| Message ID | 20230127223816.never.413-kees@kernel.org (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | iommufd: Add top-level bounds check on kernel buffer size | expand |
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? Is this just a compiler bug? Jason
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). > > Is this just a compiler bug? I don't think so. This just keeps the bounds within the buffer size now.
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
On Fri, Jan 27, 2023 at 09:13:31PM -0400, Jason Gunthorpe wrote: > 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. memset() is internally doing that via __builtin_object_size(dst + size, 1). > There are no statically computable constants at all. I think it's some logic that excludes a range based on ucmd.user_size internally to the additional checks in copy_struct_from_user(). Regardless, I think the correct fix should be with copy_struct_from_user(), so please disregard this patch. :)
diff --git a/drivers/iommu/iommufd/main.c b/drivers/iommu/iommufd/main.c index 3fbe636c3d8a..34a1785da33a 100644 --- a/drivers/iommu/iommufd/main.c +++ b/drivers/iommu/iommufd/main.c @@ -330,8 +330,9 @@ static long iommufd_fops_ioctl(struct file *filp, unsigned int cmd, return -EINVAL; ucmd.cmd = &buf; - ret = copy_struct_from_user(ucmd.cmd, op->size, ucmd.ubuffer, - ucmd.user_size); + ret = copy_struct_from_user(ucmd.cmd, + min_t(size_t, op->size, sizeof(ucmd.cmd)), + ucmd.ubuffer, ucmd.user_size); if (ret) return ret; ret = op->execute(&ucmd);
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 | ^ ../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; | ^~~ Cc: Jason Gunthorpe <jgg@ziepe.ca> Cc: Kevin Tian <kevin.tian@intel.com> Cc: Joerg Roedel <joro@8bytes.org> Cc: Will Deacon <will@kernel.org> Cc: Robin Murphy <robin.murphy@arm.com> Cc: iommu@lists.linux.dev Signed-off-by: Kees Cook <keescook@chromium.org> --- drivers/iommu/iommufd/main.c | 5 +++-- 1 file changed, 3 insertions(+), 2 deletions(-)