On Fri, Sep 10, 2021 at 09:27:46PM +0530, Shreyansh Chouhan wrote:
> On Fri, Sep 10, 2021 at 09:24:37PM +0530, Shreyansh Chouhan wrote:
> > Hi Dan,
> >
> > Sorry for the delay in the response.
> >
> > On Wed, Sep 08, 2021 at 03:54:40PM +0300, Dan Carpenter wrote:
> > > Hello Shreyansh Chouhan,
> > >
> > > The patch 72ff2bf04db2: "crypto: aesni - xts_crypt() return if
> > > walk.nbytes is 0" from Aug 22, 2021, leads to the following
> > > Smatch static checker warning:
> > >
> > > arch/x86/crypto/aesni-intel_glue.c:915 xts_crypt()
> > > warn: possible missing kernel_fpu_end()
> > >
> > > arch/x86/crypto/aesni-intel_glue.c
> > > 839 static int xts_crypt(struct skcipher_request *req, bool encrypt)
> > > 840 {
> > > 841 struct crypto_skcipher *tfm = crypto_skcipher_reqtfm(req);
> > > 842 struct aesni_xts_ctx *ctx = crypto_skcipher_ctx(tfm);
> > > 843 int tail = req->cryptlen % AES_BLOCK_SIZE;
> > > 844 struct skcipher_request subreq;
> > > 845 struct skcipher_walk walk;
> > > 846 int err;
> > > 847
> > > 848 if (req->cryptlen < AES_BLOCK_SIZE)
> > > 849 return -EINVAL;
> > > 850
> > > 851 err = skcipher_walk_virt(&walk, req, false);
> > > 852 if (!walk.nbytes)
> > > 853 return err;
> > >
> > > The patch adds this check for "walk.nbytes == 0".
> > >
> > > 854
> > > 855 if (unlikely(tail > 0 && walk.nbytes < walk.total)) {
> > > ^^^^^^^^^^^^^^^^^^^^^^^^
> > > But Smatch says that "walk.nbytes" can be set to zero inside this
> > > if statement.
> > >
> >
> > Indeed that is so, I missed it the first time around.
> >
> > > 856 int blocks = DIV_ROUND_UP(req->cryptlen, AES_BLOCK_SIZE) - 2;
> > > 857
> > > 858 skcipher_walk_abort(&walk);
> > > 859
> > > 860 skcipher_request_set_tfm(&subreq, tfm);
> > > 861 skcipher_request_set_callback(&subreq,
> > > 862 skcipher_request_flags(req),
> > > 863 NULL, NULL);
> > > 864 skcipher_request_set_crypt(&subreq, req->src, req->dst,
> > > 865 blocks * AES_BLOCK_SIZE, req->iv);
> > > 866 req = &subreq;
> > > 867
> > > 868 err = skcipher_walk_virt(&walk, req, false);
> > > 869 if (err)
> > > 870 return err;
> >
> > We can replace the above if (err) check with another if
> > (!walk.nbytes) check.
> >
> > > 871 } else {
> > > 872 tail = 0;
> > > 873 }
> > > 874
> > > 875 kernel_fpu_begin();
> > > 876
> > > 877 /* calculate first value of T */
> > > 878 aesni_enc(aes_ctx(ctx->raw_tweak_ctx), walk.iv, walk.iv);
> > > 879
> > >
> > > Leading to not entering this loop and so we don't restore kernel_fpu_end().
> > >
> > > So maybe the "if (walk.nbytes == 0)" check should be moved to right
> > > before the call to kernel_fpu_begin()?
> > >
> >
> > Instead of moving the first walk.nbytes check, I think we can have two if
> > (!walk.nbytes) checks. There was a discussion between Herbert Xu and Ard
> > Biesheuvel, and Herbert wrote in his email that most skcipher walkers are
> > not supposed to explicitly check on the err value, and should instead
> > terminate the loop whenever walk.nbytes is set to 0.
> >
> > Here is a link to that discussion:
> >
> > https://lore.kernel.org/linux-crypto/20210820125315.GB28484@xxxxxxxxxxxxxxxxxxx/
> >
>
> I can send in a patch that replaces the if (err) check with an if
> (!walk.nbytes) check if that is fine with you.
>
Yes, please!
regards,
dan carpenter
