The following commit has been merged into the objtool/core branch of tip: Commit-ID: b0b8e56b82c06b3bb6e5fb66d0e9c9c3fd3ce555 Gitweb: https://git.kernel.org/tip/b0b8e56b82c06b3bb6e5fb66d0e9c9c3fd3ce555 Author: Jann Horn <jannh@xxxxxxxxxx> AuthorDate: Tue, 29 Sep 2020 00:49:16 +02:00 Committer: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> CommitterDate: Fri, 02 Oct 2020 09:28:08 -05:00 objtool: Permit __kasan_check_{read,write} under UACCESS 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. PeterZ adds: 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 <jannh@xxxxxxxxxx> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> --- tools/objtool/check.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/tools/objtool/check.c b/tools/objtool/check.c index 2df9f76..3d14134 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",
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