Re: [PATCH] f2fs: fix use-after-free when accessing bio->bi_crypt_context

Sahitya Tummala
2020-06-15T09:53:31Z

Hi Satya,

On Mon, Jun 15, 2020 at 06:16:33AM +0000, Satya Tangirala wrote:
> On Sun, Jun 14, 2020 at 10:00:19PM -0700, Eric Biggers wrote:
> > On Mon, Jun 15, 2020 at 09:29:48AM +0530, Sahitya Tummala wrote:
> > > There could be a potential race between these two paths below,
> > > leading to use-after-free when accessing bio->bi_crypt_context.
> > >
> > > f2fs_write_cache_pages
> > > ->f2fs_do_write_data_page on page#1
> > > ->f2fs_inplace_write_data
> > > ->f2fs_merge_page_bio
> > > ->add_bio_entry
> > > ->f2fs_do_write_data_page on page#2
> > > ->f2fs_inplace_write_data
> > > ->f2fs_merge_page_bio
> > > ->f2fs_crypt_mergeable_bio
> > > ->fscrypt_mergeable_bio
> > > f2fs_write_begin on page#1
> > > ->f2fs_wait_on_page_writeback
> > > ->f2fs_submit_merged_ipu_write
> > > ->__submit_bio
> > > The bio gets completed, calling
> > > bio_endio
> > > ->bio_uninit
> > > ->bio_crypt_free_ctx
> > > ->use-after-free issue
> > >
> > > Fix this by moving f2fs_crypt_mergeable_bio() check within
> > > add_ipu_page() so that it's done under bio_list_lock to prevent
> > > the above race.
> > >
> > > Fixes: 15e76ad23e72 ("f2fs: add inline encryption support")
> > > Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
> > > ---
> > > This fix is rebased to the tip of fscrypt git -
> > > https://git.kernel.org/pub/scm/fs/fscrypt/fscrypt.git
> > > branch - inline-encryption
> > >
> > > fs/f2fs/data.c | 26 ++++++++++++++++++--------
> > > 1 file changed, 18 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > > index 0dfa8d3..3b53554 100644
> > > --- a/fs/f2fs/data.c
> > > +++ b/fs/f2fs/data.c
> > > @@ -762,9 +762,10 @@ static void del_bio_entry(struct bio_entry *be)
> > > kmem_cache_free(bio_entry_slab, be);
> > > }
> > >
> > > -static int add_ipu_page(struct f2fs_sb_info *sbi, struct bio **bio,
> > > - struct page *page)
> > > +static int add_ipu_page(struct f2fs_io_info *fio, struct bio **bio,
> > > + struct page *page, int *bio_needs_submit)
> > > {
> > > + struct f2fs_sb_info *sbi = fio->sbi;
> > > enum temp_type temp;
> > > bool found = false;
> > > int ret = -EAGAIN;
> > > @@ -780,6 +781,15 @@ static int add_ipu_page(struct f2fs_sb_info *sbi, struct bio **bio,
> > > continue;
> > >
> > > found = true;
> > > + if (*bio && (!page_is_mergeable(sbi, *bio,
> > > + *fio->last_block, fio->new_blkaddr) ||
> > > + !f2fs_crypt_mergeable_bio(*bio,
> > > + fio->page->mapping->host,
> > > + fio->page->index, fio))) {
> > > + ret = 0;
> > > + *bio_needs_submit = 1;
> > > + break;
> > > + }
> > >
> > > if (bio_add_page(*bio, page, PAGE_SIZE, 0) ==
> > > PAGE_SIZE) {
> > > @@ -864,6 +874,7 @@ int f2fs_merge_page_bio(struct f2fs_io_info *fio)
> > > struct bio *bio = *fio->bio;
> > > struct page *page = fio->encrypted_page ?
> > > fio->encrypted_page : fio->page;
> > > + int bio_needs_submit = 0;
> > >
> > > if (!f2fs_is_valid_blkaddr(fio->sbi, fio->new_blkaddr,
> > > __is_meta_io(fio) ? META_GENERIC : DATA_GENERIC))
> > > @@ -872,11 +883,6 @@ int f2fs_merge_page_bio(struct f2fs_io_info *fio)
> > > trace_f2fs_submit_page_bio(page, fio);
> > > f2fs_trace_ios(fio, 0);
> > >
> > > - if (bio && (!page_is_mergeable(fio->sbi, bio, *fio->last_block,
> > > - fio->new_blkaddr) ||
> > > - !f2fs_crypt_mergeable_bio(bio, fio->page->mapping->host,
> > > - fio->page->index, fio)))
> > > - f2fs_submit_merged_ipu_write(fio->sbi, &bio, NULL);
> > > alloc_new:
> > > if (!bio) {
> > > bio = __bio_alloc(fio, BIO_MAX_PAGES);
> > > @@ -886,8 +892,12 @@ int f2fs_merge_page_bio(struct f2fs_io_info *fio)
> > >
> > > add_bio_entry(fio->sbi, bio, page, fio->temp);
> > > } else {
> > > - if (add_ipu_page(fio->sbi, &bio, page))
> > > + if (add_ipu_page(fio, &bio, page, &bio_needs_submit))
> > > + goto alloc_new;
> > > + if (bio_needs_submit) {
> > > + f2fs_submit_merged_ipu_write(fio->sbi, &bio, NULL);
> > > goto alloc_new;
> > > + }
> > > }
> > >
> > > if (fio->io_wbc)
> >
> > Thanks, I'm still trying to understand this part of the code, but it's looking
> > like this is a real bug. Do you also have a reproducer that produces a KASAN
> > report, or did you find this another way?
> >
> > One comment: add_ipu_page() already submits the bio if it's full. Wouldn't it
> > be better to use that instead of f2fs_submit_merged_ipu_write()? I.e.:
> >
> > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > index e9dcda80e599..d7a51dbe208b 100644
> > --- a/fs/f2fs/data.c
> > +++ b/fs/f2fs/data.c
> > @@ -762,9 +762,10 @@ static void del_bio_entry(struct bio_entry *be)
> > kmem_cache_free(bio_entry_slab, be);
> > }
> >
> > -static int add_ipu_page(struct f2fs_sb_info *sbi, struct bio **bio,
> > +static int add_ipu_page(struct f2fs_io_info *fio, struct bio **bio,
> > struct page *page)
> > {
> > + struct f2fs_sb_info *sbi = fio->sbi;
> > enum temp_type temp;
> > bool found = false;
> > int ret = -EAGAIN;
> > @@ -780,14 +781,18 @@ static int add_ipu_page(struct f2fs_sb_info *sbi, struct bio **bio,
> > continue;
> >
> > found = true;
> > -
> > - if (bio_add_page(*bio, page, PAGE_SIZE, 0) ==
> > - PAGE_SIZE) {
> > + if (page_is_mergeable(sbi, *bio, *fio->last_block,
> > + fio->new_blkaddr) &&
> > + f2fs_crypt_mergeable_bio(*bio,
> > + fio->page->mapping->host,
> > + fio->page->index, fio) &&
> > + bio_add_page(*bio, page,
> > + PAGE_SIZE, 0) == PAGE_SIZE) {
> > ret = 0;
> > break;
> > }
> >
> > - /* bio is full */
> > + /* page can't be merged into bio; submit the bio */
> > del_bio_entry(be);
> > __submit_bio(sbi, *bio, DATA);
> > break;
> > @@ -872,11 +877,6 @@ int f2fs_merge_page_bio(struct f2fs_io_info *fio)
> > trace_f2fs_submit_page_bio(page, fio);
> > f2fs_trace_ios(fio, 0);
> >
> > - if (bio && (!page_is_mergeable(fio->sbi, bio, *fio->last_block,
> > - fio->new_blkaddr) ||
> > - !f2fs_crypt_mergeable_bio(bio, fio->page->mapping->host,
> > - fio->page->index, fio)))
> > - f2fs_submit_merged_ipu_write(fio->sbi, &bio, NULL);
> > alloc_new:
> > if (!bio) {
> > bio = __bio_alloc(fio, BIO_MAX_PAGES);
> > @@ -886,7 +886,7 @@ int f2fs_merge_page_bio(struct f2fs_io_info *fio)
> >
> > add_bio_entry(fio->sbi, bio, page, fio->temp);
> > } else {
> > - if (add_ipu_page(fio->sbi, &bio, page))
> > + if (add_ipu_page(fio, &bio, page))
> > goto alloc_new;
> > }
> >
> Thanks a lot for looking into this Sahitya! After reading the ipu code,
> I do think it's a bug. Regarding the patch itself, I was going to type
> out basically the same suggestion as Eric, so I definitely second his
> proposal :).
>
> Should I fold this change into the original patch? Or keep it as a
> separate patch when I send out the fscrypt/f2fs inline encryption
> patches?

It may be good to keep it seperate as we already have the base FBE patches in
several downstream kernels, so this fix can be applied easily. But I will
leave it up to you to take a call on this.