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![Aleksa Sarai](/assets/img/avatar_default.png)
Commit434ed3350f
("memfd: improve userspace warnings for missing exec-related flags") attempted to make these warnings more useful (so they would work as an incentive to get users to switch to specifying these flags -- as intended by the original MFD_NOEXEC_SEAL patchset). Unfortunately, it turns out that even INFO-level logging is too extreme to enable by default and alternative solutions to the spam issue (such as doing more extreme rate-limiting per-task) are either too ugly or overkill for something as simple as emitting a log as a developer aid. Given that the flags are new and there is no harm to not specifying them (after all, we maintain backwards compatibility) we can just drop the warnings for now until some time in the future when most programs have migrated and distributions start using vm.memfd_noexec=1 (where failing to pass the flag would result in unexpected errors for programs that use executable memfds). Link: https://lkml.kernel.org/r/20230912-memfd-reduce-spam-v2-1-7d92a4964b6a@cyphar.com Fixes:434ed3350f
("memfd: improve userspace warnings for missing exec-related flags") Fixes:2562d67b1b
("revert "memfd: improve userspace warnings for missing exec-related flags".") Signed-off-by: Aleksa Sarai <cyphar@cyphar.com> Reported-by: Damian Tometzki <dtometzki@fedoraproject.org> Reviewed-by: Christian Brauner <brauner@kernel.org> Cc: Daniel Verkamp <dverkamp@chromium.org> Cc: Jeff Xu <jeffxu@google.com> Cc: Kees Cook <keescook@chromium.org> Cc: Shuah Khan <shuah@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
390 lines
9.6 KiB
C
390 lines
9.6 KiB
C
/*
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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 <linux/pid_namespace.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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int latency = 0;
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int cache_count;
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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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cache_count = 1;
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if (!xa_is_value(page) &&
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PageTransHuge(page) && !PageHuge(page))
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cache_count = HPAGE_PMD_NR;
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if (!xa_is_value(page) &&
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page_count(page) - total_mapcount(page) != cache_count)
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xas_set_mark(xas, MEMFD_TAG_PINNED);
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if (cache_count != 1)
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xas_set(xas, page->index + cache_count);
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latency += cache_count;
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if (latency < XA_CHECK_SCHED)
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continue;
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latency = 0;
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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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int latency = 0;
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int cache_count;
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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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cache_count = 1;
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if (!xa_is_value(page) &&
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PageTransHuge(page) && !PageHuge(page))
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cache_count = HPAGE_PMD_NR;
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if (!xa_is_value(page) && cache_count !=
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page_count(page) - total_mapcount(page)) {
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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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latency += cache_count;
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if (latency < XA_CHECK_SCHED)
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continue;
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latency = 0;
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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_EXEC | \
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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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* SEAL_EXEC: Prevent modification of the exec bits in the file mode
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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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/*
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* SEAL_EXEC implys SEAL_WRITE, making W^X from the start.
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*/
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if (seals & F_SEAL_EXEC && inode->i_mode & 0111)
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seals |= F_SEAL_SHRINK|F_SEAL_GROW|F_SEAL_WRITE|F_SEAL_FUTURE_WRITE;
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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 int 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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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 | MFD_NOEXEC_SEAL | MFD_EXEC)
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static int check_sysctl_memfd_noexec(unsigned int *flags)
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{
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#ifdef CONFIG_SYSCTL
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struct pid_namespace *ns = task_active_pid_ns(current);
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int sysctl = pidns_memfd_noexec_scope(ns);
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if (!(*flags & (MFD_EXEC | MFD_NOEXEC_SEAL))) {
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if (sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL)
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*flags |= MFD_NOEXEC_SEAL;
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else
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*flags |= MFD_EXEC;
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}
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if (!(*flags & MFD_NOEXEC_SEAL) && sysctl >= MEMFD_NOEXEC_SCOPE_NOEXEC_ENFORCED) {
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pr_err_ratelimited(
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"%s[%d]: memfd_create() requires MFD_NOEXEC_SEAL with vm.memfd_noexec=%d\n",
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current->comm, task_pid_nr(current), sysctl);
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return -EACCES;
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}
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#endif
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return 0;
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}
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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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/* Invalid if both EXEC and NOEXEC_SEAL are set.*/
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if ((flags & MFD_EXEC) && (flags & MFD_NOEXEC_SEAL))
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return -EINVAL;
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error = check_sysctl_memfd_noexec(&flags);
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if (error < 0)
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return error;
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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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file = hugetlb_file_setup(name, 0, VM_NORESERVE,
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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_NOEXEC_SEAL) {
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struct inode *inode = file_inode(file);
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inode->i_mode &= ~0111;
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file_seals = memfd_file_seals_ptr(file);
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if (file_seals) {
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*file_seals &= ~F_SEAL_SEAL;
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*file_seals |= F_SEAL_EXEC;
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}
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} else if (flags & MFD_ALLOW_SEALING) {
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/* MFD_EXEC and MFD_ALLOW_SEALING are set */
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file_seals = memfd_file_seals_ptr(file);
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if (file_seals)
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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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