From patchwork Thu Apr 7 16:23:24 2022 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Alexandru Elisei X-Patchwork-Id: 12805495 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 4B999C433FE for ; Thu, 7 Apr 2022 16:40:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:MIME-Version:References:In-Reply-To: Message-Id:Date:Subject:To:From:Reply-To:Cc:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=2dUb+MjpaE9dvKvls0y7vDekEqUHF0ScVtuKjkb+9K0=; b=j3BVsYhkJY56Oe NMzq+ledjBInY1BtfsWFmK0wF+7o3AB5RFADRbnmORZqVeb51aDTjCzHSQs9BWFchFd4q/AKjhTxo xoKrW0qyHapqoDmJqLGH+EkffhwP9Fvr1EXXKRbvoRoPz5OGzaT1K+pCXHDniR/l1p0StotlBUar3 Xqfb0lXuxymfViyR87iblfSRb3jrx/YJdP2dXTaBDy5VFs0Pvc3bZkqD0VtYxBmsVHeluns7+pr0T 9yYx2+6ERytXo6nygFcxzUY9ui+1OWamtrtVLI6phaz5/7dpgyuzfLl4TORRaVyzjBWCNz19LDpwi 8Jwvdkg3LinNJ1vc/cCg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1ncV9s-00D89p-Bo; Thu, 07 Apr 2022 16:38:56 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1ncUv6-00D2Ql-P7 for linux-arm-kernel@lists.infradead.org; Thu, 07 Apr 2022 16:23:47 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id CB5E41596; Thu, 7 Apr 2022 09:23:37 -0700 (PDT) Received: from e121798.cable.virginm.net (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id 700163F73B; Thu, 7 Apr 2022 09:23:35 -0700 (PDT) From: Alexandru Elisei To: catalin.marinas@arm.com, will@kernel.org, linux-arm-kernel@lists.infradead.org, maz@kernel.org, james.morse@arm.com, suzuki.poulose@arm.com, kvmarm@lists.cs.columbia.edu, mark.rutland@arm.com Subject: [PATCH 2/5] arm64: compat: Do not treat syscall number as ESR_ELx for a bad syscall Date: Thu, 7 Apr 2022 17:23:24 +0100 Message-Id: <20220407162327.396183-3-alexandru.elisei@arm.com> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20220407162327.396183-1-alexandru.elisei@arm.com> References: <20220407162327.396183-1-alexandru.elisei@arm.com> MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20220407_092340_972377_B1C80696 X-CRM114-Status: GOOD ( 15.72 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org If a compat process tries to execute an unknown system call above the __ARM_NR_COMPAT_END number, the kernel sends a SIGILL signal to the offending process. Information about the error is printed to dmesg in compat_arm_syscall() -> arm64_notify_die() -> arm64_force_sig_fault() -> arm64_show_signal(). arm64_show_signal() interprets a non-zero value for current->thread.fault_code as an exception syndrome and displays the message associated with the ESR_ELx.EC field (bits 31:26). current->thread.fault_code is set in compat_arm_syscall() -> arm64_notify_die() with the bad syscall number instead of a valid ESR_ELx value. This means that the ESR_ELx.EC field has the value that the user set for the syscall number and the kernel can end up printing bogus exception messages*. For example, for the syscall number 0x68000000, which evaluates to ESR_ELx.EC value of 0x1A (ESR_ELx_EC_FPAC) the kernel prints this error: [ 18.349161] syscall[300]: unhandled exception: ERET/ERETAA/ERETAB, ESR 0x68000000, Oops - bad compat syscall(2) in syscall[10000+50000] [ 18.350639] CPU: 2 PID: 300 Comm: syscall Not tainted 5.18.0-rc1 #79 [ 18.351249] Hardware name: Pine64 RockPro64 v2.0 (DT) [..] which is misleading, as the bad compat syscall has nothing to do with pointer authentication. Stop arm64_show_signal() from printing exception syndrome information by having compat_arm_syscall() set the ESR_ELx value to 0, as it has no meaning for an invalid system call number. The example above now becomes: [ 19.935275] syscall[301]: unhandled exception: Oops - bad compat syscall(2) in syscall[10000+50000] [ 19.936124] CPU: 1 PID: 301 Comm: syscall Not tainted 5.18.0-rc1-00005-g7e08006d4102 #80 [ 19.936894] Hardware name: Pine64 RockPro64 v2.0 (DT) [..] which although shows less information because the syscall number, wrongfully advertised as the ESR value, is missing, it is better than showing plainly wrong information. The syscall number can be easily obtained with strace. *A 32-bit value above or equal to 0x8000_0000 is interpreted as a negative integer in compat_arm_syscal() and the condition scno < __ARM_NR_COMPAT_END evaluates to true; the syscall will exit to userspace in this case with the ENOSYS error code instead of arm64_notify_die() being called. Signed-off-by: Alexandru Elisei --- Found by code inspection. The example can be reproduced with the program (compiled for arm, not arm64): #include #include int main(void) { int fd; fd = open("/proc/sys/debug/exception-trace", O_WRONLY); if (write(fd, "1", 1) != 1) return 1; close(fd); syscall(0x1A << 26); /* ESR_ELx_EC_FPAC */ return 0; } arch/arm64/kernel/sys_compat.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/arm64/kernel/sys_compat.c b/arch/arm64/kernel/sys_compat.c index 12c6864e51e1..df14336c3a29 100644 --- a/arch/arm64/kernel/sys_compat.c +++ b/arch/arm64/kernel/sys_compat.c @@ -113,6 +113,6 @@ long compat_arm_syscall(struct pt_regs *regs, int scno) addr = instruction_pointer(regs) - (compat_thumb_mode(regs) ? 2 : 4); arm64_notify_die("Oops - bad compat syscall(2)", regs, - SIGILL, ILL_ILLTRP, addr, scno); + SIGILL, ILL_ILLTRP, addr, 0); return 0; }