| Message ID | 20200924132904.1391-4-rppt@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm: introduce memfd_secret system call to create "secret" memory areas | expand |
On Thu, 2020-09-24 at 16:29 +0300, Mike Rapoport wrote: > 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 user will create a file descriptor using the memfd_secret() > system call > where flags supplied as a parameter to this system call will define > the > desired protection mode for the memory associated with that file > descriptor. > > Currently there are two protection modes: > > * exclusive - the memory area is unmapped from the kernel direct map > and it > is present only in the page tables of the owning mm. Seems like there were some concerns raised around direct map efficiency, but in case you are going to rework this...how does this memory work for the existing kernel functionality that does things like this? get_user_pages(, &page); ptr = kmap(page); foo = *ptr; Not sure if I'm missing something, but I think apps could cause the kernel to access a not-present page and oops.
On Tue, Sep 29, 2020 at 04:58:44AM +0000, Edgecombe, Rick P wrote: > On Thu, 2020-09-24 at 16:29 +0300, Mike Rapoport wrote: > > 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 user will create a file descriptor using the memfd_secret() > > system call > > where flags supplied as a parameter to this system call will define > > the > > desired protection mode for the memory associated with that file > > descriptor. > > > > Currently there are two protection modes: > > > > * exclusive - the memory area is unmapped from the kernel direct map > > and it > > is present only in the page tables of the owning mm. > > Seems like there were some concerns raised around direct map > efficiency, but in case you are going to rework this...how does this > memory work for the existing kernel functionality that does things like > this? > > get_user_pages(, &page); > ptr = kmap(page); > foo = *ptr; > > Not sure if I'm missing something, but I think apps could cause the > kernel to access a not-present page and oops. The idea is that this memory should not be accessible by the kernel, so the sequence you describe should indeed fail. Probably oops would be to noisy and in this case the report needs to be less verbose.
On Tue, 2020-09-29 at 16:06 +0300, Mike Rapoport wrote: > On Tue, Sep 29, 2020 at 04:58:44AM +0000, Edgecombe, Rick P wrote: > > On Thu, 2020-09-24 at 16:29 +0300, Mike Rapoport wrote: > > > 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 user will create a file descriptor using the memfd_secret() > > > system call > > > where flags supplied as a parameter to this system call will > > > define > > > the > > > desired protection mode for the memory associated with that file > > > descriptor. > > > > > > Currently there are two protection modes: > > > > > > * exclusive - the memory area is unmapped from the kernel direct > > > map > > > and it > > > is present only in the page tables of the owning > > > mm. > > > > Seems like there were some concerns raised around direct map > > efficiency, but in case you are going to rework this...how does > > this > > memory work for the existing kernel functionality that does things > > like > > this? > > > > get_user_pages(, &page); > > ptr = kmap(page); > > foo = *ptr; > > > > Not sure if I'm missing something, but I think apps could cause the > > kernel to access a not-present page and oops. > > The idea is that this memory should not be accessible by the kernel, > so > the sequence you describe should indeed fail. > > Probably oops would be to noisy and in this case the report needs to > be > less verbose. I was more concerned that it could cause kernel instabilities. I see, so it should not be accessed even at the userspace address? I wonder if it should be prevented somehow then. At least get_user_pages() should be prevented I think. Blocking copy_*_user() access might not be simple. I'm also not so sure that a user would never have any possible reason to copy data from this memory into the kernel, even if it's just convenience. In which case a user setup could break if a specific kernel implementation switched to get_user_pages()/kmap() from using copy_*_user(). So seems maybe a bit thorny without fully blocking access from the kernel, or deprecating that pattern. You should probably call out these "no passing data to/from the kernel" expectations, unless I missed them somewhere.
On Tue, Sep 29, 2020 at 08:06:03PM +0000, Edgecombe, Rick P wrote: > On Tue, 2020-09-29 at 16:06 +0300, Mike Rapoport wrote: > > On Tue, Sep 29, 2020 at 04:58:44AM +0000, Edgecombe, Rick P wrote: > > > On Thu, 2020-09-24 at 16:29 +0300, Mike Rapoport wrote: > > > > 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 user will create a file descriptor using the memfd_secret() > > > > system call > > > > where flags supplied as a parameter to this system call will > > > > define > > > > the > > > > desired protection mode for the memory associated with that file > > > > descriptor. > > > > > > > > Currently there are two protection modes: > > > > > > > > * exclusive - the memory area is unmapped from the kernel direct > > > > map > > > > and it > > > > is present only in the page tables of the owning > > > > mm. > > > > > > Seems like there were some concerns raised around direct map > > > efficiency, but in case you are going to rework this...how does > > > this > > > memory work for the existing kernel functionality that does things > > > like > > > this? > > > > > > get_user_pages(, &page); > > > ptr = kmap(page); > > > foo = *ptr; > > > > > > Not sure if I'm missing something, but I think apps could cause the > > > kernel to access a not-present page and oops. > > > > The idea is that this memory should not be accessible by the kernel, > > so > > the sequence you describe should indeed fail. > > > > Probably oops would be to noisy and in this case the report needs to > > be > > less verbose. > > I was more concerned that it could cause kernel instabilities. I think kernel recovers nicely from such sort of page fault, at least on x86. > I see, so it should not be accessed even at the userspace address? I > wonder if it should be prevented somehow then. At least > get_user_pages() should be prevented I think. Blocking copy_*_user() > access might not be simple. > > I'm also not so sure that a user would never have any possible reason > to copy data from this memory into the kernel, even if it's just > convenience. In which case a user setup could break if a specific > kernel implementation switched to get_user_pages()/kmap() from using > copy_*_user(). So seems maybe a bit thorny without fully blocking > access from the kernel, or deprecating that pattern. > > You should probably call out these "no passing data to/from the kernel" > expectations, unless I missed them somewhere. You are right, I should have been more explicit in the description of the expected behavoir. Our thinking was that copy_*user() would work in the context of the process that "owns" the secretmem and gup() would not allow access in general, unless requested with certail (yet another) FOLL_ flag.
On Wed, 2020-09-30 at 13:35 +0300, Mike Rapoport wrote: > On Tue, Sep 29, 2020 at 08:06:03PM +0000, Edgecombe, Rick P wrote: > > On Tue, 2020-09-29 at 16:06 +0300, Mike Rapoport wrote: > > > On Tue, Sep 29, 2020 at 04:58:44AM +0000, Edgecombe, Rick P > > > wrote: > > > > On Thu, 2020-09-24 at 16:29 +0300, Mike Rapoport wrote: > > > > > 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 user will create a file descriptor using the > > > > > memfd_secret() > > > > > system call > > > > > where flags supplied as a parameter to this system call will > > > > > define > > > > > the > > > > > desired protection mode for the memory associated with that > > > > > file > > > > > descriptor. > > > > > > > > > > Currently there are two protection modes: > > > > > > > > > > * exclusive - the memory area is unmapped from the kernel > > > > > direct > > > > > map > > > > > and it > > > > > is present only in the page tables of the > > > > > owning > > > > > mm. > > > > > > > > Seems like there were some concerns raised around direct map > > > > efficiency, but in case you are going to rework this...how does > > > > this > > > > memory work for the existing kernel functionality that does > > > > things > > > > like > > > > this? > > > > > > > > get_user_pages(, &page); > > > > ptr = kmap(page); > > > > foo = *ptr; > > > > > > > > Not sure if I'm missing something, but I think apps could cause > > > > the > > > > kernel to access a not-present page and oops. > > > > > > The idea is that this memory should not be accessible by the > > > kernel, > > > so > > > the sequence you describe should indeed fail. > > > > > > Probably oops would be to noisy and in this case the report needs > > > to > > > be > > > less verbose. > > > > I was more concerned that it could cause kernel instabilities. > > I think kernel recovers nicely from such sort of page fault, at least > on > x86. We are talking about the kernel taking a direct map NP fault and oopsing? Hmm, I thought it should often recover, but stability should be considered reduced. How could the kernel know whether to release locks or clean up other state? Pretty sure I've seen deadlocks in this case. > > I see, so it should not be accessed even at the userspace address? > > I > > wonder if it should be prevented somehow then. At least > > get_user_pages() should be prevented I think. Blocking > > copy_*_user() > > access might not be simple. > > > > I'm also not so sure that a user would never have any possible > > reason > > to copy data from this memory into the kernel, even if it's just > > convenience. In which case a user setup could break if a specific > > kernel implementation switched to get_user_pages()/kmap() from > > using > > copy_*_user(). So seems maybe a bit thorny without fully blocking > > access from the kernel, or deprecating that pattern. > > > > You should probably call out these "no passing data to/from the > > kernel" > > expectations, unless I missed them somewhere. > > You are right, I should have been more explicit in the description of > the expected behavoir. > > Our thinking was that copy_*user() would work in the context of the > process that "owns" the secretmem and gup() would not allow access in > general, unless requested with certail (yet another) FOLL_ flag. Hmm, yes. I think one easier thing about this design over the series Kirill sent out is that the actual page will never transition to and from unmapped while it's mapped in userspace. If it could transition, you'd have to worry about a race window between get_user_pages(FOLL_foo) and the kmap() where the page might get unmapped. Without the ability to transition pages though, using this for KVM guests memory remains a not completely worked through problem since it has the get_user_pages()/kmap() pattern quite a bit. Did you have an idea for that? (I thought I saw that use case mentioned somewhere).
On Wed, Sep 30, 2020 at 08:11:28PM +0000, Edgecombe, Rick P wrote: > On Wed, 2020-09-30 at 13:35 +0300, Mike Rapoport wrote: > > > > Our thinking was that copy_*user() would work in the context of the > > process that "owns" the secretmem and gup() would not allow access in > > general, unless requested with certail (yet another) FOLL_ flag. > > Hmm, yes. I think one easier thing about this design over the series > Kirill sent out is that the actual page will never transition to and > from unmapped while it's mapped in userspace. If it could transition, > you'd have to worry about a race window between > get_user_pages(FOLL_foo) and the kmap() where the page might get > unmapped. > > Without the ability to transition pages though, using this for KVM > guests memory remains a not completely worked through problem since it > has the get_user_pages()/kmap() pattern quite a bit. Did you have an > idea for that? (I thought I saw that use case mentioned somewhere). I've mentioned the KVM usecase because it was dicussed at the hallway track at KVM Forum last year and also after looking at Kirill's patches I though that "KVM protected" memory could be implemented on top of secretmem. Can't say I have enough expertise in KVM to have a completely worked through solution for that.
diff --git a/arch/Kconfig b/arch/Kconfig index af14a567b493..d3f11b2d03e8 100644 --- a/arch/Kconfig +++ b/arch/Kconfig @@ -975,6 +975,13 @@ config HAVE_SPARSE_SYSCALL_NR config ARCH_HAS_VDSO_DATA bool +config HAVE_SECRETMEM_UNCACHED + bool + help + An architecture can select this if its semantics of non-cached + mappings can be used to prevent speculative loads and it is + useful for secret protection. + source "kernel/gcov/Kconfig" source "scripts/gcc-plugins/Kconfig" diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index 7101ac64bb20..38ead8bd9909 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -220,6 +220,7 @@ config X86 select HAVE_UNSTABLE_SCHED_CLOCK select HAVE_USER_RETURN_NOTIFIER select HAVE_GENERIC_VDSO + select HAVE_SECRETMEM_UNCACHED select HOTPLUG_SMT if SMP select IRQ_FORCED_THREADING select NEED_SG_DMA_LENGTH diff --git a/include/uapi/linux/magic.h b/include/uapi/linux/magic.h index f3956fc11de6..35687dcb1a42 100644 --- a/include/uapi/linux/magic.h +++ b/include/uapi/linux/magic.h @@ -97,5 +97,6 @@ #define DEVMEM_MAGIC 0x454d444d /* "DMEM" */ #define Z3FOLD_MAGIC 0x33 #define PPC_CMM_MAGIC 0xc7571590 +#define SECRETMEM_MAGIC 0x5345434d /* "SECM" */ #endif /* __LINUX_MAGIC_H__ */ diff --git a/include/uapi/linux/secretmem.h b/include/uapi/linux/secretmem.h new file mode 100644 index 000000000000..2b9675f5dea9 --- /dev/null +++ b/include/uapi/linux/secretmem.h @@ -0,0 +1,8 @@ +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */ +#ifndef _UAPI_LINUX_SECRERTMEM_H +#define _UAPI_LINUX_SECRERTMEM_H + +/* secretmem operation modes */ +#define SECRETMEM_UNCACHED 0x1 + +#endif /* _UAPI_LINUX_SECRERTMEM_H */ diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c index 4d59775ea79c..8ae8d0c2d381 100644 --- a/kernel/sys_ni.c +++ b/kernel/sys_ni.c @@ -349,6 +349,8 @@ COND_SYSCALL(pkey_mprotect); COND_SYSCALL(pkey_alloc); COND_SYSCALL(pkey_free); +/* memfd_secret */ +COND_SYSCALL(memfd_secret); /* * Architecture specific weak syscall entries. diff --git a/mm/Kconfig b/mm/Kconfig index 6c974888f86f..d2fc73ccc183 100644 --- a/mm/Kconfig +++ b/mm/Kconfig @@ -868,4 +868,8 @@ config ARCH_HAS_HUGEPD config MAPPING_DIRTY_HELPERS bool +config SECRETMEM + def_bool ARCH_HAS_SET_DIRECT_MAP && !EMBEDDED + select GENERIC_ALLOCATOR + endmenu diff --git a/mm/Makefile b/mm/Makefile index d5649f1c12c0..cae063dc8298 100644 --- a/mm/Makefile +++ b/mm/Makefile @@ -121,3 +121,4 @@ obj-$(CONFIG_MEMFD_CREATE) += memfd.o obj-$(CONFIG_MAPPING_DIRTY_HELPERS) += mapping_dirty_helpers.o obj-$(CONFIG_PTDUMP_CORE) += ptdump.o obj-$(CONFIG_PAGE_REPORTING) += page_reporting.o +obj-$(CONFIG_SECRETMEM) += secretmem.o diff --git a/mm/secretmem.c b/mm/secretmem.c new file mode 100644 index 000000000000..3293f761076e --- /dev/null +++ b/mm/secretmem.c @@ -0,0 +1,264 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * Copyright IBM Corporation, 2020 + * + * Author: Mike Rapoport <rppt@linux.ibm.com> + */ + +#include <linux/mm.h> +#include <linux/fs.h> +#include <linux/mount.h> +#include <linux/memfd.h> +#include <linux/bitops.h> +#include <linux/printk.h> +#include <linux/pagemap.h> +#include <linux/syscalls.h> +#include <linux/pseudo_fs.h> +#include <linux/set_memory.h> +#include <linux/sched/signal.h> + +#include <uapi/linux/secretmem.h> +#include <uapi/linux/magic.h> + +#include <asm/tlbflush.h> + +#include "internal.h" + +#undef pr_fmt +#define pr_fmt(fmt) "secretmem: " fmt + +/* + * Secret memory areas are always exclusive to owning mm and they are + * removed from the direct map. + */ +#ifdef CONFIG_HAVE_SECRETMEM_UNCACHED +#define SECRETMEM_MODE_MASK (SECRETMEM_UNCACHED) +#else +#define SECRETMEM_MODE_MASK (0x0) +#endif + +#define SECRETMEM_FLAGS_MASK SECRETMEM_MODE_MASK + +struct secretmem_ctx { + unsigned int mode; +}; + +static struct page *secretmem_alloc_page(gfp_t gfp) +{ + /* + * FIXME: use a cache of large pages to reduce the direct map + * fragmentation + */ + return alloc_page(gfp); +} + +static vm_fault_t secretmem_fault(struct vm_fault *vmf) +{ + struct address_space *mapping = vmf->vma->vm_file->f_mapping; + struct inode *inode = file_inode(vmf->vma->vm_file); + pgoff_t offset = vmf->pgoff; + unsigned long addr; + struct page *page; + int ret = 0; + + if (((loff_t)vmf->pgoff << PAGE_SHIFT) >= i_size_read(inode)) + return vmf_error(-EINVAL); + + page = find_get_entry(mapping, offset); + if (!page) { + page = secretmem_alloc_page(vmf->gfp_mask); + if (!page) + return vmf_error(-ENOMEM); + + ret = add_to_page_cache(page, mapping, offset, vmf->gfp_mask); + if (unlikely(ret)) + goto err_put_page; + + ret = set_direct_map_invalid_noflush(page); + if (ret) + goto err_del_page_cache; + + addr = (unsigned long)page_address(page); + flush_tlb_kernel_range(addr, addr + PAGE_SIZE); + + __SetPageUptodate(page); + + ret = VM_FAULT_LOCKED; + } + + vmf->page = page; + return ret; + +err_del_page_cache: + delete_from_page_cache(page); +err_put_page: + put_page(page); + return vmf_error(ret); +} + +static const struct vm_operations_struct secretmem_vm_ops = { + .fault = secretmem_fault, +}; + +static int secretmem_mmap(struct file *file, struct vm_area_struct *vma) +{ + struct secretmem_ctx *ctx = file->private_data; + unsigned long len = vma->vm_end - vma->vm_start; + + if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0) + return -EINVAL; + + if (mlock_future_check(vma->vm_mm, vma->vm_flags | VM_LOCKED, len)) + return -EAGAIN; + + if (ctx->mode & SECRETMEM_UNCACHED) + vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot); + + vma->vm_ops = &secretmem_vm_ops; + vma->vm_flags |= VM_LOCKED; + + return 0; +} + +const struct file_operations secretmem_fops = { + .mmap = secretmem_mmap, +}; + +static bool secretmem_isolate_page(struct page *page, isolate_mode_t mode) +{ + return false; +} + +static int secretmem_migratepage(struct address_space *mapping, + struct page *newpage, struct page *page, + enum migrate_mode mode) +{ + return -EBUSY; +} + +static void secretmem_freepage(struct page *page) +{ + set_direct_map_default_noflush(page); +} + +static const struct address_space_operations secretmem_aops = { + .freepage = secretmem_freepage, + .migratepage = secretmem_migratepage, + .isolate_page = secretmem_isolate_page, +}; + +static struct vfsmount *secretmem_mnt; + +static struct file *secretmem_file_create(unsigned long flags) +{ + struct file *file = ERR_PTR(-ENOMEM); + struct secretmem_ctx *ctx; + struct inode *inode; + + inode = alloc_anon_inode(secretmem_mnt->mnt_sb); + if (IS_ERR(inode)) + return ERR_CAST(inode); + + ctx = kzalloc(sizeof(*ctx), GFP_KERNEL); + if (!ctx) + goto err_free_inode; + + file = alloc_file_pseudo(inode, secretmem_mnt, "secretmem", + O_RDWR, &secretmem_fops); + if (IS_ERR(file)) + goto err_free_ctx; + + mapping_set_unevictable(inode->i_mapping); + + inode->i_mapping->private_data = ctx; + inode->i_mapping->a_ops = &secretmem_aops; + + /* pretend we are a normal file with zero size */ + inode->i_mode |= S_IFREG; + inode->i_size = 0; + + file->private_data = ctx; + + ctx->mode = flags & SECRETMEM_MODE_MASK; + + return file; + +err_free_ctx: + kfree(ctx); +err_free_inode: + iput(inode); + return file; +} + +SYSCALL_DEFINE1(memfd_secret, unsigned long, flags) +{ + struct file *file; + int fd, err; + + /* make sure local flags do not confict with global fcntl.h */ + BUILD_BUG_ON(SECRETMEM_FLAGS_MASK & O_CLOEXEC); + + if (flags & ~(SECRETMEM_FLAGS_MASK | O_CLOEXEC)) + return -EINVAL; + + fd = get_unused_fd_flags(flags & O_CLOEXEC); + if (fd < 0) + return fd; + + file = secretmem_file_create(flags); + if (IS_ERR(file)) { + err = PTR_ERR(file); + goto err_put_fd; + } + + file->f_flags |= O_LARGEFILE; + + fd_install(fd, file); + return fd; + +err_put_fd: + put_unused_fd(fd); + return err; +} + +static void secretmem_evict_inode(struct inode *inode) +{ + struct secretmem_ctx *ctx = inode->i_private; + + truncate_inode_pages_final(&inode->i_data); + clear_inode(inode); + kfree(ctx); +} + +static const struct super_operations secretmem_super_ops = { + .evict_inode = secretmem_evict_inode, +}; + +static int secretmem_init_fs_context(struct fs_context *fc) +{ + struct pseudo_fs_context *ctx = init_pseudo(fc, SECRETMEM_MAGIC); + + if (!ctx) + return -ENOMEM; + ctx->ops = &secretmem_super_ops; + + return 0; +} + +static struct file_system_type secretmem_fs = { + .name = "secretmem", + .init_fs_context = secretmem_init_fs_context, + .kill_sb = kill_anon_super, +}; + +static int secretmem_init(void) +{ + int ret = 0; + + secretmem_mnt = kern_mount(&secretmem_fs); + if (IS_ERR(secretmem_mnt)) + ret = PTR_ERR(secretmem_mnt); + + return ret; +} +fs_initcall(secretmem_init);