On Wed, Jun 20, 2018 at 4:57 PM, Eric Biggers <ebiggers3@xxxxxxxxx> wrote: > On Wed, Jun 20, 2018 at 12:04:06PM -0700, Kees Cook wrote: >> In the quest to remove all stack VLA usage from the kernel[1], this uses >> the newly defined max alignment to perform unaligned hashing to avoid >> VLAs, and drops the helper function while adding sanity checks on the >> resulting buffer sizes. >> >> [1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@xxxxxxxxxxxxxx >> >> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx> >> --- >> crypto/shash.c | 21 ++++++++++----------- >> 1 file changed, 10 insertions(+), 11 deletions(-) >> >> diff --git a/crypto/shash.c b/crypto/shash.c >> index ab6902c6dae7..1bb58209330a 100644 >> --- a/crypto/shash.c >> +++ b/crypto/shash.c >> @@ -73,13 +73,6 @@ int crypto_shash_setkey(struct crypto_shash *tfm, const u8 *key, >> } >> EXPORT_SYMBOL_GPL(crypto_shash_setkey); >> >> -static inline unsigned int shash_align_buffer_size(unsigned len, >> - unsigned long mask) >> -{ >> - typedef u8 __aligned_largest u8_aligned; >> - return len + (mask & ~(__alignof__(u8_aligned) - 1)); >> -} >> - >> static int shash_update_unaligned(struct shash_desc *desc, const u8 *data, >> unsigned int len) >> { >> @@ -88,11 +81,14 @@ 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[CRYPTO_ALG_MAX_ALIGNMASK] >> + __aligned(CRYPTO_ALG_MAX_ALIGNMASK + 1); >> u8 *buf = PTR_ALIGN(&ubuf[0], alignmask + 1); >> int err; > > Are you sure that __attribute__((aligned(64))) works correctly on stack > variables on all architectures? > > And if it is expected to work, then why is the buffer still aligned by hand on > the very next line? I really don't know -- the existing code was doing both the __align bit and the manual alignment, so I was trying to simplify it while removing the VLA. I'm totally open to suggestions here. BTW, these are also the only users of __aligned_largest() in the kernel, and the only use of unsized __attribute__((aligned)) -Kees -- Kees Cook Pixel Security
