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mm: enable maple tree RCU mode by default. 

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
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
include/linux/mm_types.h
View File

@ -725,7 +725,8 @@ struct mm_struct {
unsigned long cpu_bitmap[]; 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; extern struct mm_struct init_mm;
/* Pointer magic because the dynamic array size confuses some compilers. */ /* 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) if (retval)
goto out; goto out;
mt_clear_in_rcu(mas.tree);
mas_for_each(&old_mas, mpnt, ULONG_MAX) { mas_for_each(&old_mas, mpnt, ULONG_MAX) {
struct file *file; struct file *file;
@ -703,6 +704,8 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
retval = arch_dup_mmap(oldmm, mm); retval = arch_dup_mmap(oldmm, mm);
loop_out: loop_out:
mas_destroy(&mas); mas_destroy(&mas);
if (!retval)
mt_set_in_rcu(mas.tree);
out: out:
mmap_write_unlock(mm); mmap_write_unlock(mm);
flush_tlb_mm(oldmm); 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 count = 0;
int error = -ENOMEM; int error = -ENOMEM;
MA_STATE(mas_detach, &mt_detach, 0, 0); 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); mt_set_external_lock(&mt_detach, &mm->mmap_lock);
if (mas_preallocate(mas, vma, GFP_KERNEL)) 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); set_bit(MMF_OOM_SKIP, &mm->flags);
mmap_write_lock(mm); mmap_write_lock(mm);
mt_clear_in_rcu(&mm->mm_mt);
free_pgtables(&tlb, &mm->mm_mt, vma, FIRST_USER_ADDRESS, free_pgtables(&tlb, &mm->mm_mt, vma, FIRST_USER_ADDRESS,
USER_PGTABLES_CEILING); USER_PGTABLES_CEILING);
tlb_finish_mmu(&tlb); tlb_finish_mmu(&tlb);