Messages in this thread | | | From | Kees Cook <> | Date | Wed, 9 May 2018 10:01:41 -0700 | Subject | Re: [RFC][PATCH 00/13] Provide saturating helpers for allocation |
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On Wed, May 9, 2018 at 9:08 AM, Laura Abbott <labbott@redhat.com> wrote: > On 05/08/2018 05:42 PM, Kees Cook wrote: >> >> This is a stab at providing three new helpers for allocation size >> calculation: >> >> struct_size(), array_size(), and array3_size(). >> >> These are implemented on top of Rasmus's overflow checking functions, >> and the last 8 patches are all treewide conversions of open-coded >> multiplications into the various combinations of the helper functions. >> >> -Kees >> >> > Obvious question (that might indicate this deserves documentation?) > > What's the difference between > > kmalloc_array(cnt, sizeof(struct blah), GFP_KERNEL); > > and > > kmalloc(array_size(cnt, struct blah), GFP_KERNEL); > > > and when would you use one over the other?
If I'm understanding the intentions here, the next set of treewide changes would be to remove *calloc() and *_array() in favor of using the array_size() helper. (i.e. reducing proliferation of allocator helpers in favor of using the *_size() helpers.
There are, however, some cases that don't map well to {struct,array,array3}_size(), specifically cases of additions in finding a count. For example, stuff like:
kmalloc(sizeof(header) + sizeof(trailing_array) * (count + SOMETHING), gfp...)
This gets currently mapped to:
kmalloc(struct_size(header, trailing_array, (count + SOMETHING), gfp...)
But we run the risk in some cases of having even the addition overflow. I think we need to have a "saturating add" too. Something like:
kmalloc(struct_size(header, trailing_array, sat_add(count, SOMETHING), gfp...)
It's a bit ugly, but it would cover nearly all the remaining cases...
-Kees
-- Kees Cook Pixel Security
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