On Sat, Jun 24, 2017 at 2:11 AM, Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx> wrote: > On Fri, Jun 23, 2017 at 01:59:55PM -0700, Kees Cook wrote: >> This is v2 (to refresh the 5 patches in -mm) for moving ELF_ET_DYN_BASE >> safely lower. Changes are clarifications in the commit logs (suggested >> by mpe), a compat think-o fix for arm64 (thanks to Ard), and to add >> Rik and mpe's Acks. >> >> Quoting patch 1/5: >> >> The ELF_ET_DYN_BASE position was originally intended to keep loaders >> away from ET_EXEC binaries. (For example, running "/lib/ld-linux.so.2 >> /bin/cat" might cause the subsequent load of /bin/cat into where the >> loader had been loaded.) With the advent of PIE (ET_DYN binaries with >> an INTERP Program Header), ELF_ET_DYN_BASE continued to be used since >> the kernel was only looking at ET_DYN. However, since ELF_ET_DYN_BASE >> is traditionally set at the top 1/3rd of the TASK_SIZE, a substantial >> portion of the address space is unused. > > With existing kernels on ARM: > > 00010000-00017000 r-xp 00000000 08:01 270810 /bin/cat > 00026000-00027000 r--p 00006000 08:01 270810 /bin/cat > 00027000-00028000 rw-p 00007000 08:01 270810 /bin/cat > 7f661000-7f679000 r-xp 00000000 08:01 281659 /lib/arm-linux-gnueabihf/ld-2.23.so > 7f688000-7f689000 r--p 00017000 08:01 281659 /lib/arm-linux-gnueabihf/ld-2.23.so > 7f689000-7f68a000 rw-p 00018000 08:01 281659 /lib/arm-linux-gnueabihf/ld-2.23.so > > If the loader is loaded at 4MB, this means the size of an ET_EXEC > program is limited to less than 4MB - and distros aren't yet > building everything as PIE on ARM. The loader isn't loaded at 4MB; that's what patch 1 changes: loaders are moved into the mmap region so they will not collide with either ET_EXEC nor PIE (ET_DYN-with-INTERP). (After this patch, the name "ELF_ET_DYN_BASE" becomes a bit misleading...) -Kees -- Kees Cook Pixel Security
