The PCIOCLISTVPD ioctl on /dev/pci is used to fetch a list of VPD key-value pairs for a specific PCI function. It is used by 'pciconf -l -V'. The list is stored in a userland-supplied buffer as an array of variable-length structures where the key and data length are stored in a fixed-size header followed by the variable-length value as a byte array. To facilitate walking this array in userland, <sys/pciio.h> provides a PVE_NEXT() helper macro to return a pointer to the next array element by reading the the length out of the current header and using it to compute the address of the next header.
To simplify the implementation, the ioctl handler was also using PVE_NEXT() when on the user address of the user buffer to compute the user address of the next array element. However, the PVE_NEXT() macro when used with a user address was reading the value's length by indirecting the user pointer. The value was ready after the current record had been copied out to the user buffer, so it appeared to work on architectures where user addresses are directly dereferencable from the kernel (all but powerpc and i386 after the 4:4 split). The recent enablement of SMAP on amd64 caught this violation however. To fix, add a variant of PVE_NEXT() for use in the ioctl handler that takes an explicit value length.
Reported by: Jeffrey Pieper @ Intel