~(unsigned int)F_ALL_SEALS) |
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return -EINVAL; |
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inode_lock(inode); |
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file_seals = memfd_file_seals_ptr(file); |
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if (!file_seals) { |
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error = -EINVAL; |
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goto unlock; |
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} |
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if (*file_seals & F_SEAL_SEAL) { |
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error = -EPERM; |
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goto unlock; |
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} |
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if ((seals & F_SEAL_WRITE) && !(*file_seals & F_SEAL_WRITE)) { |
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error = mapping_deny_writable(file->f_mapping); |
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if (error) |
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goto unlock; |
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error = memfd_wait_for_pins(file->f_mapping); |
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if (error) { |
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mapping_allow_writable(file->f_mapping); |
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goto unlock; |
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} |
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} |
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*file_seals |= seals; |
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error = 0; |
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unlock: |
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inode_unlock(inode); |
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return error; |
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} |
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static int memfd_get_seals(struct file *file) |
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{ |
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unsigned int *seals = memfd_file_seals_ptr(file); |
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return seals ? *seals : -EINVAL; |
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} |
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long memfd_fcntl(struct file *file, unsigned int cmd, unsigned long arg) |
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{ |
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long error; |
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switch (cmd) { |
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case F_ADD_SEALS: |
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/* disallow upper 32bit */ |
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if (arg > UINT_MAX) |
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return -EINVAL; |
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error = memfd_add_seals(file, arg); |
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break; |
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case F_GET_SEALS: |
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error = memfd_get_seals(file); |
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break; |
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default: |
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error = -EINVAL; |
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break; |
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} |
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return error; |
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} |
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#define MFD_NAME_PREFIX "memfd:" |
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#define MFD_NAME_PREFIX_LEN (sizeof(MFD_NAME_PREFIX) - 1) |
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#define MFD_NAME_MAX_LEN (NAME_MAX - MFD_NAME_PREFIX_LEN) |
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#define MFD_ALL_FLAGS (MFD_CLOEXEC | MFD_ALLOW_SEALING | MFD_HUGETLB) |
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SYSCALL_DEFINE2(memfd_create, |
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const char __user *, uname, |
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unsigned int, flags) |
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{ |
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unsigned int *file_seals; |
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struct file *file; |
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int fd, error; |
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char *name; |
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long len; |
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if (!(flags & MFD_HUGETLB)) { |
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if (flags & ~(unsigned int)MFD_ALL_FLAGS) |
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return -EINVAL; |
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} else { |
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/* Allow huge page size encoding in flags. */ |
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if (flags & ~(unsigned int)(MFD_ALL_FLAGS | |
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(MFD_HUGE_MASK << MFD_HUGE_SHIFT))) |
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return -EINVAL; |
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} |
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/* length includes terminating zero */ |
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len = strnlen_user(uname, MFD_NAME_MAX_LEN + 1); |
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if (len <= 0) |
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return -EFAULT; |
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if (len > MFD_NAME_MAX_LEN + 1) |
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return -EINVAL; |
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name = kmalloc(len + MFD_NAME_PREFIX_LEN, GFP_KERNEL); |
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if (!name) |
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return -ENOMEM; |
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strcpy(name, MFD_NAME_PREFIX); |
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if (copy_from_user(&name[MFD_NAME_PREFIX_LEN], uname, len)) { |
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error = -EFAULT; |
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goto err_name; |
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} |
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/* terminating-zero may have changed after strnlen_user() returned */ |
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if (name[len + MFD_NAME_PREFIX_LEN - 1]) { |
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error = -EFAULT; |
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goto err_name; |
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} |
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fd = get_unused_fd_flags((flags & MFD_CLOEXEC) ? O_CLOEXEC : 0); |
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if (fd < 0) { |
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error = fd; |
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goto err_name; |
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} |
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if (flags & MFD_HUGETLB) { |
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struct ucounts *ucounts = NULL; |
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file = hugetlb_file_setup(name, 0, VM_NORESERVE, &ucounts, |
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HUGETLB_ANONHUGE_INODE, |
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(flags >> MFD_HUGE_SHIFT) & |
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MFD_HUGE_MASK); |
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} else |
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file = shmem_file_setup(name, 0, VM_NORESERVE); |
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if (IS_ERR(file)) { |
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error = PTR_ERR(file); |
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goto err_fd; |
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} |
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file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE; |
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file->f_flags |= O_LARGEFILE; |
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if (flags & MFD_ALLOW_SEALING) { |
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file_seals = memfd_file_seals_ptr(file); |
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*file_seals &= ~F_SEAL_SEAL; |
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} |
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fd_install(fd, file); |
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kfree(name); |
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return fd; |
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err_fd: |
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put_unused_fd(fd); |
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err_name: |
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kfree(name); |
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return error; |
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}
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