2017-03-22 0:16 GMT+03:00 Adam Borowski <kilobyte@xxxxxxxxxx>: > On Tue, Mar 21, 2017 at 08:47:11PM +0300, Dmitry Safonov wrote: >> After my changes to mmap(), its code now relies on the bitness of >> performing syscall. According to that, it chooses the base of allocation: >> mmap_base for 64-bit mmap() and mmap_compat_base for 32-bit syscall. >> It was done by: >> commit 1b028f784e8c ("x86/mm: Introduce mmap_compat_base() for >> 32-bit mmap()"). >> >> The code afterwards relies on in_compat_syscall() returning true for >> 32-bit syscalls. It's usually so while we're in context of application >> that does 32-bit syscalls. But during exec() it is not valid for x32 ELF. >> The reason is that the application hasn't yet done any syscall, so x32 >> bit has not being set. >> That results in -ENOMEM for x32 ELF files as there fired BAD_ADDR() >> in elf_map(), that is called from do_execve()->load_elf_binary(). >> For i386 ELFs it works as SET_PERSONALITY() sets TS_COMPAT flag. >> >> Set x32 bit before first return to userspace, during setting personality >> at exec(). This way we can rely on in_compat_syscall() during exec(). >> Do also the reverse: drop x32 syscall bit at SET_PERSONALITY for 64-bits. >> >> Fixes: commit 1b028f784e8c ("x86/mm: Introduce mmap_compat_base() for >> 32-bit mmap()") > > Tested: > with bash:x32, mksh:amd64, posh:i386, zsh:armhf (binfmt:qemu), fork+exec > works for every parent-child combination. > > Contrary to my naive initial reading of your fix, mixing syscalls from a > process of the wrong ABI also works as it did before. While using a glibc > wrapper will call the right version, x32 processes calling amd64 syscalls is > surprisingly common -- this brings seccomp joy. JFI: x32 mmap() syscall in 64 ELF should work even better - it has returned addresses over 4Gb in ia32 mmap()s, so I expect it did the same in x32 top-down allocation. (I guess you've mentioned the fixes-for patch). So the thing to check not also that mmap() returned address, but at least verify-dereference it with `mov' e.g. (or better - to parse /proc/self/maps) > I've attached a freestanding test case for write() and mmap(); it's > freestanding asm as most of you don't have an x32 toolchain at hand, sorry > for unfriendly error messages. > > So with these two patches: > x86/tls: Forcibly set the accessed bit in TLS segments > x86/mm: set x32 syscall bit in SET_PERSONALITY() > everything appears to be fine. Big thanks for the testing work, Adam! -- Dmitry -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>