[PATCH 5.10 447/717] mm/gup: prevent gup_fast from racing with COW during fork

Greg Kroah-Hartman
2020-12-28T14:49:04Z

From: Jason Gunthorpe <jgg@nvidia.com>

[ Upstream commit 57efa1fe5957694fa541c9062de0a127f0b9acb0 ]

Since commit 70e806e4e645 ("mm: Do early cow for pinned pages during
fork() for ptes") pages under a FOLL_PIN will not be write protected
during COW for fork. This means that pages returned from
pin_user_pages(FOLL_WRITE) should not become write protected while the pin
is active.

However, there is a small race where get_user_pages_fast(FOLL_PIN) can
establish a FOLL_PIN at the same time copy_present_page() is write
protecting it:

CPU 0 CPU 1
get_user_pages_fast()
internal_get_user_pages_fast()
copy_page_range()
pte_alloc_map_lock()
copy_present_page()
atomic_read(has_pinned) == 0
page_maybe_dma_pinned() == false
atomic_set(has_pinned, 1);
gup_pgd_range()
gup_pte_range()
pte_t pte = gup_get_pte(ptep)
pte_access_permitted(pte)
try_grab_compound_head()
pte = pte_wrprotect(pte)
set_pte_at();
pte_unmap_unlock()
// GUP now returns with a write protected page

The first attempt to resolve this by using the write protect caused
problems (and was missing a barrrier), see commit f3c64eda3e50 ("mm: avoid
early COW write protect games during fork()")

Instead wrap copy_p4d_range() with the write side of a seqcount and check
the read side around gup_pgd_range(). If there is a collision then
get_user_pages_fast() fails and falls back to slow GUP.

Slow GUP is safe against this race because copy_page_range() is only
called while holding the exclusive side of the mmap_lock on the src
mm_struct.

[akpm@linux-foundation.org: coding style fixes]
Link: https://lore.kernel.org/r/CAHk-=wi=iCnYCARbPGjkVJu9eyYeZ13N64tZYLdOB8CP5Q_PLw@mail.gmail.com

Link: https://lkml.kernel.org/r/2-v4-908497cf359a+4782-gup_fork_jgg@nvidia.com
Fixes: f3c64eda3e50 ("mm: avoid early COW write protect games during fork()")
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Peter Xu <peterx@redhat.com>
Acked-by: "Ahmed S. Darwish" <a.darwish@linutronix.de> [seqcount_t parts]
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Kirill Shutemov <kirill@shutemov.name>
Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Leon Romanovsky <leonro@nvidia.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
arch/x86/kernel/tboot.c | 1 +
drivers/firmware/efi/efi.c | 1 +
include/linux/mm_types.h | 8 ++++++++
kernel/fork.c | 1 +
mm/gup.c | 18 ++++++++++++++++++
mm/init-mm.c | 1 +
mm/memory.c | 13 ++++++++++++-
7 files changed, 42 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/tboot.c b/arch/x86/kernel/tboot.c
index ae64f98ec2ab6..4c09ba1102047 100644
--- a/arch/x86/kernel/tboot.c
+++ b/arch/x86/kernel/tboot.c
@@ -93,6 +93,7 @@ static struct mm_struct tboot_mm = {
.pgd = swapper_pg_dir,
.mm_users = ATOMIC_INIT(2),
.mm_count = ATOMIC_INIT(1),
+ .write_protect_seq = SEQCNT_ZERO(tboot_mm.write_protect_seq),
MMAP_LOCK_INITIALIZER(init_mm)
.page_table_lock = __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
.mmlist = LIST_HEAD_INIT(init_mm.mmlist),
diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index 6c6eec044a978..df3f9bcab581c 100644
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -57,6 +57,7 @@ struct mm_struct efi_mm = {
.mm_rb = RB_ROOT,
.mm_users = ATOMIC_INIT(2),
.mm_count = ATOMIC_INIT(1),
+ .write_protect_seq = SEQCNT_ZERO(efi_mm.write_protect_seq),
MMAP_LOCK_INITIALIZER(efi_mm)
.page_table_lock = __SPIN_LOCK_UNLOCKED(efi_mm.page_table_lock),
.mmlist = LIST_HEAD_INIT(efi_mm.mmlist),
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 5a9238f6caad9..915f4f100383b 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -14,6 +14,7 @@
#include <linux/uprobes.h>
#include <linux/page-flags-layout.h>
#include <linux/workqueue.h>
+#include <linux/seqlock.h>

#include <asm/mmu.h>

@@ -446,6 +447,13 @@ struct mm_struct {
*/
atomic_t has_pinned;

+ /**
+ * @write_protect_seq: Locked when any thread is write
+ * protecting pages mapped by this mm to enforce a later COW,
+ * for instance during page table copying for fork().
+ */
+ seqcount_t write_protect_seq;
+
#ifdef CONFIG_MMU
atomic_long_t pgtables_bytes; /* PTE page table pages */
#endif
diff --git a/kernel/fork.c b/kernel/fork.c
index 6d266388d3804..dc55f68a6ee36 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1007,6 +1007,7 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p,
mm->vmacache_seqnum = 0;
atomic_set(&mm->mm_users, 1);
atomic_set(&mm->mm_count, 1);
+ seqcount_init(&mm->write_protect_seq);
mmap_init_lock(mm);
INIT_LIST_HEAD(&mm->mmlist);
mm->core_state = NULL;
diff --git a/mm/gup.c b/mm/gup.c
index c7e24301860ab..9c6a2f5001c5c 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -2684,11 +2684,18 @@ static unsigned long lockless_pages_from_mm(unsigned long start,
{
unsigned long flags;
int nr_pinned = 0;
+ unsigned seq;

if (!IS_ENABLED(CONFIG_HAVE_FAST_GUP) ||
!gup_fast_permitted(start, end))
return 0;

+ if (gup_flags & FOLL_PIN) {
+ seq = raw_read_seqcount(&current->mm->write_protect_seq);
+ if (seq & 1)
+ return 0;
+ }
+
/*
* Disable interrupts. The nested form is used, in order to allow full,
* general purpose use of this routine.
@@ -2703,6 +2710,17 @@ static unsigned long lockless_pages_from_mm(unsigned long start,
local_irq_save(flags);
gup_pgd_range(start, end, gup_flags, pages, &nr_pinned);
local_irq_restore(flags);
+
+ /*
+ * When pinning pages for DMA there could be a concurrent write protect
+ * from fork() via copy_page_range(), in this case always fail fast GUP.
+ */
+ if (gup_flags & FOLL_PIN) {
+ if (read_seqcount_retry(&current->mm->write_protect_seq, seq)) {
+ unpin_user_pages(pages, nr_pinned);
+ return 0;
+ }
+ }
return nr_pinned;
}

diff --git a/mm/init-mm.c b/mm/init-mm.c
index 3a613c85f9ede..153162669f806 100644
--- a/mm/init-mm.c
+++ b/mm/init-mm.c
@@ -31,6 +31,7 @@ struct mm_struct init_mm = {
.pgd = swapper_pg_dir,
.mm_users = ATOMIC_INIT(2),
.mm_count = ATOMIC_INIT(1),
+ .write_protect_seq = SEQCNT_ZERO(init_mm.write_protect_seq),
MMAP_LOCK_INITIALIZER(init_mm)
.page_table_lock = __SPIN_LOCK_UNLOCKED(init_mm.page_table_lock),
.arg_lock = __SPIN_LOCK_UNLOCKED(init_mm.arg_lock),
diff --git a/mm/memory.c b/mm/memory.c
index c48f8df6e5026..50632c4366b8a 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1171,6 +1171,15 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_PAGE,
0, src_vma, src_mm, addr, end);
mmu_notifier_invalidate_range_start(&range);
+ /*
+ * Disabling preemption is not needed for the write side, as
+ * the read side doesn't spin, but goes to the mmap_lock.
+ *
+ * Use the raw variant of the seqcount_t write API to avoid
+ * lockdep complaining about preemptibility.
+ */
+ mmap_assert_write_locked(src_mm);
+ raw_write_seqcount_begin(&src_mm->write_protect_seq);
}

ret = 0;
@@ -1187,8 +1196,10 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
}
} while (dst_pgd++, src_pgd++, addr = next, addr != end);

- if (is_cow)
+ if (is_cow) {
+ raw_write_seqcount_end(&src_mm->write_protect_seq);
mmu_notifier_invalidate_range_end(&range);
+ }
return ret;
}

--
2.27.0