[PATCH] iommufd: Add top-level bounds check on kernel buffer size

[Kees Cook <keescook@chromium.org>]

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(-)

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);
-- 
2.34.1