mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
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1507f51255
Introduce "memfd_secret" system call with the ability to create memory
areas visible only in the context of the owning process and not mapped not
only to other processes but in the kernel page tables as well.
The secretmem feature is off by default and the user must explicitly
enable it at the boot time.
Once secretmem is enabled, the user will be able to create a file
descriptor using the memfd_secret() system call. The memory areas created
by mmap() calls from this file descriptor will be unmapped from the kernel
direct map and they will be only mapped in the page table of the processes
that have access to the file descriptor.
Secretmem is designed to provide the following protections:
* Enhanced protection (in conjunction with all the other in-kernel
attack prevention systems) against ROP attacks. Seceretmem makes
"simple" ROP insufficient to perform exfiltration, which increases the
required complexity of the attack. Along with other protections like
the kernel stack size limit and address space layout randomization which
make finding gadgets is really hard, absence of any in-kernel primitive
for accessing secret memory means the one gadget ROP attack can't work.
Since the only way to access secret memory is to reconstruct the missing
mapping entry, the attacker has to recover the physical page and insert
a PTE pointing to it in the kernel and then retrieve the contents. That
takes at least three gadgets which is a level of difficulty beyond most
standard attacks.
* Prevent cross-process secret userspace memory exposures. Once the
secret memory is allocated, the user can't accidentally pass it into the
kernel to be transmitted somewhere. The secreremem pages cannot be
accessed via the direct map and they are disallowed in GUP.
* Harden against exploited kernel flaws. In order to access secretmem,
a kernel-side attack would need to either walk the page tables and
create new ones, or spawn a new privileged uiserspace process to perform
secrets exfiltration using ptrace.
The file descriptor based memory has several advantages over the
"traditional" mm interfaces, such as mlock(), mprotect(), madvise(). File
descriptor approach allows explicit and controlled sharing of the memory
areas, it allows to seal the operations. Besides, file descriptor based
memory paves the way for VMMs to remove the secret memory range from the
userspace hipervisor process, for instance QEMU. Andy Lutomirski says:
"Getting fd-backed memory into a guest will take some possibly major
work in the kernel, but getting vma-backed memory into a guest without
mapping it in the host user address space seems much, much worse."
memfd_secret() is made a dedicated system call rather than an extension to
memfd_create() because it's purpose is to allow the user to create more
secure memory mappings rather than to simply allow file based access to
the memory. Nowadays a new system call cost is negligible while it is way
simpler for userspace to deal with a clear-cut system calls than with a
multiplexer or an overloaded syscall. Moreover, the initial
implementation of memfd_secret() is completely distinct from
memfd_create() so there is no much sense in overloading memfd_create() to
begin with. If there will be a need for code sharing between these
implementation it can be easily achieved without a need to adjust user
visible APIs.
The secret memory remains accessible in the process context using uaccess
primitives, but it is not exposed to the kernel otherwise; secret memory
areas are removed from the direct map and functions in the
follow_page()/get_user_page() family will refuse to return a page that
belongs to the secret memory area.
Once there will be a use case that will require exposing secretmem to the
kernel it will be an opt-in request in the system call flags so that user
would have to decide what data can be exposed to the kernel.
Removing of the pages from the direct map may cause its fragmentation on
architectures that use large pages to map the physical memory which
affects the system performance. However, the original Kconfig text for
CONFIG_DIRECT_GBPAGES said that gigabyte pages in the direct map "... can
improve the kernel's performance a tiny bit ..." (commit 00d1c5e057
("x86: add gbpages switches")) and the recent report [1] showed that "...
although 1G mappings are a good default choice, there is no compelling
evidence that it must be the only choice". Hence, it is sufficient to
have secretmem disabled by default with the ability of a system
administrator to enable it at boot time.
Pages in the secretmem regions are unevictable and unmovable to avoid
accidental exposure of the sensitive data via swap or during page
migration.
Since the secretmem mappings are locked in memory they cannot exceed
RLIMIT_MEMLOCK. Since these mappings are already locked independently
from mlock(), an attempt to mlock()/munlock() secretmem range would fail
and mlockall()/munlockall() will ignore secretmem mappings.
However, unlike mlock()ed memory, secretmem currently behaves more like
long-term GUP: secretmem mappings are unmovable mappings directly consumed
by user space. With default limits, there is no excessive use of
secretmem and it poses no real problem in combination with
ZONE_MOVABLE/CMA, but in the future this should be addressed to allow
balanced use of large amounts of secretmem along with ZONE_MOVABLE/CMA.
A page that was a part of the secret memory area is cleared when it is
freed to ensure the data is not exposed to the next user of that page.
The following example demonstrates creation of a secret mapping (error
handling is omitted):
fd = memfd_secret(0);
ftruncate(fd, MAP_SIZE);
ptr = mmap(NULL, MAP_SIZE, PROT_READ | PROT_WRITE,
MAP_SHARED, fd, 0);
[1] https://lore.kernel.org/linux-mm/213b4567-46ce-f116-9cdf-bbd0c884eb3c@linux.intel.com/
[akpm@linux-foundation.org: suppress Kconfig whine]
Link: https://lkml.kernel.org/r/20210518072034.31572-5-rppt@kernel.org
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Hagen Paul Pfeifer <hagen@jauu.net>
Acked-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christopher Lameter <cl@linux.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Elena Reshetova <elena.reshetova@intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Palmer Dabbelt <palmerdabbelt@google.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: Roman Gushchin <guro@fb.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tycho Andersen <tycho@tycho.ws>
Cc: Will Deacon <will@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: kernel test robot <lkp@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
102 lines
3.7 KiB
C
102 lines
3.7 KiB
C
/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
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#ifndef __LINUX_MAGIC_H__
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#define __LINUX_MAGIC_H__
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#define ADFS_SUPER_MAGIC 0xadf5
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#define AFFS_SUPER_MAGIC 0xadff
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#define AFS_SUPER_MAGIC 0x5346414F
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#define AUTOFS_SUPER_MAGIC 0x0187
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#define CODA_SUPER_MAGIC 0x73757245
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#define CRAMFS_MAGIC 0x28cd3d45 /* some random number */
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#define CRAMFS_MAGIC_WEND 0x453dcd28 /* magic number with the wrong endianess */
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#define DEBUGFS_MAGIC 0x64626720
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#define SECURITYFS_MAGIC 0x73636673
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#define SELINUX_MAGIC 0xf97cff8c
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#define SMACK_MAGIC 0x43415d53 /* "SMAC" */
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#define RAMFS_MAGIC 0x858458f6 /* some random number */
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#define TMPFS_MAGIC 0x01021994
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#define HUGETLBFS_MAGIC 0x958458f6 /* some random number */
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#define SQUASHFS_MAGIC 0x73717368
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#define ECRYPTFS_SUPER_MAGIC 0xf15f
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#define EFS_SUPER_MAGIC 0x414A53
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#define EROFS_SUPER_MAGIC_V1 0xE0F5E1E2
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#define EXT2_SUPER_MAGIC 0xEF53
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#define EXT3_SUPER_MAGIC 0xEF53
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#define XENFS_SUPER_MAGIC 0xabba1974
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#define EXT4_SUPER_MAGIC 0xEF53
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#define BTRFS_SUPER_MAGIC 0x9123683E
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#define NILFS_SUPER_MAGIC 0x3434
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#define F2FS_SUPER_MAGIC 0xF2F52010
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#define HPFS_SUPER_MAGIC 0xf995e849
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#define ISOFS_SUPER_MAGIC 0x9660
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#define JFFS2_SUPER_MAGIC 0x72b6
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#define XFS_SUPER_MAGIC 0x58465342 /* "XFSB" */
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#define PSTOREFS_MAGIC 0x6165676C
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#define EFIVARFS_MAGIC 0xde5e81e4
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#define HOSTFS_SUPER_MAGIC 0x00c0ffee
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#define OVERLAYFS_SUPER_MAGIC 0x794c7630
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#define MINIX_SUPER_MAGIC 0x137F /* minix v1 fs, 14 char names */
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#define MINIX_SUPER_MAGIC2 0x138F /* minix v1 fs, 30 char names */
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#define MINIX2_SUPER_MAGIC 0x2468 /* minix v2 fs, 14 char names */
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#define MINIX2_SUPER_MAGIC2 0x2478 /* minix v2 fs, 30 char names */
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#define MINIX3_SUPER_MAGIC 0x4d5a /* minix v3 fs, 60 char names */
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#define MSDOS_SUPER_MAGIC 0x4d44 /* MD */
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#define NCP_SUPER_MAGIC 0x564c /* Guess, what 0x564c is :-) */
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#define NFS_SUPER_MAGIC 0x6969
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#define OCFS2_SUPER_MAGIC 0x7461636f
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#define OPENPROM_SUPER_MAGIC 0x9fa1
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#define QNX4_SUPER_MAGIC 0x002f /* qnx4 fs detection */
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#define QNX6_SUPER_MAGIC 0x68191122 /* qnx6 fs detection */
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#define AFS_FS_MAGIC 0x6B414653
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#define REISERFS_SUPER_MAGIC 0x52654973 /* used by gcc */
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/* used by file system utilities that
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look at the superblock, etc. */
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#define REISERFS_SUPER_MAGIC_STRING "ReIsErFs"
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#define REISER2FS_SUPER_MAGIC_STRING "ReIsEr2Fs"
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#define REISER2FS_JR_SUPER_MAGIC_STRING "ReIsEr3Fs"
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#define SMB_SUPER_MAGIC 0x517B
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#define CGROUP_SUPER_MAGIC 0x27e0eb
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#define CGROUP2_SUPER_MAGIC 0x63677270
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#define RDTGROUP_SUPER_MAGIC 0x7655821
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#define STACK_END_MAGIC 0x57AC6E9D
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#define TRACEFS_MAGIC 0x74726163
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#define V9FS_MAGIC 0x01021997
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#define BDEVFS_MAGIC 0x62646576
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#define DAXFS_MAGIC 0x64646178
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#define BINFMTFS_MAGIC 0x42494e4d
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#define DEVPTS_SUPER_MAGIC 0x1cd1
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#define BINDERFS_SUPER_MAGIC 0x6c6f6f70
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#define FUTEXFS_SUPER_MAGIC 0xBAD1DEA
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#define PIPEFS_MAGIC 0x50495045
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#define PROC_SUPER_MAGIC 0x9fa0
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#define SOCKFS_MAGIC 0x534F434B
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#define SYSFS_MAGIC 0x62656572
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#define USBDEVICE_SUPER_MAGIC 0x9fa2
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#define MTD_INODE_FS_MAGIC 0x11307854
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#define ANON_INODE_FS_MAGIC 0x09041934
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#define BTRFS_TEST_MAGIC 0x73727279
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#define NSFS_MAGIC 0x6e736673
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#define BPF_FS_MAGIC 0xcafe4a11
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#define AAFS_MAGIC 0x5a3c69f0
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#define ZONEFS_MAGIC 0x5a4f4653
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/* Since UDF 2.01 is ISO 13346 based... */
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#define UDF_SUPER_MAGIC 0x15013346
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#define BALLOON_KVM_MAGIC 0x13661366
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#define ZSMALLOC_MAGIC 0x58295829
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#define DMA_BUF_MAGIC 0x444d4142 /* "DMAB" */
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#define DEVMEM_MAGIC 0x454d444d /* "DMEM" */
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#define Z3FOLD_MAGIC 0x33
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#define PPC_CMM_MAGIC 0xc7571590
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#define SECRETMEM_MAGIC 0x5345434d /* "SECM" */
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#endif /* __LINUX_MAGIC_H__ */
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