Messages in this thread | | | Date | Fri, 27 Jan 2023 21:13:31 -0400 | From | Jason Gunthorpe <> | Subject | Re: [PATCH] iommufd: Add top-level bounds check on kernel buffer size |
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On Fri, Jan 27, 2023 at 04:57:26PM -0800, Kees Cook wrote: > On Fri, Jan 27, 2023 at 08:47:34PM -0400, Jason Gunthorpe wrote: > > On Fri, Jan 27, 2023 at 02:38:17PM -0800, Kees Cook wrote: > > > While the op->size assignments are already bounds-checked at static > > > initializer time, these limits aren't aggregated and tracked when doing > > > later variable range checking under -Warray-bounds. Help the compiler > > > see that we know what we're talking about, and we'll never ask to > > > write more that sizeof(ucmd.cmd) bytes during the memset() inside > > > copy_struct_from_user(). 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 > > > > This seems strange to me > > > > I thought the way gcc handled this was if it knew the value must be in > > a certain range then it would check it > > > > If it couldn't figure out any ranges it would not make a warning. > > > > So why did it decide "rest" was in that really weird range? > > It's because it got bounds-checked at the lower end (for the minimum > size test).
Where? There is no sizeof(ucmd.ubuffer) in this code.
There are no statically computable constants at all.
The minimum size test loads from a struct:
if (ucmd.user_size < op->min_size) return -EINVAL;
So, either gcc can't see through that and thus has no idea what the bound check is
Or, gcc has figured out that struct iommufd_ioctl_op::min_size has a finite set of values
If the latter, why doesn't it also know that iommufd_ioctl_op::size has finite set too?
Combined with the weird report that the upper end of that range is -2 (not UINT_MAX), something very strange is going on inside gcc.
Jason
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