Re: [PATCH v2] f2fs: avoid out-of-range memory access

[Ocean Chen]

Hi YuChao,

  Yes, we got externel researcher reports this security vulnerability.

And dump info is better when blk_off is invalid. I'll prepare the next
patch for it.

On Thu, Jul 04, 2019 at 03:11:27PM +0800, Chao Yu wrote:
> Hi Ocean,
> 
> On 2019/7/3 23:03, Ocean Chen wrote:
> > Hi Yu Chao,
> > 
> > The cur_data_segno only was checked in mount process. In terms of
> > security concern, it's better to check value before using it. I know the
> 
> Could you explain more about security concern.. Do you get any report from user
> or tools that complaining f2fs issue/codes?
> 
> I'm not against sanity check for basic core data of filesystem in run-time, but,
> in order to troubleshoot root cause of this issue we can trigger panic directly
> to dump more info under F2FS_CHECK_FS macro.
> 
> So, maybe we can change as below?
> 
> blk_off = le16_to_cpu(ckpt->cur_data_blkoff[i]);
> +if (blk_off > ENTRIES_IN_SUM) {
> +	f2fs_bug_on(1);
> +	f2fs_put_page(page, 1);
> +	return -EFAULT;
> +}
> 
> Thanks,
> 
> > risk is low. IMHO, it can be safer.
> > BTW, I found we can only check blk_off before for loop instead of
> > checking 'j' in each iteratoin.
> > 
> > On Wed, Jul 03, 2019 at 10:07:11AM +0800, Chao Yu wrote:
> >> Hi Ocean,
> >>
> >> If filesystem is corrupted, it should fail mount due to below check in
> >> f2fs_sanity_check_ckpt(), so we are safe in read_compacted_summaries() to access
> >> entries[0,blk_off], right?
> >>
> >> 	for (i = 0; i < NR_CURSEG_DATA_TYPE; i++) {
> >> 		if (le32_to_cpu(ckpt->cur_data_segno[i]) >= main_segs ||
> >> 			le16_to_cpu(ckpt->cur_data_blkoff[i]) >= blocks_per_seg)
> >> 			return 1;
> >>
> >> Thanks,
> >>
> >> On 2019/7/2 16:05, Ocean Chen wrote:
> >>> blk_off might over 512 due to fs corrupt.
> >>> Use ENTRIES_IN_SUM to protect invalid memory access.
> >>>
> >>> v2:
> >>> - fix typo
> >>> Signed-off-by: Ocean Chen <oceanchen@xxxxxxxxxx>
> >>> ---
> >>>  fs/f2fs/segment.c | 2 ++
> >>>  1 file changed, 2 insertions(+)
> >>>
> >>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> >>> index 8dee063c833f..a5e8af0bd62e 100644
> >>> --- a/fs/f2fs/segment.c
> >>> +++ b/fs/f2fs/segment.c
> >>> @@ -3403,6 +3403,8 @@ static int read_compacted_summaries(struct f2fs_sb_info *sbi)
> >>>  
> >>>  		for (j = 0; j < blk_off; j++) {
> >>>  			struct f2fs_summary *s;
> >>> +			if (j >= ENTRIES_IN_SUM)
> >>> +				return -EFAULT;
> >>>  			s = (struct f2fs_summary *)(kaddr + offset);
> >>>  			seg_i->sum_blk->entries[j] = *s;
> >>>  			offset += SUMMARY_SIZE;
> >>>
> > .
> >