Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <85bd8614-9a55-3113-e5a8-b7e73f636135@openvz.org>
Date:   Thu, 30 Jun 2022 08:58:36 +0300
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.9.1
From:   Vasily Averin <vvs@openvz.org>
Subject: [PATCH] x86/fault: ignore RSVD flag in error code if P flag is 0
To:     Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>
Cc:     kernel@openvz.org, linux-kernel@vger.kernel.org,
        Konstantin Khorenko <khorenko@virtuozzo.com>,
        steve.sipes@comandsolutions.com
References: <9bc8de5f-fd80-57fe-0169-0ec942638299@openvz.org>
Content-Language: en-US
In-Reply-To: <9bc8de5f-fd80-57fe-0169-0ec942638299@openvz.org>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

Some older Intel CPUs have errata:
"Not-Present Page Faults May Set the RSVD Flag in the Error Code

Problem:
An attempt to access a page that is not marked present causes a page
fault. Such a page fault delivers an error code in which both the
P flag (bit 0) and the RSVD flag (bit 3) are 0. Due to this erratum,
not-present page faults may deliver an error code in which the P flag
is 0 but the RSVD flag is 1.

Implication:
Software may erroneously infer that a page fault was due to a
reserved-bit violation when it was actually due to an attempt
to access a not-present page.

Workaround: Page-fault handlers should ignore the RSVD flag in the error
code if the P flag is 0."

This issues was observed on several nodes crashed with messages
httpd: Corrupted page table at address 7f62d5b48e68
PGD 80000002e92bf067 PUD 1c99c5067 PMD 195015067 PTE 7fffffffb78b680
Bad pagetable: 000c [#1] SMP

Let's follow the recommendation and will ignore the RSVD flag in the
error code if the P flag is 0

Link: https://lore.kernel.org/all/aae9c7c6-989c-0261-470a-252537493b53@openvz.org
Signed-off-by: Vasily Averin <vvs@openvz.org>
---
 arch/x86/mm/fault.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index fe10c6d76bac..ffc6d6bd2a22 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1481,6 +1481,15 @@ handle_page_fault(struct pt_regs *regs, unsigned long error_code,
 	if (unlikely(kmmio_fault(regs, address)))
 		return;
 
+	/*
+	 * Some older Intel CPUs have errata
+	 * "Not-Present Page Faults May Set the RSVD Flag in the Error Code"
+	 * It is recommended to ignore the RSVD flag (bit 3) in the error code
+	 * if the P flag (bit 0) is 0.
+	 */
+	if (unlikely((error_code & X86_PF_RSVD) && !(error_code & X86_PF_PROT)))
+		error_code &= ~X86_PF_RSVD;
+
 	/* Was the fault on kernel-controlled part of the address space? */
 	if (unlikely(fault_in_kernel_space(address))) {
 		do_kern_addr_fault(regs, error_code, address);
-- 
2.36.1