Messages in this thread | | | Subject | Re: [tip: x86/fpu] x86/fpu/xstate: Define new functions for clearing fpregs and xstates | From | "Yu, Yu-cheng" <> | Date | Tue, 25 May 2021 10:44:08 -0700 |
| |
On 5/24/2021 9:34 AM, Andy Lutomirski wrote: > On Sat, May 16, 2020 at 8:10 AM tip-bot2 for Fenghua Yu > <tip-bot2@linutronix.de> wrote: >> >> The following commit has been merged into the x86/fpu branch of tip: >> >> Commit-ID: b860eb8dce5906b14e3a7f3c771e0b3d6ef61b94 >> Gitweb: https://git.kernel.org/tip/b860eb8dce5906b14e3a7f3c771e0b3d6ef61b94 >> Author: Fenghua Yu <fenghua.yu@intel.com> >> AuthorDate: Tue, 12 May 2020 07:54:39 -07:00 >> Committer: Borislav Petkov <bp@suse.de> >> CommitterDate: Wed, 13 May 2020 13:41:50 +02:00 >> >> x86/fpu/xstate: Define new functions for clearing fpregs and xstates > > syzbot says this is busted. I've made no effort to identify the > precise bug that is making syzbot complain, but: > >> /* >> - * Clear FPU registers by setting them up from >> - * the init fpstate: >> + * Clear FPU registers by setting them up from the init fpstate. >> + * Caller must do fpregs_[un]lock() around it. >> */ >> -static inline void copy_init_fpstate_to_fpregs(void) >> +static inline void copy_init_fpstate_to_fpregs(u64 features_mask) >> { >> - fpregs_lock(); >> - > > > >> if (use_xsave()) >> - copy_kernel_to_xregs(&init_fpstate.xsave, -1); >> + copy_kernel_to_xregs(&init_fpstate.xsave, features_mask); >> else if (static_cpu_has(X86_FEATURE_FXSR)) >> copy_kernel_to_fxregs(&init_fpstate.fxsave); >> else >> @@ -307,9 +305,6 @@ static inline void copy_init_fpstate_to_fpregs(void) >> >> if (boot_cpu_has(X86_FEATURE_OSPKE)) >> copy_init_pkru_to_fpregs(); > > if (boot_cpu_has(X86_FEATURE_OSPKE) && (features_mask & PKRU)), perhaps? > >> - >> - fpregs_mark_activate(); >> - fpregs_unlock(); >> } >> >> /* >> @@ -318,18 +313,40 @@ static inline void copy_init_fpstate_to_fpregs(void) >> * Called by sys_execve(), by the signal handler code and by various >> * error paths. >> */ >> -void fpu__clear(struct fpu *fpu) >> +static void fpu__clear(struct fpu *fpu, bool user_only) >> { >> - WARN_ON_FPU(fpu != ¤t->thread.fpu); /* Almost certainly an anomaly */ >> + WARN_ON_FPU(fpu != ¤t->thread.fpu); >> >> - fpu__drop(fpu); >> + if (!static_cpu_has(X86_FEATURE_FPU)) { >> + fpu__drop(fpu); >> + fpu__initialize(fpu); >> + return; >> + } >> >> - /* >> - * Make sure fpstate is cleared and initialized. >> - */ >> - fpu__initialize(fpu); >> - if (static_cpu_has(X86_FEATURE_FPU)) >> - copy_init_fpstate_to_fpregs(); >> + fpregs_lock(); >> + >> + if (user_only) { >> + if (!fpregs_state_valid(fpu, smp_processor_id()) && >> + xfeatures_mask_supervisor()) >> + copy_kernel_to_xregs(&fpu->state.xsave, >> + xfeatures_mask_supervisor()); > > This looks correct to me. > > So I'm guessing that syzbot may have misattributed the problem. But > we definitely need to clean up the XRSTOR #GP handling before CET > lands. >
From the crash dump, the system is doing syscall_exit_to_user_mode() for __x64_sys_futex(). The futex syscall does not seem to modify xstates, but upon returning to user mode, XRSTORS gets a GP. Can this be some memory corruption? fpu__clear() is merely helping to clear the mess and seems to be innocent.
I also run the syz repro on my Tiger Lake machine, and it only produces segfaults (no Bad FPU state, etc.).
Yu-cheng
| |