On Mon, Apr 21, 2014 at 6:47 PM, H. Peter Anvin <h...@linux.intel.com> wrote: > This is a prototype of espfix for the 64-bit kernel. espfix is a > workaround for the architectural definition of IRET, which fails to > restore bits [31:16] of %esp when returning to a 16-bit stack > segment. We have a workaround for the 32-bit kernel, but that > implementation doesn't work for 64 bits.
Hi, A comment: The main purpose of espfix is to prevent attackers from learning sensitive addresses, right? But as far as I can tell, this mini-stack becomes itself somewhat sensitive: - The user can put arbitrary data in registers before returning to the LDT in order to get it saved at a known address accessible from the kernel. With SMAP and KASLR this might otherwise be difficult. - If the iret faults, kernel addresses will get stored there (and not cleared). If a vulnerability could return data from an arbitrary specified address to the user, this would be harmful. I guess with the current KASLR implementation you could get the same effects via brute force anyway, by filling up and browsing memory, respectively, but ideally there wouldn't be any virtual addresses guaranteed not to fault. - If a vulnerability allowed overwriting data at an arbitrary specified address, the exception frame could get overwritten at exactly the right moment between the copy and iret (or right after the iret to mess up fixup_exception)? You probably know better than I whether or not caches prevent this from actually being possible. Just raising the issue. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
