On Fri, Mar 23, 2018 at 05:48:06AM -0700, Matthew Wilcox wrote: > On Thu, Mar 22, 2018 at 07:36:36PM +0300, Ilya Smith wrote: > > Current implementation doesn't randomize address returned by mmap. > > All the entropy ends with choosing mmap_base_addr at the process > > creation. After that mmap build very predictable layout of address > > space. It allows to bypass ASLR in many cases. This patch make > > randomization of address on any mmap call. > > Why should this be done in the kernel rather than libc? libc is perfectly > capable of specifying random numbers in the first argument of mmap. Generally libc does not have a view of the current vm maps, and thus in passing "random numbers", they would have to be uniform across the whole vm space and thus non-uniform once the kernel rounds up to avoid existing mappings. Also this would impose requirements that libc be aware of the kernel's use of the virtual address space and what's available to userspace -- for example, on 32-bit archs whether 2GB, 3GB, or full 4GB (for 32-bit-user-on-64-bit-kernel) is available, and on 64-bit archs where fewer than the full 64 bits are actually valid in addresses, what the actual usable pointer size is. There is currently no clean way of conveying this information to userspace. Rich
