On Thu, Feb 9, 2017 at 3:05 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > On Thu, Feb 9, 2017 at 11:31 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote: >> On Thu, Feb 9, 2017 at 10:33 AM, Thomas Garnier <thgarnie@xxxxxxxxxx> wrote: >>> This patch prevents a syscall to modify the address limit of the >>> caller. The address limit is kept by the syscall wrapper and restored >>> just after the syscall ends. >>> >>> For example, it would mitigation this bug: >>> >>> - https://bugs.chromium.org/p/project-zero/issues/detail?id=990 >>> >>> Signed-off-by: Thomas Garnier <thgarnie@xxxxxxxxxx> >>> --- >>> Based on next-20170209 >>> --- >>> include/linux/syscalls.h | 5 ++++- >>> 1 file changed, 4 insertions(+), 1 deletion(-) >>> >>> diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h >>> index 91a740f6b884..a1b6a62a9849 100644 >>> --- a/include/linux/syscalls.h >>> +++ b/include/linux/syscalls.h >>> @@ -198,7 +198,10 @@ extern struct trace_event_functions exit_syscall_print_funcs; >>> asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__)); \ >>> asmlinkage long SyS##name(__MAP(x,__SC_LONG,__VA_ARGS__)) \ >>> { \ >>> - long ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__)); \ >>> + long ret; \ >>> + mm_segment_t fs = get_fs(); \ >>> + ret = SYSC##name(__MAP(x,__SC_CAST,__VA_ARGS__)); \ >>> + set_fs(fs); \ >>> __MAP(x,__SC_TEST,__VA_ARGS__); \ >>> __PROTECT(x, ret,__MAP(x,__SC_ARGS,__VA_ARGS__)); \ >>> return ret; \ >>> -- >>> 2.11.0.483.g087da7b7c-goog >>> >> >> I have a memory of Andy looking at this before, and there was some >> problem with how a bunch of compat code would set fs and then re-call >> the syscall... but I can't quite find the conversation. Andy, do you >> remember the details? >> >> This seems like an entirely reasonable thing to enforce for syscalls, >> though I'm sure there's a gotcha somewhere. :) > > This sounds vaguely familiar, but that's about all. > > Anyway, it seems reasonable that the SyS_foobar wrappers are genuinely > only used for syscalls and not for other things, so the code should > *work*. That being said, I think there's room for several > improvements. > > 1. Why save the old "fs" value? For that matter, why restore it? > IOW, I'd rather see BUG_ON(get_fs() != USER_DS) at the end. > I guess that make sense in the wrapper. > 2. I'd rather see the mechanism be more general. If we had, effectively: > > asmlinkage long SyS_foo(...) { > sys_foo(); > verify_pre_usermode_state(); > } > > and let verify_pre_usermode_state() potentially do more things, we'd > get a more flexible mechanism. On arches like x86_32, we could save a > decent amount of code size by moving verify_pre_usermode_state() into > prepare_exit_to_usermode(), but that would have to be a per-arch > opt-in. x86_64 probably would *not* select this due to the fast path > (or it would do it in asm. hmm.). > I will look into that. I like this design better. > 3. If this thing gets factored out, then arch code can call it for > non-syscall entries, too. > Yes, it makes sense. > 4. Can we make this configurable? > > > For x86, a nice implementation might be: > > select ARCH_NO_SYSCALL_VERIFY_PRE_USERMODE_STATE > > ... in prepare_exit_to_usermode(): > > verify_pre_usermode_state(); // right at the beginning > > ... in the asm syscall fast path: > > #ifdef CONFIG_VERIFY_PRE_USERMODE_STATE > call verify_pre_usermode_staet > #endif > > (or just inline the interesting bit) > So by default it is in the wrapper. If selected, an architecture can disable the wrapper put it in the best places. Understood correctly? > --Andy -- Thomas -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
