Messages in this thread | | | From | Kees Cook <> | Date | Sun, 1 Jul 2018 10:04:59 -0700 | Subject | Re: [dm-devel] [PATCH v3 9/9] crypto: shash: Remove VLA usage in unaligned hashing |
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On Sat, Jun 30, 2018 at 12:03 AM, Eric Biggers <ebiggers3@gmail.com> wrote: > On Thu, Jun 28, 2018 at 05:28:43PM -0700, Kees Cook wrote: >> @@ -88,11 +81,13 @@ static int shash_update_unaligned(struct shash_desc *desc, const u8 *data, >> unsigned long alignmask = crypto_shash_alignmask(tfm); >> unsigned int unaligned_len = alignmask + 1 - >> ((unsigned long)data & alignmask); >> - u8 ubuf[shash_align_buffer_size(unaligned_len, alignmask)] >> - __aligned_largest; >> + u8 ubuf[MAX_ALGAPI_ALIGNMASK + 1]; >> u8 *buf = PTR_ALIGN(&ubuf[0], alignmask + 1); >> int err; >> >> + if (WARN_ON(buf + unaligned_len > ubuf + sizeof(ubuf))) >> + return -EINVAL; >> + > > How is 'ubuf' guaranteed to be large enough? You removed the __aligned > attribute, so 'ubuf' can have any alignment. So the aligned pointer 'buf' may > be as high as '&ubuf[alignmask]'. Then, up to 'alignmask' bytes of data will be > copied into 'buf'... resulting in up to '2 * alignmask' bytes needed in 'ubuf'. > But you've only guaranteed 'alignmask + 1' bytes.
Hm, good point. Adding __aligned(MAX_ALGAPI_ALIGNMASK + 1) looks to fix this, yes?
Also, if __aligned() is used here, can't PTR_ALIGN() be dropped? (I think you pointed this out earlier.)
Also, is "unaligned_len" being calculated correctly? Let's say alignmask is 63. If data is binary ...111111, then unaligned_len will be 64 - 63 == 1, which is fine: we copy 1 byte out, bump the address by 1, and we're happily aligned to ...000000. If data is ...000000, then unaligned_len will be 64. But it should be 0. Shouldn't this be:
unsigned int unaligned_len;
unaligned_len = (unsigned long)data & alignmask; if (unaligned_len) unaligned_len = alignmask + 1 - unaligned_len;
And then ubuf only needs to be MAX_ALGAPI_ALIGNMASK, without the +1?
-Kees
-- Kees Cook Pixel Security
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