Re: [PATCH 1/5] btrfs: extent_io: Do extra check for extent buffer read write functions

[Nikolay Borisov <nborisov@suse.com>]

On 10.07.19 г. 11:02 ч., Qu Wenruo wrote:
> Although we have start, len check for extent buffer reader/write (e.g.
> read_extent_buffer()), those checks has its limitations:
> - No overflow check
>   Values like start = 1024 len = -1024 can still pass the basic
>    (start + len) > eb->len check.
> 
> - Checks are not consistent
>   For read_extent_buffer() we only check (start + len) against eb->len.
>   While for memcmp_extent_buffer() we also check start against eb->len.
> 
> - Different error reporting mechanism
>   We use WARN() in read_extent_buffer() but BUG() in
>   memcpy_extent_buffer().
> 
> - Still modify memory if the request is obviously wrong
>   In read_extent_buffer() even we find (start + len) > eb->len, we still
>   call memset(dst, 0, len), which can eaisly cause memory access error
>   if start + len overflows.
> 
> To address above problems, this patch creates a new common function to
> check such access, check_eb_range().
> - Add overflow check
>   This function checks start, start + len against eb->len and overflow
>   check.
> 
> - Unified checks
> 
> - Unified error reports
>   Will call WARN() if CONFIG_BTRFS_DEBUG is configured.
>   And also do btrfs_warn() message for non-debug build.
> 
> - Exit ASAP if check fails
>   No more possible memory corruption.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=202817
> [ Inspired by above report, the report itself is already addressed ]
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
>  fs/btrfs/extent_io.c | 79 +++++++++++++++++++++++---------------------
>  1 file changed, 42 insertions(+), 37 deletions(-)
> 
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index db337e53aab3..b072ef5bceac 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -5429,6 +5429,31 @@ int read_extent_buffer_pages(struct extent_buffer *eb, int wait, int mirror_num)
>  	return ret;
>  }
>  
> +/*
> + * Check if the [start, start + len) range is valid before reading/writing
> + * the eb.
> + *
> + * Caller should not touch the dst/src memory if this function returns error.
> + */
> +static int check_eb_range(const struct extent_buffer *eb, unsigned long start,
> +			  unsigned long len)
> +{
> +	unsigned long end;
> +
> +	/* start, start + len should not go beyond eb->len nor overflow */
> +	if (unlikely(start > eb->len || start + len > eb->len ||

I think your check here is wrong, it should be start + len > start +
eb->len. start is the logical address hence it can be a lot bigger than
the size of the eb which is 16k by default.

> +		     check_add_overflow(start, len, &end))) {
> +		WARN(IS_ENABLED(CONFIG_BTRFS_DEBUG), KERN_ERR
> +"btrfs: bad eb rw request, eb bytenr=%llu len=%lu rw start=%lu len=%lu\n",
> +		     eb->start, eb->len, start, len);
> +		btrfs_warn(eb->fs_info,
> +"btrfs: bad eb rw request, eb bytenr=%llu len=%lu rw start=%lu len=%lu\n",
> +			   eb->start, eb->len, start, len);

If CONFIG_BTRFS_DEBUG is enabled then we will print the warning text
twice. Simply make  it:

WARN_ON(IS_ENABLED()) and leave the btrfs_Warn to always print the text.

> +		return -EINVAL;
> +	}
> +	return 0;
> +}
> +
>  void read_extent_buffer(const struct extent_buffer *eb, void *dstv,
>  			unsigned long start, unsigned long len)
>  {
> @@ -5440,12 +5465,8 @@ void read_extent_buffer(const struct extent_buffer *eb, void *dstv,
>  	size_t start_offset = offset_in_page(eb->start);
>  	unsigned long i = (start_offset + start) >> PAGE_SHIFT;
>  
> -	if (start + len > eb->len) {
> -		WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, wanted %lu %lu\n",
> -		     eb->start, eb->len, start, len);
> -		memset(dst, 0, len);
> +	if (check_eb_range(eb, start, len))
>  		return;
> -	}
>  
>  	offset = offset_in_page(start_offset + start);
>  
> @@ -5554,8 +5575,8 @@ int memcmp_extent_buffer(const struct extent_buffer *eb, const void *ptrv,
>  	unsigned long i = (start_offset + start) >> PAGE_SHIFT;
>  	int ret = 0;
>  
> -	WARN_ON(start > eb->len);
> -	WARN_ON(start + len > eb->start + eb->len);
> +	if (check_eb_range(eb, start, len))
> +		return -EINVAL;
>  
>  	offset = offset_in_page(start_offset + start);
>  
> @@ -5609,8 +5630,8 @@ void write_extent_buffer(struct extent_buffer *eb, const void *srcv,
>  	size_t start_offset = offset_in_page(eb->start);
>  	unsigned long i = (start_offset + start) >> PAGE_SHIFT;
>  
> -	WARN_ON(start > eb->len);
> -	WARN_ON(start + len > eb->start + eb->len);
> +	if (check_eb_range(eb, start, len))
> +		return;
>  
>  	offset = offset_in_page(start_offset + start);
>  
> @@ -5639,8 +5660,8 @@ void memzero_extent_buffer(struct extent_buffer *eb, unsigned long start,
>  	size_t start_offset = offset_in_page(eb->start);
>  	unsigned long i = (start_offset + start) >> PAGE_SHIFT;
>  
> -	WARN_ON(start > eb->len);
> -	WARN_ON(start + len > eb->start + eb->len);
> +	if (check_eb_range(eb, start, len))
> +		return;
>  
>  	offset = offset_in_page(start_offset + start);
>  
> @@ -5684,6 +5705,10 @@ void copy_extent_buffer(struct extent_buffer *dst, struct extent_buffer *src,
>  	size_t start_offset = offset_in_page(dst->start);
>  	unsigned long i = (start_offset + dst_offset) >> PAGE_SHIFT;
>  
> +	if (check_eb_range(dst, dst_offset, len) ||
> +	    check_eb_range(src, src_offset, len))
> +		return;
> +
>  	WARN_ON(src->len != dst_len);
>  
>  	offset = offset_in_page(start_offset + dst_offset);
> @@ -5872,7 +5897,6 @@ static void copy_pages(struct page *dst_page, struct page *src_page,
>  void memcpy_extent_buffer(struct extent_buffer *dst, unsigned long dst_offset,
>  			   unsigned long src_offset, unsigned long len)
>  {
> -	struct btrfs_fs_info *fs_info = dst->fs_info;
>  	size_t cur;
>  	size_t dst_off_in_page;
>  	size_t src_off_in_page;
> @@ -5880,18 +5904,9 @@ void memcpy_extent_buffer(struct extent_buffer *dst, unsigned long dst_offset,
>  	unsigned long dst_i;
>  	unsigned long src_i;
>  
> -	if (src_offset + len > dst->len) {
> -		btrfs_err(fs_info,
> -			"memmove bogus src_offset %lu move len %lu dst len %lu",
> -			 src_offset, len, dst->len);
> -		BUG();
> -	}
> -	if (dst_offset + len > dst->len) {
> -		btrfs_err(fs_info,
> -			"memmove bogus dst_offset %lu move len %lu dst len %lu",
> -			 dst_offset, len, dst->len);
> -		BUG();
> -	}
> +	if (check_eb_range(dst, dst_offset, len) ||
> +	    check_eb_range(dst, src_offset, len))
> +		return;
>  
>  	while (len > 0) {
>  		dst_off_in_page = offset_in_page(start_offset + dst_offset);
> @@ -5917,7 +5932,6 @@ void memcpy_extent_buffer(struct extent_buffer *dst, unsigned long dst_offset,
>  void memmove_extent_buffer(struct extent_buffer *dst, unsigned long dst_offset,
>  			   unsigned long src_offset, unsigned long len)
>  {
> -	struct btrfs_fs_info *fs_info = dst->fs_info;
>  	size_t cur;
>  	size_t dst_off_in_page;
>  	size_t src_off_in_page;
> @@ -5927,18 +5941,9 @@ void memmove_extent_buffer(struct extent_buffer *dst, unsigned long dst_offset,
>  	unsigned long dst_i;
>  	unsigned long src_i;
>  
> -	if (src_offset + len > dst->len) {
> -		btrfs_err(fs_info,
> -			  "memmove bogus src_offset %lu move len %lu len %lu",
> -			  src_offset, len, dst->len);
> -		BUG();
> -	}
> -	if (dst_offset + len > dst->len) {
> -		btrfs_err(fs_info,
> -			  "memmove bogus dst_offset %lu move len %lu len %lu",
> -			  dst_offset, len, dst->len);
> -		BUG();
> -	}
> +	if (check_eb_range(dst, dst_offset, len) ||
> +	    check_eb_range(dst, src_offset, len))
> +		return;
>  	if (dst_offset < src_offset) {
>  		memcpy_extent_buffer(dst, dst_offset, src_offset, len);
>  		return;
>