[dm-devel] [BUG] BLKZEROOUT on dm-crypt container cause OOM / kernel panic

[Tom Yan]

Hi again,

I think I have sort of identified the problem. It appears to me that both the block layer and dm-crypt is defected on handling this.

First of all, check out the "fallback" zero out implementation, which is used in this case, here:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/block/blk-lib.c?h=v4.12#n309

>From the outer loop, it seem to imply that this should be done in multiple "bio"s, if the request (original "nr_sects") is larger than BIO_MAX_PAGES:

...
while (nr_sects != 0) {
	bio = next_bio(bio, min(nr_sects, (sector_t)BIO_MAX_PAGES),
			gfp_mask);
...
}
...

However, there is a inner loop:

...
	while (nr_sects != 0) {
		sz = min((sector_t) PAGE_SIZE >> 9 , nr_sects);
		bi_size = bio_add_page(bio, ZERO_PAGE(0), sz << 9, 0);
		nr_sects -= bi_size >> 9;
		sector += bi_size >> 9;
		if (bi_size < (sz << 9))
			break;
	}
...

which apparently would loop over the whole request on its own, making the outer loop a bogus one.

The request ends up being done in a single (huge) bio. When the bio is passed on to dm-crypt, it appears that dm-crypt will not split the bio either when it allocates buffer for conversion/encryption:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/md/dm-crypt.c?h=v4.12#n1694

which leads to possible enormous uptake of memory, causing OOM / kernel panic.

There seems to be some measure that is suppose to split large bio though:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/md/dm-crypt.c?h=v4.12#n2787

Apparently it is called before kcryptd_crypt_write_convert() / crypt_alloc_buffer(). However, I don't really parse dm_accept_partial_bio() (or the comment about it) so I don't really know what it actually does or how it does it. Neither can I see it helps in reality anyway.

Here is another test case that shows the problem:
https://ptpb.pw/BWWo.png
https://ptpb.pw/aRTE.png

Regards,
Tom Yan


From: Tom Yan
Sent: Monday, August 7, 2017 9:58 AM
To: dm-devel at redhat.com
Subject: [BUG] BLKZEROOUT on dm-crypt container cause OOM / kernel panic
    
Hi all,

When I do the following:

cryptsetup open /dev/sdX[Y] rand --type plain --key-file /dev/random;
blkdiscard -z /dev/mapper/rand

Some OOM killings and a kernel panic occur. Here is a screenshot:
https://gitlab.com/cryptsetup/cryptsetup/uploads/207ffdada52f3172f54a014c67159625/DSC_0129.JPGhttps://gitlab.com/cryptsetup/cryptsetup/uploads/207ffdada52f3172f54a014c67159625/DSC_0129.JPG 


Similar issue is NOT found in doing `cat /dev/zero > /dev/mapper/rand`, or `blkdiscard -z /dev/sdX[Y]` (without opening a dm-crypt on it), on the same hardware and configuration.

Note that `blkdiscard -z` does not trigger the BLKDISCARD ioctl but the BLKZEROOUT ioctl. And the devices I have been testing on are USB devices that does NOT support WRITE SAME or UNMAP.

Regards,
Tom Yan