On Fri, Jan 13, 2023 at 09:54:41PM +0000, Sean Christopherson wrote: > On Fri, Dec 02, 2022, Chao Peng wrote: > > The system call is currently wired up for x86 arch. > > Building on other architectures (except for arm64 for some reason) yields: > > CALL /.../scripts/checksyscalls.sh > <stdin>:1565:2: warning: #warning syscall memfd_restricted not implemented [-Wcpp] > > Do we care? It's the only such warning, which makes me think we either need to > wire this up for all architectures, or explicitly document that it's unsupported. I'm a bit conservative and prefer enabling only on x86 where we know the exact usecase. For the warning we can get rid of by changing scripts/checksyscalls.sh, just like __IGNORE_memfd_secret: https://lkml.kernel.org/r/20210518072034.31572-7-rppt@xxxxxxxxxx > > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> > > Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx> > > --- > > ... > > > diff --git a/include/linux/restrictedmem.h b/include/linux/restrictedmem.h > > new file mode 100644 > > index 000000000000..c2700c5daa43 > > --- /dev/null > > +++ b/include/linux/restrictedmem.h > > @@ -0,0 +1,71 @@ > > +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */ > > +#ifndef _LINUX_RESTRICTEDMEM_H > > Missing > > #define _LINUX_RESTRICTEDMEM_H > > which causes fireworks if restrictedmem.h is included more than once. > > > +#include <linux/file.h> > > +#include <linux/magic.h> > > +#include <linux/pfn_t.h> > > ... > > > +static inline int restrictedmem_get_page(struct file *file, pgoff_t offset, > > + struct page **pagep, int *order) > > +{ > > + return -1; > > This should be a proper -errno, though in the current incarnation of things it's > a moot point because no stub is needed. KVM can (and should) easily provide its > own stub for this one. > > > +} > > + > > +static inline bool file_is_restrictedmem(struct file *file) > > +{ > > + return false; > > +} > > + > > +static inline void restrictedmem_error_page(struct page *page, > > + struct address_space *mapping) > > +{ > > +} > > + > > +#endif /* CONFIG_RESTRICTEDMEM */ > > + > > +#endif /* _LINUX_RESTRICTEDMEM_H */ > > ... > > > diff --git a/mm/restrictedmem.c b/mm/restrictedmem.c > > new file mode 100644 > > index 000000000000..56953c204e5c > > --- /dev/null > > +++ b/mm/restrictedmem.c > > @@ -0,0 +1,318 @@ > > +// SPDX-License-Identifier: GPL-2.0 > > +#include "linux/sbitmap.h" > > +#include <linux/pagemap.h> > > +#include <linux/pseudo_fs.h> > > +#include <linux/shmem_fs.h> > > +#include <linux/syscalls.h> > > +#include <uapi/linux/falloc.h> > > +#include <uapi/linux/magic.h> > > +#include <linux/restrictedmem.h> > > + > > +struct restrictedmem_data { > > Any objection to simply calling this "restrictedmem"? And then using either "rm" > or "rmem" for local variable names? I kept reading "data" as the underyling data > being written to the page, as opposed to the metadata describing the restrictedmem > instance. > > > + struct mutex lock; > > + struct file *memfd; > > + struct list_head notifiers; > > +}; > > + > > +static void restrictedmem_invalidate_start(struct restrictedmem_data *data, > > + pgoff_t start, pgoff_t end) > > +{ > > + struct restrictedmem_notifier *notifier; > > + > > + mutex_lock(&data->lock); > > This can be a r/w semaphore instead of a mutex, that way punching holes at multiple > points in the file can at least run the notifiers in parallel. The actual allocation > by shmem will still be serialized, but I think it's worth the simple optimization > since zapping and flushing in KVM may be somewhat slow. > > > + list_for_each_entry(notifier, &data->notifiers, list) { > > + notifier->ops->invalidate_start(notifier, start, end); > > Two major design issues that we overlooked long ago: > > 1. Blindly invoking notifiers will not scale. E.g. if userspace configures a > VM with a large number of convertible memslots that are all backed by a > single large restrictedmem instance, then converting a single page will > result in a linear walk through all memslots. I don't expect anyone to > actually do something silly like that, but I also never expected there to be > a legitimate usecase for thousands of memslots. > > 2. This approach fails to provide the ability for KVM to ensure a guest has > exclusive access to a page. As discussed in the past, the kernel can rely > on hardware (and maybe ARM's pKVM implementation?) for those guarantees, but > only for SNP and TDX VMs. For VMs where userspace is trusted to some extent, > e.g. SEV, there is value in ensuring a 1:1 association. > > And probably more importantly, relying on hardware for SNP and TDX yields a > poor ABI and complicates KVM's internals. If the kernel doesn't guarantee a > page is exclusive to a guest, i.e. if userspace can hand out the same page > from a restrictedmem instance to multiple VMs, then failure will occur only > when KVM tries to assign the page to the second VM. That will happen deep > in KVM, which means KVM needs to gracefully handle such errors, and it means > that KVM's ABI effectively allows plumbing garbage into its memslots. It may not be a valid usage, but in my TDX environment I do meet below issue. kvm_set_user_memory AddrSpace#0 Slot#0 flags=0x4 gpa=0x0 size=0x80000000 ua=0x7fe1ebfff000 ret=0 kvm_set_user_memory AddrSpace#0 Slot#1 flags=0x4 gpa=0xffc00000 size=0x400000 ua=0x7fe271579000 ret=0 kvm_set_user_memory AddrSpace#0 Slot#2 flags=0x4 gpa=0xfeda0000 size=0x20000 ua=0x7fe1ec09f000 ret=-22 Slot#2('SMRAM') is actually an alias into system memory(Slot#0) in QEMU and slot#2 fails due to below exclusive check. Currently I changed QEMU code to mark these alias slots as shared instead of private but I'm not 100% confident this is correct fix. > > Rather than use a simple list of notifiers, this appears to be yet another > opportunity to use an xarray. Supporting sharing of restrictedmem will be > non-trivial, but IMO we should punt that to the future since it's still unclear > exactly how sharing will work. > > An xarray will solve #1 by notifying only the consumers (memslots) that are bound > to the affected range. > > And for #2, it's relatively straightforward (knock wood) to detect existing > entries, i.e. if the user wants exclusive access to memory, then the bind operation > can be reject if there's an existing entry. > > VERY lightly tested code snippet at the bottom (will provide link to fully worked > code in cover letter). > > > > +static long restrictedmem_punch_hole(struct restrictedmem_data *data, int mode, > > + loff_t offset, loff_t len) > > +{ > > + int ret; > > + pgoff_t start, end; > > + struct file *memfd = data->memfd; > > + > > + if (!PAGE_ALIGNED(offset) || !PAGE_ALIGNED(len)) > > + return -EINVAL; > > + > > + start = offset >> PAGE_SHIFT; > > + end = (offset + len) >> PAGE_SHIFT; > > + > > + restrictedmem_invalidate_start(data, start, end); > > + ret = memfd->f_op->fallocate(memfd, mode, offset, len); > > + restrictedmem_invalidate_end(data, start, end); > > The lock needs to be end for the entire duration of the hole punch, i.e. needs to > be taken before invalidate_start() and released after invalidate_end(). If a user > (un)binds/(un)registers after invalidate_state(), it will see an unpaired notification, > e.g. could leave KVM with incorrect notifier counts. > > > + > > + return ret; > > +} > > What I ended up with for an xarray-based implementation. I'm very flexible on > names and whatnot, these are just what made sense to me. > > static long restrictedmem_punch_hole(struct restrictedmem *rm, int mode, > loff_t offset, loff_t len) > { > struct restrictedmem_notifier *notifier; > struct file *memfd = rm->memfd; > unsigned long index; > pgoff_t start, end; > int ret; > > if (!PAGE_ALIGNED(offset) || !PAGE_ALIGNED(len)) > return -EINVAL; > > start = offset >> PAGE_SHIFT; > end = (offset + len) >> PAGE_SHIFT; > > /* > * Bindings must stable across invalidation to ensure the start+end > * are balanced. > */ > down_read(&rm->lock); > > xa_for_each_range(&rm->bindings, index, notifier, start, end) > notifier->ops->invalidate_start(notifier, start, end); > > ret = memfd->f_op->fallocate(memfd, mode, offset, len); > > xa_for_each_range(&rm->bindings, index, notifier, start, end) > notifier->ops->invalidate_end(notifier, start, end); > > up_read(&rm->lock); > > return ret; > } > > int restrictedmem_bind(struct file *file, pgoff_t start, pgoff_t end, > struct restrictedmem_notifier *notifier, bool exclusive) > { > struct restrictedmem *rm = file->f_mapping->private_data; > int ret = -EINVAL; > > down_write(&rm->lock); > > /* Non-exclusive mappings are not yet implemented. */ > if (!exclusive) > goto out_unlock; > > if (!xa_empty(&rm->bindings)) { > if (exclusive != rm->exclusive) > goto out_unlock; > > if (exclusive && xa_find(&rm->bindings, &start, end, XA_PRESENT)) > goto out_unlock; > } > > xa_store_range(&rm->bindings, start, end, notifier, GFP_KERNEL); > rm->exclusive = exclusive; > ret = 0; > out_unlock: > up_write(&rm->lock); > return ret; > } > EXPORT_SYMBOL_GPL(restrictedmem_bind); > > void restrictedmem_unbind(struct file *file, pgoff_t start, pgoff_t end, > struct restrictedmem_notifier *notifier) > { > struct restrictedmem *rm = file->f_mapping->private_data; > > down_write(&rm->lock); > xa_store_range(&rm->bindings, start, end, NULL, GFP_KERNEL); > synchronize_rcu(); > up_write(&rm->lock); > } > EXPORT_SYMBOL_GPL(restrictedmem_unbind);
