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userfaultfd: provide unmasked address on page-fault

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Userfaultfd is supposed to provide the full address (i.e., unmasked) of
the faulting access back to userspace.  However, that is not the case for
quite some time.

Even running "userfaultfd_demo" from the userfaultfd man page provides the
wrong output (and contradicts the man page).  Notice that
"UFFD_EVENT_PAGEFAULT event" shows the masked address (7fc5e30b3000) and
not the first read address (0x7fc5e30b300f).

	Address returned by mmap() = 0x7fc5e30b3000

	fault_handler_thread():
	    poll() returns: nready = 1; POLLIN = 1; POLLERR = 0
	    UFFD_EVENT_PAGEFAULT event: flags = 0; address = 7fc5e30b3000
		(uffdio_copy.copy returned 4096)
	Read address 0x7fc5e30b300f in main(): A
	Read address 0x7fc5e30b340f in main(): A
	Read address 0x7fc5e30b380f in main(): A
	Read address 0x7fc5e30b3c0f in main(): A

The exact address is useful for various reasons and specifically for
prefetching decisions.  If it is known that the memory is populated by
certain objects whose size is not page-aligned, then based on the faulting
address, the uffd-monitor can decide whether to prefetch and prefault the
adjacent page.

This bug has been for quite some time in the kernel: since commit
1a29d85eb0 ("mm: use vmf->address instead of of vmf->virtual_address")
vmf->virtual_address"), which dates back to 2016.  A concern has been
raised that existing userspace application might rely on the old/wrong
behavior in which the address is masked.  Therefore, it was suggested to
provide the masked address unless the user explicitly asks for the exact
address.

Add a new userfaultfd feature UFFD_FEATURE_EXACT_ADDRESS to direct
userfaultfd to provide the exact address.  Add a new "real_address" field
to vmf to hold the unmasked address.  Provide the address to userspace
accordingly.

Initialize real_address in various code-paths to be consistent with
address, even when it is not used, to be on the safe side.

[namit@vmware.com: initialize real_address on all code paths, per Jan]
  Link: https://lkml.kernel.org/r/20220226022655.350562-1-namit@vmware.com
[akpm@linux-foundation.org: fix typo in comment, per Jan]

Link: https://lkml.kernel.org/r/20220218041003.3508-1-namit@vmware.com
Signed-off-by: Nadav Amit <namit@vmware.com>
Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Nadav Amit 2022-03-22 14:45:32 -07:00 committed by Linus Torvalds
parent 87d2762e22
commit 824ddc601a
6 changed files with 19 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
fs/userfaultfd.c
View File

@ -198,6 +198,9 @@ static inline struct uffd_msg userfault_msg(unsigned long address,
struct uffd_msg msg;
msg_init(&msg);
msg.event = UFFD_EVENT_PAGEFAULT;
if (!(features & UFFD_FEATURE_EXACT_ADDRESS))
address &= PAGE_MASK;
msg.arg.pagefault.address = address;
/*
* These flags indicate why the userfault occurred:
@ -482,7 +485,7 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
uwq.wq.private = current;
uwq.msg = userfault_msg(vmf->address, vmf->flags, reason,
uwq.msg = userfault_msg(vmf->real_address, vmf->flags, reason,
ctx->features);
uwq.ctx = ctx;
uwq.waken = false;

3
include/linux/mm.h
View File

@ -478,7 +478,8 @@ struct vm_fault {
struct vm_area_struct *vma; /* Target VMA */
gfp_t gfp_mask; /* gfp mask to be used for allocations */
pgoff_t pgoff; /* Logical page offset based on vma */
unsigned long address; /* Faulting virtual address */
unsigned long address; /* Faulting virtual address - masked */
unsigned long real_address; /* Faulting virtual address - unmasked */
};
enum fault_flag flags; /* FAULT_FLAG_xxx flags
* XXX: should really be 'const' */

8
include/uapi/linux/userfaultfd.h
View File

@ -32,7 +32,8 @@
UFFD_FEATURE_SIGBUS | \
UFFD_FEATURE_THREAD_ID | \
UFFD_FEATURE_MINOR_HUGETLBFS | \
UFFD_FEATURE_MINOR_SHMEM)
UFFD_FEATURE_MINOR_SHMEM | \
UFFD_FEATURE_EXACT_ADDRESS)
#define UFFD_API_IOCTLS \
((__u64)1 << _UFFDIO_REGISTER | \
(__u64)1 << _UFFDIO_UNREGISTER | \
@ -189,6 +190,10 @@ struct uffdio_api {
*
* UFFD_FEATURE_MINOR_SHMEM indicates the same support as
* UFFD_FEATURE_MINOR_HUGETLBFS, but for shmem-backed pages instead.
*
* UFFD_FEATURE_EXACT_ADDRESS indicates that the exact address of page
* faults would be provided and the offset within the page would not be
* masked.
*/
#define UFFD_FEATURE_PAGEFAULT_FLAG_WP (1<<0)
#define UFFD_FEATURE_EVENT_FORK (1<<1)
@ -201,6 +206,7 @@ struct uffdio_api {
#define UFFD_FEATURE_THREAD_ID (1<<8)
#define UFFD_FEATURE_MINOR_HUGETLBFS (1<<9)
#define UFFD_FEATURE_MINOR_SHMEM (1<<10)
#define UFFD_FEATURE_EXACT_ADDRESS (1<<11)
__u64 features;
__u64 ioctls;

6
mm/hugetlb.c
View File

@ -5341,6 +5341,7 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
pgoff_t idx,
unsigned int flags,
unsigned long haddr,
unsigned long addr,
unsigned long reason)
{
vm_fault_t ret;
@ -5348,6 +5349,7 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
struct vm_fault vmf = {
.vma = vma,
.address = haddr,
.real_address = addr,
.flags = flags,
/*
@ -5416,7 +5418,7 @@ retry:
/* Check for page in userfault range */
if (userfaultfd_missing(vma)) {
ret = hugetlb_handle_userfault(vma, mapping, idx,
flags, haddr,
flags, haddr, address,
VM_UFFD_MISSING);
goto out;
}
@ -5480,7 +5482,7 @@ retry:
unlock_page(page);
put_page(page);
ret = hugetlb_handle_userfault(vma, mapping, idx,
flags, haddr,
flags, haddr, address,
VM_UFFD_MINOR);
goto out;
}

1
mm/memory.c
View File

@ -4633,6 +4633,7 @@ static vm_fault_t __handle_mm_fault(struct vm_area_struct *vma,
struct vm_fault vmf = {
.vma = vma,
.address = address & PAGE_MASK,
.real_address = address,
.flags = flags,
.pgoff = linear_page_index(vma, address),
.gfp_mask = __get_fault_gfp_mask(vma),

1
mm/swapfile.c
View File

@ -1951,6 +1951,7 @@ static int unuse_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
struct vm_fault vmf = {
.vma = vma,
.address = addr,
.real_address = addr,
.pmd = pmd,
};