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

[Eric Biggers <ebiggers@kernel.org>]

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;
 	}
 


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