Messages in this thread | | | Date | Fri, 3 Feb 2023 22:27:54 +0000 | From | Kees Cook <> | Subject | Re: [PATCH] uaccess: Add minimum bounds check on kernel buffer size |
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On Fri, Feb 03, 2023 at 10:23:13PM +0100, Arnd Bergmann wrote: > On Fri, Feb 3, 2023, at 20:35, Kees Cook wrote: > > While there is logic about the difference between ksize and usize, > > copy_struct_from_user() didn't check the size of the destination buffer > > (when it was known) against ksize. Add this check so there is an upper > > bounds check on the possible memset() call, otherwise lower bounds > > checks made by callers will trigger bounds warnings under -Warray-bounds. > > Seen under GCC 13: > > > > In function 'copy_struct_from_user', > > inlined from 'iommufd_fops_ioctl' at > > ../drivers/iommu/iommufd/main.c:333:8: > > ../include/linux/fortify-string.h:59:33: warning: '__builtin_memset' > > offset [57, 4294967294] is out of the bounds [0, 56] of object 'buf' > > with type 'union ucmd_buffer' [-Warray-bounds=] > > 59 | #define __underlying_memset __builtin_memset > > | ^ > > ../include/linux/fortify-string.h:453:9: note: in expansion of macro > > '__underlying_memset' > > 453 | __underlying_memset(p, c, __fortify_size); \ > > | ^~~~~~~~~~~~~~~~~~~ > > ../include/linux/fortify-string.h:461:25: note: in expansion of macro > > '__fortify_memset_chk' > > 461 | #define memset(p, c, s) __fortify_memset_chk(p, c, s, \ > > | ^~~~~~~~~~~~~~~~~~~~ > > ../include/linux/uaccess.h:334:17: note: in expansion of macro 'memset' > > 334 | memset(dst + size, 0, rest); > > | ^~~~~~ > > ../drivers/iommu/iommufd/main.c: In function 'iommufd_fops_ioctl': > > ../drivers/iommu/iommufd/main.c:311:27: note: 'buf' declared here > > 311 | union ucmd_buffer buf; > > | ^~~ > > > > Hi Kees, > > I started building with gcc-13.0.1 myself but ran into a lot of > other -Warray-bounds warnings in randconfig builds, so I ended up > turning it off once more with CONFIG_CC_NO_ARRAY_BOUNDS in order > to keep building without warnings.
Understood. AFAIK, all the open bugs I (and you) filed with GCC 13 have been fixed related to -Warray-bounds. The most recent was the misbehavior between CONFIG_UBSAN_SHIFT and -Warray-bounds. (Though the shift checking still exposes some warnings since it introduces an implicit bounds check on the shift variable, but they're not _wrong_ any more.)
> Is there anything else I need to do to get to the point of > just addressing actual issues instead of false positives? > Do you already have a patch series for fixing the others?
I've been working through the list that I see when building with -Warray-bounds and -fstrict-flex-arrays=3. Some are real bugs, as usual.
> > diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h > > index afb18f198843..ab9728138ad6 100644 > > --- a/include/linux/uaccess.h > > +++ b/include/linux/uaccess.h > > @@ -329,6 +329,10 @@ copy_struct_from_user(void *dst, size_t ksize, > > const void __user *src, > > size_t size = min(ksize, usize); > > size_t rest = max(ksize, usize) - size; > > > > + /* Double check if ksize is larger than a known object size. */ > > + if (WARN_ON_ONCE(ksize > __builtin_object_size(dst, 1))) > > + return -E2BIG; > > + > > WARN_ON_ONCE() may be a little expensive since that adds two > comparisons and a static variable to each copy, but it's probably > fine.
Yeah. IMO, copy_struct_from_user() is not fast path and having better bounds checking when coming from userspace is well worth it.
-- Kees Cook
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