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mm: enable maple tree RCU mode by default.

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commit 3dd4432549 upstream.

Use the maple tree in RCU mode for VMA tracking.

The maple tree tracks the stack and is able to update the pivot
(lower/upper boundary) in-place to allow the page fault handler to write
to the tree while holding just the mmap read lock.  This is safe as the
writes to the stack have a guard VMA which ensures there will always be
a NULL in the direction of the growth and thus will only update a pivot.

It is possible, but not recommended, to have VMAs that grow up/down
without guard VMAs.  syzbot has constructed a testcase which sets up a
VMA to grow and consume the empty space.  Overwriting the entire NULL
entry causes the tree to be altered in a way that is not safe for
concurrent readers; the readers may see a node being rewritten or one
that does not match the maple state they are using.

Enabling RCU mode allows the concurrent readers to see a stable node and
will return the expected result.

Link: https://lkml.kernel.org/r/20230227173632.3292573-9-surenb@google.com
Cc: stable@vger.kernel.org
Fixes: d4af56c5c7 ("mm: start tracking VMAs with maple tree")
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reported-by: syzbot+8d95422d3537159ca390@syzkaller.appspotmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Liam R. Howlett 2023-04-11 11:10:55 -04:00 committed by Greg Kroah-Hartman
parent 9b6627bc36
commit 1c87a6f82a
3 changed files with 7 additions and 2 deletions
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3
include/linux/mm_types.h
View file

@ -725,7 +725,8 @@ struct mm_struct {
unsigned long cpu_bitmap[];
};
#define MM_MT_FLAGS (MT_FLAGS_ALLOC_RANGE | MT_FLAGS_LOCK_EXTERN)
#define MM_MT_FLAGS (MT_FLAGS_ALLOC_RANGE | MT_FLAGS_LOCK_EXTERN | \
MT_FLAGS_USE_RCU)
extern struct mm_struct init_mm;
/* Pointer magic because the dynamic array size confuses some compilers. */

3
kernel/fork.c
View file

@ -617,6 +617,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
if (retval)
goto out;
mt_clear_in_rcu(mas.tree);
mas_for_each(&old_mas, mpnt, ULONG_MAX) {
struct file *file;
@ -703,6 +704,8 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
retval = arch_dup_mmap(oldmm, mm);
loop_out:
mas_destroy(&mas);
if (!retval)
mt_set_in_rcu(mas.tree);
out:
mmap_write_unlock(mm);
flush_tlb_mm(oldmm);

3
mm/mmap.c
View file

@ -2308,7 +2308,7 @@ do_mas_align_munmap(struct ma_state *mas, struct vm_area_struct *vma,
int count = 0;
int error = -ENOMEM;
MA_STATE(mas_detach, &mt_detach, 0, 0);
mt_init_flags(&mt_detach, MT_FLAGS_LOCK_EXTERN);
mt_init_flags(&mt_detach, mas->tree->ma_flags & MT_FLAGS_LOCK_MASK);
mt_set_external_lock(&mt_detach, &mm->mmap_lock);
if (mas_preallocate(mas, vma, GFP_KERNEL))
@ -3095,6 +3095,7 @@ void exit_mmap(struct mm_struct *mm)
*/
set_bit(MMF_OOM_SKIP, &mm->flags);
mmap_write_lock(mm);
mt_clear_in_rcu(&mm->mm_mt);
free_pgtables(&tlb, &mm->mm_mt, vma, FIRST_USER_ADDRESS,
USER_PGTABLES_CEILING);
tlb_finish_mmu(&tlb);