From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=linaro.org header.i=@linaro.org header.b="pEoza8wV" Received: from mail-pg1-x535.google.com (mail-pg1-x535.google.com [IPv6:2607:f8b0:4864:20::535]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 4C8A110FC for ; Fri, 8 Dec 2023 19:15:26 -0800 (PST) Received: by mail-pg1-x535.google.com with SMTP id 41be03b00d2f7-5c210e34088so2238089a12.2 for ; Fri, 08 Dec 2023 19:15:26 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; t=1702091726; x=1702696526; darn=vger.kernel.org; h=mime-version:message-id:date:in-reply-to:subject:cc:to:from :user-agent:references:from:to:cc:subject:date:message-id:reply-to; bh=2b+6n+ULng3a579Szt1kcCIxHSrZ+D7LmH/Z+vqyyTE=; b=pEoza8wVnM6tjIXWzzXHyURLxumLQKhtVzfubbHMHLXafo2aNEaDG1BKQ6qKIcjqDs PBizxwWF777KEHW4PdK2suQqyYH+3EsToGtXPnPDKV40E4CCvzzmTGmT7Ysgev9/RbKz j3mnjTIxzaK46S1EEvPtDI3j7Y/N6siH5PWGzL8CHn3ocgcD57KfSfja+wD1nyiyjeCP ZzfLXRBoE+yBTbAY6njOwXDki228RkGszl44Upz9uCcBaPbLU8CHlOnQeavbIHx84lkj 1V8tljMWPcVkl4GVwI7X1g3OP9IDEckBY0lFz+PMH+xqYh3x9zhN6a7eR4sAgfUtpc0O fCfw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1702091726; x=1702696526; h=mime-version:message-id:date:in-reply-to:subject:cc:to:from :user-agent:references:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=2b+6n+ULng3a579Szt1kcCIxHSrZ+D7LmH/Z+vqyyTE=; b=lpZh0xil7EhGCpVMgJWuU8I3cC1Y8KOyH1qvIK6JsZaiPPiUtZ/hhmtMaMukbFwtxK 6ybxRdx70LHT7uhg7cKQMCNmkNcwAUV98wt5lNrD0d2GriV9nThdJ2oNXdUVkaU2F680 VvTIWmtjRstQn/kDkTEM/ylEvkHMlqyhk6xMXYx6/g1+LbsiQNMEMev9oeom8E8kGsdj fz2RlGUfsLwZP3aAX4Uu6whslMwvND2u4bF02t41HPOAGUcXvm1kYG3nFkmB12Eln3V+ dp/WKI8Uxfsf1QaciQDe5c00Zje2IPGN/Bh/EZFAxZYRl8HTC5jH/QHCC2cSde6oyUtn ZudA== X-Gm-Message-State: AOJu0Yzlm82RooS1zjA2zl1gsk1velRN+mfnIJvvukG69oLfOwglES7o VB3en9uUDY1khbMoTI6JD64wSw== X-Google-Smtp-Source: AGHT+IEaUnkmb6VMOC1ZmV7RhknH3M28z+3B6TqVq+yqgnnJz1v02mq63uTBtrSHn4oV4teABtH6uA== X-Received: by 2002:a05:6a20:f390:b0:190:6920:e14b with SMTP id qr16-20020a056a20f39000b001906920e14bmr1018200pzb.122.1702091725612; Fri, 08 Dec 2023 19:15:25 -0800 (PST) Received: from localhost ([2804:14d:7e39:8470:4c58:a216:27d2:2ff]) by smtp.gmail.com with ESMTPSA id x22-20020a056a00271600b006be5af77f06sm2330693pfv.2.2023.12.08.19.15.24 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 08 Dec 2023 19:15:25 -0800 (PST) References: <20231122-arm64-gcs-v7-0-201c483bd775@kernel.org> <20231122-arm64-gcs-v7-24-201c483bd775@kernel.org> User-agent: mu4e 1.10.8; emacs 29.1 From: Thiago Jung Bauermann To: Mark Brown Cc: Catalin Marinas , Will Deacon , Jonathan Corbet , Andrew Morton , Marc Zyngier , Oliver Upton , James Morse , Suzuki K Poulose , Arnd Bergmann , Oleg Nesterov , Eric Biederman , Kees Cook , Shuah Khan , "Rick P. Edgecombe" , Deepak Gupta , Ard Biesheuvel , Szabolcs Nagy , "H.J. Lu" , Paul Walmsley , Palmer Dabbelt , Albert Ou , Florian Weimer , Christian Brauner , linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, kvmarm@lists.linux.dev, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org Subject: Re: [PATCH v7 24/39] arm64/signal: Set up and restore the GCS context for signal handlers In-reply-to: <20231122-arm64-gcs-v7-24-201c483bd775@kernel.org> Date: Sat, 09 Dec 2023 00:15:22 -0300 Message-ID: <8734wcgj79.fsf@linaro.org> Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Mark Brown writes: > +static bool gcs_signal_cap_valid(u64 addr, u64 val) > +{ > + /* > + * The top bit should be set, this is an invalid address for > + * EL0 and will only be set for caps created by signals. > + */ > + if (!(val & GCS_SIGNAL_CAP_FLAG)) > + return false; > + > + /* The rest should be a standard architectural cap token. */ > + val &= ~GCS_SIGNAL_CAP_FLAG; > + > + /* The cap must have the low bits set to a token value */ > + if (GCS_CAP_TOKEN(val) != 0) > + return false; I found the comment above a little confusing, since the if condition actually checks that low bits aren't set at all. Perhaps reword to something like "The token value of a signal cap must be 0"? > + > + /* The cap must store the VA the cap was stored at */ > + if (GCS_CAP_ADDR(addr) != GCS_CAP_ADDR(val)) > + return false; > + > + return true; > +} > +#endif > + > /* > * Do a signal return; undo the signal stack. These are aligned to 128-bit. > */ > @@ -815,6 +847,45 @@ static int restore_sigframe(struct pt_regs *regs, > return err; > } > > +#ifdef CONFIG_ARM64_GCS > +static int gcs_restore_signal(void) > +{ > + u64 gcspr_el0, cap; > + int ret; > + > + if (!system_supports_gcs()) > + return 0; > + > + if (!(current->thread.gcs_el0_mode & PR_SHADOW_STACK_ENABLE)) > + return 0; > + > + gcspr_el0 = read_sysreg_s(SYS_GCSPR_EL0); > + > + /* > + * GCSPR_EL0 should be pointing at a capped GCS, read the cap... > + */ > + gcsb_dsync(); > + ret = copy_from_user(&cap, (__user void*)gcspr_el0, sizeof(cap)); > + if (ret) > + return -EFAULT; > + > + /* > + * ...then check that the cap is the actual GCS before > + * restoring it. > + */ > + if (!gcs_signal_cap_valid(gcspr_el0, cap)) > + return -EINVAL; > + > + current->thread.gcspr_el0 = gcspr_el0 + sizeof(cap); > + write_sysreg_s(current->thread.gcspr_el0, SYS_GCSPR_EL0); At this point, there's an inactive but valid cap just below the GCS. Over time, as different signals are received when the GCSPR is pointing at different locations of the stack, there could be a number of valid inactive caps available for misuse. I'm still not proficient enough in GCS to know how exactly this could be abused (e.g., somehow writing the desired return location right above one of these inactive caps and arranging for GCSPR to point to the cap before returning from a signal) but to be safe or paranoid, perhaps zero the location of the cap before returning? > + > + return 0; > +} > + > +#else > +static int gcs_restore_signal(void) { return 0; } > +#endif > + > SYSCALL_DEFINE0(rt_sigreturn) > { > struct pt_regs *regs = current_pt_regs(); > @@ -841,6 +912,9 @@ SYSCALL_DEFINE0(rt_sigreturn) > if (restore_altstack(&frame->uc.uc_stack)) > goto badframe; > > + if (gcs_restore_signal()) > + goto badframe; > + > return regs->regs[0]; > > badframe: > @@ -1071,7 +1145,50 @@ static int get_sigframe(struct rt_sigframe_user_layout *user, > return 0; > } > > -static void setup_return(struct pt_regs *regs, struct k_sigaction *ka, > +#ifdef CONFIG_ARM64_GCS > + > +static int gcs_signal_entry(__sigrestore_t sigtramp, struct ksignal *ksig) The ksig argument is unused, so it can be removed. > +{ > + unsigned long __user *gcspr_el0; > + int ret = 0; > + > + if (!system_supports_gcs()) > + return 0; > + > + if (!task_gcs_el0_enabled(current)) > + return 0; > + > + /* > + * We are entering a signal handler, current register state is > + * active. > + */ > + gcspr_el0 = (unsigned long __user *)read_sysreg_s(SYS_GCSPR_EL0); > + > + /* > + * Push a cap and the GCS entry for the trampoline onto the GCS. > + */ > + put_user_gcs((unsigned long)sigtramp, gcspr_el0 - 2, &ret); > + put_user_gcs(GCS_SIGNAL_CAP(gcspr_el0 - 1), gcspr_el0 - 1, &ret); > + if (ret != 0) > + return ret; > + > + gcsb_dsync(); > + > + gcspr_el0 -= 2; > + write_sysreg_s((unsigned long)gcspr_el0, SYS_GCSPR_EL0); > + > + return 0; > +} > +#else > + > +static int gcs_signal_entry(__sigrestore_t sigtramp, struct ksignal *ksig) > +{ > + return 0; > +} > + > +#endif > + > +static int setup_return(struct pt_regs *regs, struct ksignal *ksig, > struct rt_sigframe_user_layout *user, int usig) Since the ksig argument isn't used by gcs_signal_entry(), setup_return() can keep the ka argument and the changes below from ka to ksic->ka are unnecessary. > { > __sigrestore_t sigtramp; > @@ -1079,7 +1196,7 @@ static void setup_return(struct pt_regs *regs, struct k_sigaction *ka, > regs->regs[0] = usig; > regs->sp = (unsigned long)user->sigframe; > regs->regs[29] = (unsigned long)&user->next_frame->fp; > - regs->pc = (unsigned long)ka->sa.sa_handler; > + regs->pc = (unsigned long)ksig->ka.sa.sa_handler; > > /* > * Signal delivery is a (wacky) indirect function call in > @@ -1119,12 +1236,14 @@ static void setup_return(struct pt_regs *regs, struct k_sigaction *ka, > sme_smstop(); > } > > - if (ka->sa.sa_flags & SA_RESTORER) > - sigtramp = ka->sa.sa_restorer; > + if (ksig->ka.sa.sa_flags & SA_RESTORER) > + sigtramp = ksig->ka.sa.sa_restorer; > else > sigtramp = VDSO_SYMBOL(current->mm->context.vdso, sigtramp); > > regs->regs[30] = (unsigned long)sigtramp; > + > + return gcs_signal_entry(sigtramp, ksig); > } > > static int setup_rt_frame(int usig, struct ksignal *ksig, sigset_t *set, > @@ -1147,7 +1266,7 @@ static int setup_rt_frame(int usig, struct ksignal *ksig, sigset_t *set, > err |= __save_altstack(&frame->uc.uc_stack, regs->sp); > err |= setup_sigframe(&user, regs, set); > if (err == 0) { > - setup_return(regs, &ksig->ka, &user, usig); > + err = setup_return(regs, ksig, &user, usig); > if (ksig->ka.sa.sa_flags & SA_SIGINFO) { > err |= copy_siginfo_to_user(&frame->info, &ksig->info); > regs->regs[1] = (unsigned long)&frame->info; > diff --git a/arch/arm64/mm/gcs.c b/arch/arm64/mm/gcs.c > index 02f8f6046c10..6f51429c5a46 100644 > --- a/arch/arm64/mm/gcs.c > +++ b/arch/arm64/mm/gcs.c > @@ -6,6 +6,7 @@ > #include > > #include > +#include > #include This is #include isn't needed by this patch. Probably better as part of another one. -- Thiago