[PATCH] ima: dynamically allocate shash_desc

[Mimi Zohar]

On Tue, 2019-06-18 at 20:06 +0200, Arnd Bergmann wrote:
> On Tue, Jun 18, 2019 at 3:55 PM Mimi Zohar <zohar at linux.ibm.com> wrote:
> >
> > On Mon, 2019-06-17 at 22:08 +0200, Arnd Bergmann wrote:
> > > On Mon, Jun 17, 2019 at 8:08 PM Mimi Zohar <zohar at linux.ibm.com> wrote:
> > > >
> > > > On Mon, 2019-06-17 at 11:55 -0400, Mimi Zohar wrote:
> > > > > On Mon, 2019-06-17 at 13:20 +0200, Arnd Bergmann wrote:
> > > > > > On 32-bit ARM, we get a warning about excessive stack usage when
> > > > > > building with clang.
> > > > > >
> > > > > > security/integrity/ima/ima_crypto.c:504:5: error: stack frame size
> > > > > > of 1152 bytes in function 'ima_calc_field_array_hash' [-Werror,-
> > > > > > Wframe-larger-than=]
> > > > >
> > > > > I'm definitely not seeing this.  Is this problem a result of non
> > > > > upstreamed patches?  For sha1, currently the only possible hash
> > > > > algorithm, I'm seeing 664.
> > >
> > > You won't see it with gcc, only with clang in some randconfig builds,
> > > I suppose only when KASAN is enabled.
> > >
> > > > Every time a measurement is added to the measurement list, the memory
> > > > would be allocated/freed.  The frequency of new measurements is policy
> > > > dependent.  For performance reasons, I'd prefer if the allocation
> > > > remains on the stack.
> > >
> > > Is there a way to preallocate the shash_desc instead? That would
> > > avoid the overhead.
> >
> > There are 3 other SHASH_DESC_ON_STACK definitions in just
> > ima_crypto.c, with a total of ~55 other places in the kernel.  Before
> > fixing this particular function, I'd like to know if the "excessive
> > stack usage" warning is limited to ima_calc_field_array_hash_tfm().
> >  If so, what is so special about its usage of SHASH_DESC_ON_STACK?
> 
> SHASH_DESC_ON_STACK() uses at least 512 bytes of stack
> everywhere, which is half of the warning limit for a function on
> 32 bit kernels.
> 
> With KASAN, a small redzone is put around it so we can detect out
> of bounds access to a variable that is passed by reference.
> clang makes that buffer larger than gcc, so we end up with something
> like 768 bytes for each instance of SHASH_DESC_ON_STACK().
> 
> Most other users still stay below the 1024 byte warning level though,
> because typical functions only use a few bytes of stack space.
> In case of ima_calc_field_array_hash_tfm(), the is also the buffer[]
> array of 255 bytes that gets another large redzone.
> 
> I fixed up all the (randconfig) warnings I get for arm32, arm64 and
> x86 kernels, and I think there were four to five that were because of
> SHASH_DESC_ON_STACK(). It might make sense to convert all
> three instances in ima to preallocate the descriptor if we do it for
> one of them, even when it's not actually needed.

"buffer" is only used for the original "ima" template format, which is
limited to sha1.  Rather than allocating shash, I would prefer
"buffer" be allocated, if needed, and only the first time.

Mimi