Re: [PATCH v3] zram: fix invalid memory references during disk write

[Minchan Kim <minchan@kernel.org>]

Hi Greg,

I would like to tidy zram_bvec_write up but it needs more churn
than needed to fix this bug and makes review hard. So I want you to pick up
this patch asap because it's a candidate of stable.

Thanks.

On Thu, Nov 29, 2012 at 01:18:48AM -0800, Nitin Gupta wrote:
> Changelog v3 vs v2:
>  - Further changes to changelog message to explain the bug case
> of incorrect use of kunmapped buffer
> Changelog v2 vs v1:
>  - Changelog message now correctly explains the problem
> 
> Fixes a bug introduced by commit c8f2f0db1 ("zram: Fix handling
> of incompressible pages") which caused invalid memory references
> during disk write. Invalid references could occur in two cases:
>  - Incoming data expands on compression: In this case, reference was
> made to kunmap()'ed bio page.
>  - Partial (non PAGE_SIZE) write with incompressible data: In this
> case, reference was made to a kfree()'ed buffer.
> 
> Fixes bug 50081:
> https://bugzilla.kernel.org/show_bug.cgi?id=50081
> 
> Signed-off-by: Nitin Gupta <ngupta@vflare.org>
> Reported-by: Mihail Kasadjikov <hamer.mk@gmail.com>
> Reported-by: Tomas M <tomas@slax.org>
> Reviewed-by: Minchan Kim <minchan@kernel.org>
> ---
>  drivers/staging/zram/zram_drv.c |   39 ++++++++++++++++++++++++---------------
>  1 file changed, 24 insertions(+), 15 deletions(-)
> 
> diff --git a/drivers/staging/zram/zram_drv.c b/drivers/staging/zram/zram_drv.c
> index fb4a7c9..f2a73bd 100644
> --- a/drivers/staging/zram/zram_drv.c
> +++ b/drivers/staging/zram/zram_drv.c
> @@ -265,7 +265,7 @@ out_cleanup:
>  static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
>  			   int offset)
>  {
> -	int ret;
> +	int ret = 0;
>  	size_t clen;
>  	unsigned long handle;
>  	struct page *page;
> @@ -286,10 +286,8 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
>  			goto out;
>  		}
>  		ret = zram_decompress_page(zram, uncmem, index);
> -		if (ret) {
> -			kfree(uncmem);
> +		if (ret)
>  			goto out;
> -		}
>  	}
> 
>  	/*
> @@ -302,16 +300,18 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
> 
>  	user_mem = kmap_atomic(page);
> 
> -	if (is_partial_io(bvec))
> +	if (is_partial_io(bvec)) {
>  		memcpy(uncmem + offset, user_mem + bvec->bv_offset,
>  		       bvec->bv_len);
> -	else
> +		kunmap_atomic(user_mem);
> +		user_mem = NULL;
> +	} else {
>  		uncmem = user_mem;
> +	}
> 
>  	if (page_zero_filled(uncmem)) {
> -		kunmap_atomic(user_mem);
> -		if (is_partial_io(bvec))
> -			kfree(uncmem);
> +		if (!is_partial_io(bvec))
> +			kunmap_atomic(user_mem);
>  		zram_stat_inc(&zram->stats.pages_zero);
>  		zram_set_flag(zram, index, ZRAM_ZERO);
>  		ret = 0;
> @@ -321,9 +321,11 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
>  	ret = lzo1x_1_compress(uncmem, PAGE_SIZE, src, &clen,
>  			       zram->compress_workmem);
> 
> -	kunmap_atomic(user_mem);
> -	if (is_partial_io(bvec))
> -			kfree(uncmem);
> +	if (!is_partial_io(bvec)) {
> +		kunmap_atomic(user_mem);
> +		user_mem = NULL;
> +		uncmem = NULL;
> +	}
> 
>  	if (unlikely(ret != LZO_E_OK)) {
>  		pr_err("Compression failed! err=%d\n", ret);
> @@ -332,8 +334,10 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
> 
>  	if (unlikely(clen > max_zpage_size)) {
>  		zram_stat_inc(&zram->stats.bad_compress);
> -		src = uncmem;
>  		clen = PAGE_SIZE;
> +		src = NULL;
> +		if (is_partial_io(bvec))
> +			src = uncmem;
>  	}
> 
>  	handle = zs_malloc(zram->mem_pool, clen);
> @@ -345,7 +349,11 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
>  	}
>  	cmem = zs_map_object(zram->mem_pool, handle, ZS_MM_WO);
> 
> +	if ((clen == PAGE_SIZE) && !is_partial_io(bvec))
> +		src = kmap_atomic(page);
>  	memcpy(cmem, src, clen);
> +	if ((clen == PAGE_SIZE) && !is_partial_io(bvec))
> +		kunmap_atomic(src);
> 
>  	zs_unmap_object(zram->mem_pool, handle);
> 
> @@ -358,9 +366,10 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
>  	if (clen <= PAGE_SIZE / 2)
>  		zram_stat_inc(&zram->stats.good_compress);
> 
> -	return 0;
> -
>  out:
> +	if (is_partial_io(bvec))
> +		kfree(uncmem);
> +
>  	if (ret)
>  		zram_stat64_inc(zram, &zram->stats.failed_writes);
>  	return ret;
> --
> 1.7.10.4
> 
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-- 
Kind regards,
Minchan Kim