[PATCH 2/3] x86: Move CPU feature test out of uaccess region

Marco Elver
2019-05-29T14:23:44Z

This patch is a pre-requisite for enabling KASAN bitops instrumentation:
moves boot_cpu_has feature test out of the uaccess region, as
boot_cpu_has uses test_bit. With instrumentation, the KASAN check would
otherwise be flagged by objtool.

This approach is preferred over adding the explicit kasan_check_*
functions to the uaccess whitelist of objtool, as the case here appears
to be the only one.

Signed-off-by: Marco Elver <elver@google.com>
---
v1:
* This patch replaces patch: 'tools/objtool: add kasan_check_* to
uaccess whitelist'
---
arch/x86/ia32/ia32_signal.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
index 629d1ee05599..12264e3c9c43 100644
--- a/arch/x86/ia32/ia32_signal.c
+++ b/arch/x86/ia32/ia32_signal.c
@@ -333,6 +333,7 @@ int ia32_setup_rt_frame(int sig, struct ksignal *ksig,
void __user *restorer;
int err = 0;
void __user *fpstate = NULL;
+ bool has_xsave;

/* __copy_to_user optimizes that into a single 8 byte store */
static const struct {
@@ -352,13 +353,19 @@ int ia32_setup_rt_frame(int sig, struct ksignal *ksig,
if (!access_ok(frame, sizeof(*frame)))
return -EFAULT;

+ /*
+ * Move non-uaccess accesses out of uaccess region if not strictly
+ * required; this also helps avoid objtool flagging these accesses with
+ * instrumentation enabled.
+ */
+ has_xsave = boot_cpu_has(X86_FEATURE_XSAVE);
put_user_try {
put_user_ex(sig, &frame->sig);
put_user_ex(ptr_to_compat(&frame->info), &frame->pinfo);
put_user_ex(ptr_to_compat(&frame->uc), &frame->puc);

/* Create the ucontext. */
- if (boot_cpu_has(X86_FEATURE_XSAVE))
+ if (has_xsave)
put_user_ex(UC_FP_XSTATE, &frame->uc.uc_flags);
else
put_user_ex(0, &frame->uc.uc_flags);
--
2.22.0.rc1.257.g3120a18244-goog