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SubjectRe: [syzbot] BUG: sleeping function called from invalid context in __fdget_pos
On Wed, 30 Jun 2021 at 10:10, Herbert Xu <herbert@gondor.apana.org.au> wrote:
>
> Hi Ard:
>
> On Wed, Jun 30, 2021 at 09:42:14AM +0200, Ard Biesheuvel wrote:
> >
> > > There's one suspect-looking site in xts_crypt():
> > >
> > > > kernel_fpu_begin();
> > > >
> > > > /* calculate first value of T */
> > > > aesni_enc(aes_ctx(ctx->raw_tweak_ctx), walk.iv, walk.iv);
> > > >
> > > > while (walk.nbytes > 0) {
> > > > int nbytes = walk.nbytes;
> > > >
> > > > ...
> > > >
> > > > err = skcipher_walk_done(&walk, walk.nbytes - nbytes);
> > > >
> > > > kernel_fpu_end();
> > > >
> > > > if (walk.nbytes > 0)
> > > > kernel_fpu_begin();
> > > > }
> > >
> > > I wonder if a slab allocation failure could leave us with walk.nbytes==0.
> >
> > The code is actually the other way around: kernel_fpu_end() comes
> > before the call to skcipher_walk_done().
> >
> > So IIUC, this code forces an allocation failure, and checks whether
> > the code deals with this gracefully, right?
> >
> > The skcipher walk API guarantees that walk.nbytes == 0 if an error is
> > returned, so the pairing of FPU begin/end looks correct to me. And
> > skcipher_walk_next() should not invoke anything that might sleep from
> > this particular context.
> >
> > Herbert, any ideas?
>
> xts_crypt looks buggy to me. In particular, if the second
> skcipher_walk_virt call (the one in the if clause) fails, then
> we will return without calling kernel_fpu_end.
>
> Another issue, we are not checking for errors on the first
> skcipher_walk_virt call, this may cause a double-free with
> the subsequent skcipher_walk_abort inside the if clause.
>
> With skcikpher_walk_virt, you must check for errors explicitly
> *unless* you use it in a loop construct which exits on !walk->nbytes.
>

So something like this, I suppose?

--- a/arch/x86/crypto/aesni-intel_glue.c
+++ b/arch/x86/crypto/aesni-intel_glue.c
@@ -849,6 +849,8 @@
return -EINVAL;

err = skcipher_walk_virt(&walk, req, false);
+ if (err)
+ return err;

if (unlikely(tail > 0 && walk.nbytes < walk.total)) {
int blocks = DIV_ROUND_UP(req->cryptlen, AES_BLOCK_SIZE) - 2;
@@ -862,7 +864,10 @@
skcipher_request_set_crypt(&subreq, req->src, req->dst,
blocks * AES_BLOCK_SIZE, req->iv);
req = &subreq;
+
err = skcipher_walk_virt(&walk, req, false);
+ if (err)
+ return err;
} else {
tail = 0;
}
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