Andres L-C has pointed out that the single-page unmap_mapping_range()
fallback in truncate_inode_page() cannot protect against the case when
a huge page was faulted in after the full-range unmap_mapping_range():
because page_mapped(page) checks tail page's mapcount, not the head's.
So, there's a danger that hole-punching (and maybe even truncation)
can free pages while they are mapped into userspace with a huge pmd.
And I don't believe that the CVE-2014-4171 protection in shmem_fault()
can fully protect from this, although it does make it much harder.
Fix that by adding a duplicate single-page unmap_mapping_range()
into shmem_disband_hugeteam() (called when punching or truncating
a PageTeam), at the point when we also hold the head's page lock
(without which there would still be races): which will then split
all huge pmd mappings covering the page into team pte mappings.
This is also just what's needed to handle memory_failure() correctly:
provide custom shmem_error_remove_page(), call shmem_disband_hugeteam()
from that before proceeding to generic_error_remove_page(), then this
additional unmap_mapping_range() will remap team by ptes as needed.
(There is an unlikely case that we're racing with another disbander,
or disband didn't get trylock on head page at first: memory_failure()
has almost finished with the page, so it's safe to unlock and relock
before retrying.)
But there is one further change needed in hwpoison_user_mappings():
it must recognize a hugely mapped team before concluding that the
page is not mapped. (And still no support for soft_offline(),
which will have to wait for page migration of teams.)
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
---
mm/memory-failure.c | 7 ++++++-
mm/shmem.c | 30 +++++++++++++++++++++++++++++-
2 files changed, 35 insertions(+), 2 deletions(-)
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -45,6 +45,7 @@
#include <linux/rmap.h>
#include <linux/export.h>
#include <linux/pagemap.h>
+#include <linux/pageteam.h>
#include <linux/swap.h>
#include <linux/backing-dev.h>
#include <linux/migrate.h>
@@ -902,6 +903,7 @@ static int hwpoison_user_mappings(struct
enum ttu_flags ttu = TTU_UNMAP | TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS;
struct address_space *mapping;
LIST_HEAD(tokill);
+ bool mapped;
int ret;
int kill = 1, forcekill;
struct page *hpage = *hpagep;
@@ -919,7 +921,10 @@ static int hwpoison_user_mappings(struct
* This check implies we don't kill processes if their pages
* are in the swap cache early. Those are always late kills.
*/
- if (!page_mapped(hpage))
+ mapped = page_mapped(hpage);
+ if (PageTeam(p) && team_pmd_mapped(team_head(p)))
+ mapped = true;
+ if (!mapped)
return SWAP_SUCCESS;
if (PageKsm(p)) {
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -605,6 +605,19 @@ static void shmem_disband_hugeteam(struc
}
/*
+ * truncate_inode_page() will unmap page if page_mapped(page),
+ * but there's a race by which the team could be hugely mapped,
+ * with page_mapped(page) saying false. So check here if the
+ * head is hugely mapped, and if so unmap page to remap team.
+ * Use a loop because there is no good locking against a
+ * concurrent remap_team_by_ptes().
+ */
+ while (team_pmd_mapped(head)) {
+ unmap_mapping_range(page->mapping,
+ (loff_t)page->index << PAGE_SHIFT, PAGE_SIZE, 0);
+ }
+
+ /*
* Disable preemption because truncation may end up spinning until a
* tail PageTeam has been cleared: we hold the lock as briefly as we
* can (splitting disband in two stages), but better not be preempted.
@@ -1305,6 +1318,21 @@ static int shmem_getattr(struct vfsmount
return 0;
}
+static int shmem_error_remove_page(struct address_space *mapping,
+ struct page *page)
+{
+ if (PageTeam(page)) {
+ shmem_disband_hugeteam(page);
+ while (unlikely(PageTeam(page))) {
+ unlock_page(page);
+ cond_resched();
+ lock_page(page);
+ shmem_disband_hugeteam(page);
+ }
+ }
+ return generic_error_remove_page(mapping, page);
+}
+
static int shmem_setattr(struct dentry *dentry, struct iattr *attr)
{
struct inode *inode = d_inode(dentry);
@@ -4088,7 +4116,7 @@ static const struct address_space_operat
#ifdef CONFIG_MIGRATION
.migratepage = migrate_page,
#endif
- .error_remove_page = generic_error_remove_page,
+ .error_remove_page = shmem_error_remove_page,
};
static const struct file_operations shmem_file_operations = {
--
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