Messages in this thread | | | Date | Fri, 03 Feb 2023 22:23:13 +0100 | From | "Arnd Bergmann" <> | Subject | Re: [PATCH] uaccess: Add minimum bounds check on kernel buffer size |
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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.
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?
> 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.
Arnd
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