Re: Kernel panic - encryption/decryption failed when open file on Arm64

From: xiakaixu
Date: Mon Sep 12 2016 - 22:06:15 EST

On 12 September 2016 at 03:16, liushuoran <liushuoran@xxxxxxxxxx> wrote:
Hi Ard,

Thanks for the prompt reply. With the patch, there is no panic anymore. But it seems that the encryption/decryption is not successful anyway.

As Herbert points out, "If the page allocation fails in blkcipher_walk_next it'll simply switch over to processing it block by block". So does that mean the encryption/decryption should be successful even if the page allocation fails? Please correct me if I misunderstand anything. Thanks in advance.

Perhaps Herbert can explain: I don't see how the 'n = 0' assignment
results in the correct path being taken; this chunk (blkcipher.c:252)

if (unlikely(n < bsize)) {
err = blkcipher_next_slow(desc, walk, bsize, walk->alignmask);
goto set_phys_lowmem;

is skipped due to the fact that n == 0 and therefore bsize == 0, and
so the condition is always false for n == 0

Therefore we end up here (blkcipher.c:257)

walk->nbytes = n;
if (walk->flags & BLKCIPHER_WALK_COPY) {
err = blkcipher_next_copy(walk);
goto set_phys_lowmem;

where blkcipher_next_copy() unconditionally calls memcpy() with
walk->page as destination (even though we ended up here due to the
fact that walk->page == NULL)

So to me, it seems like we should be taking the blkcipher_next_slow()
path, which does a kmalloc() and bails with -ENOMEM if that fails.

Hi Ard,

Thanks for such a detailed reply.

According to your reply, I just make a little change to take the
blkcipher_next_slow() path. I test it on arm64 board, there is
no panic anymore and seems the encryption/decryption is successful.

diff --git a/crypto/blkcipher.c b/crypto/blkcipher.c
index 0122bec..5389d40 100644
--- a/crypto/blkcipher.c
+++ b/crypto/blkcipher.c
@@ -240,12 +240,13 @@ static int blkcipher_walk_next(struct blkcipher_desc *desc,
walk->flags |= BLKCIPHER_WALK_COPY;
if (!walk->page) {
walk->page = (void *)__get_free_page(GFP_ATOMIC);
+ walk->page = NULL;
if (!walk->page)
n = 0;

- bsize = min(walk->walk_blocksize, n);
+ bsize = walk->walk_blocksize;
n = scatterwalk_clamp(&walk->in, n);
n = scatterwalk_clamp(&walk->out, n);

It is just a trial and not sure it makes sense. But anyway, we can do
something here to fix the crash result from the page allocation failure.

What's your opinions, Herbert?

Kaixu Xia