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authorLorenzo Stoakes2023-05-04 22:27:53 +0100
committerAndrew Morton2023-06-09 16:25:57 -0700
commita6e79df92e4a371ba7b751a20e3a31fde0f528b8 (patch)
tree2556f57cea3ed09fdb14e22b7bf7e866bf1b08d2 /mm/gup.c
parent8ac268436e6d52f3bb4d212df9395aec97afaa00 (diff)
mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings
Writing to file-backed dirty-tracked mappings via GUP is inherently broken as we cannot rule out folios being cleaned and then a GUP user writing to them again and possibly marking them dirty unexpectedly. This is especially egregious for long-term mappings (as indicated by the use of the FOLL_LONGTERM flag), so we disallow this case in GUP-fast as we have already done in the slow path. We have access to less information in the fast path as we cannot examine the VMA containing the mapping, however we can determine whether the folio is anonymous or belonging to a whitelisted filesystem - specifically hugetlb and shmem mappings. We take special care to ensure that both the folio and mapping are safe to access when performing these checks and document folio_fast_pin_allowed() accordingly. It's important to note that there are no APIs allowing users to specify FOLL_FAST_ONLY for a PUP-fast let alone with FOLL_LONGTERM, so we can always rely on the fact that if we fail to pin on the fast path, the code will fall back to the slow path which can perform the more thorough check. Link: https://lkml.kernel.org/r/a27d39b87ded7f3dad5fd4181edb106393660453.1683235180.git.lstoakes@gmail.com Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com> Suggested-by: David Hildenbrand <david@redhat.com> Suggested-by: Kirill A . Shutemov <kirill@shutemov.name> Suggested-by: Peter Zijlstra <peterz@infradead.org> Reviewed-by: Jan Kara <jack@suse.cz> Acked-by: David Hildenbrand <david@redhat.com> Cc: Jason Gunthorpe <jgg@nvidia.com> Cc: John Hubbard <jhubbard@nvidia.com> Cc: Mika Penttilä <mpenttil@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'mm/gup.c')
-rw-r--r--mm/gup.c101
1 files changed, 101 insertions, 0 deletions
diff --git a/mm/gup.c b/mm/gup.c
index 67999dd43d83..a718b956edbe 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -18,6 +18,7 @@
#include <linux/migrate.h>
#include <linux/mm_inline.h>
#include <linux/sched/mm.h>
+#include <linux/shmem_fs.h>
#include <asm/mmu_context.h>
#include <asm/tlbflush.h>
@@ -2351,6 +2352,82 @@ EXPORT_SYMBOL(get_user_pages_unlocked);
*/
#ifdef CONFIG_HAVE_FAST_GUP
+/*
+ * Used in the GUP-fast path to determine whether a pin is permitted for a
+ * specific folio.
+ *
+ * This call assumes the caller has pinned the folio, that the lowest page table
+ * level still points to this folio, and that interrupts have been disabled.
+ *
+ * Writing to pinned file-backed dirty tracked folios is inherently problematic
+ * (see comment describing the writable_file_mapping_allowed() function). We
+ * therefore try to avoid the most egregious case of a long-term mapping doing
+ * so.
+ *
+ * This function cannot be as thorough as that one as the VMA is not available
+ * in the fast path, so instead we whitelist known good cases and if in doubt,
+ * fall back to the slow path.
+ */
+static bool folio_fast_pin_allowed(struct folio *folio, unsigned int flags)
+{
+ struct address_space *mapping;
+ unsigned long mapping_flags;
+
+ /*
+ * If we aren't pinning then no problematic write can occur. A long term
+ * pin is the most egregious case so this is the one we disallow.
+ */
+ if ((flags & (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE)) !=
+ (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE))
+ return true;
+
+ /* The folio is pinned, so we can safely access folio fields. */
+
+ if (WARN_ON_ONCE(folio_test_slab(folio)))
+ return false;
+
+ /* hugetlb mappings do not require dirty-tracking. */
+ if (folio_test_hugetlb(folio))
+ return true;
+
+ /*
+ * GUP-fast disables IRQs. When IRQS are disabled, RCU grace periods
+ * cannot proceed, which means no actions performed under RCU can
+ * proceed either.
+ *
+ * inodes and thus their mappings are freed under RCU, which means the
+ * mapping cannot be freed beneath us and thus we can safely dereference
+ * it.
+ */
+ lockdep_assert_irqs_disabled();
+
+ /*
+ * However, there may be operations which _alter_ the mapping, so ensure
+ * we read it once and only once.
+ */
+ mapping = READ_ONCE(folio->mapping);
+
+ /*
+ * The mapping may have been truncated, in any case we cannot determine
+ * if this mapping is safe - fall back to slow path to determine how to
+ * proceed.
+ */
+ if (!mapping)
+ return false;
+
+ /* Anonymous folios pose no problem. */
+ mapping_flags = (unsigned long)mapping & PAGE_MAPPING_FLAGS;
+ if (mapping_flags)
+ return mapping_flags & PAGE_MAPPING_ANON;
+
+ /*
+ * At this point, we know the mapping is non-null and points to an
+ * address_space object. The only remaining whitelisted file system is
+ * shmem.
+ */
+ return shmem_mapping(mapping);
+}
+
static void __maybe_unused undo_dev_pagemap(int *nr, int nr_start,
unsigned int flags,
struct page **pages)
@@ -2436,6 +2513,11 @@ static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
goto pte_unmap;
}
+ if (!folio_fast_pin_allowed(folio, flags)) {
+ gup_put_folio(folio, 1, flags);
+ goto pte_unmap;
+ }
+
if (!pte_write(pte) && gup_must_unshare(NULL, flags, page)) {
gup_put_folio(folio, 1, flags);
goto pte_unmap;
@@ -2628,6 +2710,11 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
return 0;
}
+ if (!folio_fast_pin_allowed(folio, flags)) {
+ gup_put_folio(folio, refs, flags);
+ return 0;
+ }
+
if (!pte_write(pte) && gup_must_unshare(NULL, flags, &folio->page)) {
gup_put_folio(folio, refs, flags);
return 0;
@@ -2694,6 +2781,10 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
return 0;
}
+ if (!folio_fast_pin_allowed(folio, flags)) {
+ gup_put_folio(folio, refs, flags);
+ return 0;
+ }
if (!pmd_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) {
gup_put_folio(folio, refs, flags);
return 0;
@@ -2734,6 +2825,11 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
return 0;
}
+ if (!folio_fast_pin_allowed(folio, flags)) {
+ gup_put_folio(folio, refs, flags);
+ return 0;
+ }
+
if (!pud_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) {
gup_put_folio(folio, refs, flags);
return 0;
@@ -2774,6 +2870,11 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
return 0;
}
+ if (!folio_fast_pin_allowed(folio, flags)) {
+ gup_put_folio(folio, refs, flags);
+ return 0;
+ }
+
*nr += refs;
folio_set_referenced(folio);
return 1;