On Sun, Jul 1, 2018 at 10:20 AM, Eric Biggers <ebiggers3@xxxxxxxxx> wrote: > On Sun, Jul 01, 2018 at 10:04:59AM -0700, Kees Cook wrote: >> On Sat, Jun 30, 2018 at 12:03 AM, Eric Biggers <ebiggers3@xxxxxxxxx> 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.) > > Sure, I'm just not sure whether __aligned() with such a large alignment is > guaranteed to work on stack variables on all architectures. See e.g. > https://patchwork.kernel.org/patch/9507697/. That's terrible. :( That seems like a compiler bug, but okay. >> 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? > > shash_update_unaligned() is only called when 'data & alignmask'. > Similarly with shash_final_unaligned(). Ah! I see that now. > Though, calculating 'unaligned_len' could be simplified to > > unsigned int unaligned_len = -(unsigned long)data & alignmask; > > which works either way. So, since we can't depend on __aligned() working, I'll just keep the PTR_ALIGN and add MAX_ALGAPI_ALIGNMASK to each array. That'll be less memory-efficient, but it'll actually get aligned correctly. -Kees -- Kees Cook Pixel Security
