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aio: fix race in ring buffer page lookup introduced by page migration support

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Prior to the introduction of page migration support in "fs/aio: Add support
to aio ring pages migration" / 36bc08cc01,
mapping of the ring buffer pages was done via get_user_pages() while
retaining mmap_sem held for write.  This avoided possible races with userland
racing an munmap() or mremap().  The page migration patch, however, switched
to using mm_populate() to prime the page mapping.  mm_populate() cannot be
called with mmap_sem held.

Instead of dropping the mmap_sem, revert to the old behaviour and simply
drop the use of mm_populate() since get_user_pages() will cause the pages to
get mapped anyways.  Thanks to Al Viro for spotting this issue.

Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
This commit is contained in:
Benjamin LaHaise 2013-09-09 11:57:59 -04:00
parent 77d30b14d2
commit d6c355c7da
1 changed files with 12 additions and 3 deletions
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15
fs/aio.c
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@ -307,16 +307,25 @@ static int aio_setup_ring(struct kioctx *ctx)
aio_free_ring(ctx);
return -EAGAIN;
}
up_write(&mm->mmap_sem);
mm_populate(ctx->mmap_base, populate);
pr_debug("mmap address: 0x%08lx\n", ctx->mmap_base);
/* We must do this while still holding mmap_sem for write, as we
* need to be protected against userspace attempting to mremap()
* or munmap() the ring buffer.
*/
ctx->nr_pages = get_user_pages(current, mm, ctx->mmap_base, nr_pages,
1, 0, ctx->ring_pages, NULL);
/* Dropping the reference here is safe as the page cache will hold
* onto the pages for us. It is also required so that page migration
* can unmap the pages and get the right reference count.
*/
for (i = 0; i < ctx->nr_pages; i++)
put_page(ctx->ring_pages[i]);
up_write(&mm->mmap_sem);
if (unlikely(ctx->nr_pages != nr_pages)) {
aio_free_ring(ctx);
return -EAGAIN;