On Tue, Sep 29, 2020 at 12:49:16AM +0200, Jann Horn wrote: > Building linux-next with JUMP_LABEL=n and KASAN=y, I got this objtool > warning: > > arch/x86/lib/copy_mc.o: warning: objtool: copy_mc_to_user()+0x22: call to > __kasan_check_read() with UACCESS enabled > > What happens here is that copy_mc_to_user() branches on a static key in a > UACCESS region: > > __uaccess_begin(); > if (static_branch_unlikely(©_mc_fragile_key)) > ret = copy_mc_fragile(to, from, len); > ret = copy_mc_generic(to, from, len); > __uaccess_end(); > > and the !CONFIG_JUMP_LABEL version of static_branch_unlikely() uses > static_key_enabled(), which uses static_key_count(), which uses > atomic_read(), which calls instrument_atomic_read(), which uses > kasan_check_read(), which is __kasan_check_read(). > > Let's permit these KASAN helpers in UACCESS regions - static keys should > probably work under UACCESS, I think.
It's not a matter of permitting, it's a matter of being safe and correct. In this case it is, because it's a thin wrapper around check_memory_region() which was already marked safe. check_memory_region() is correct because the only thing it ends up calling is kasa_report() and that is also marked safe because that is annotated with user_access_save/restore() before it does anything else. On top of that, all of KASAN is noinstr, so nothing in here will end up in tracing and/or call schedule() before the user_access_save(). > Signed-off-by: Jann Horn <[email protected]> Acked-by: Peter Zijlstra (Intel) <[email protected]> > --- > Calling atomic_read() on a global under UACCESS should probably be fine, > right? Yes, per the above. > tools/objtool/check.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/tools/objtool/check.c b/tools/objtool/check.c > index a88fb05242d5..1141a8e26c1e 100644 > --- a/tools/objtool/check.c > +++ b/tools/objtool/check.c > @@ -583,6 +583,8 @@ static const char *uaccess_safe_builtin[] = { > "__asan_store4_noabort", > "__asan_store8_noabort", > "__asan_store16_noabort", > + "__kasan_check_read", > + "__kasan_check_write", > /* KASAN in-line */ > "__asan_report_load_n_noabort", > "__asan_report_load1_noabort", > > base-commit: 0248dedd12d43035bf53c326633f0610a49d7134 > -- > 2.28.0.709.gb0816b6eb0-goog >
