linux-kernel - [PATCH 01/13] x86/fault: Check user_mode(regs) when avoiding an mmap

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <4b89b542e8ceba9bd6abde2f386afed6d99244a9.1542667307.git.luto@kernel.org>
Date:   Mon, 19 Nov 2018 14:45:25 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH 01/13] x86/fault: Check user_mode(regs) when avoiding an mmap_sem deadlock

The fault-handling code that takes mmap_sem needs to avoid a
deadlock that could occur if the kernel took a bad (OOPS-worthy)
page fault on a user address while holding mmap_sem.  This can only
happen if the faulting instruction was in the kernel
(i.e. user_mode(regs)).  Rather than checking the sw_error_code
(which will have the USER bit set if the fault was a USER-permission
access *or* if user_mode(regs)), just check user_mode(regs)
directly.

The old code would have malfunctioned if the kernel executed a bogus
WRUSS instruction while holding mmap_sem.  Fortunately, that is
extremely unlikely in current kernels, which don't use WRUSS.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 arch/x86/mm/fault.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 71d4b9d4d43f..91d4d2722f2e 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1344,13 +1344,10 @@ void do_user_addr_fault(struct pt_regs *regs,
 	 * Only do the expensive exception table search when we might be at
 	 * risk of a deadlock.  This happens if we
 	 * 1. Failed to acquire mmap_sem, and
-	 * 2. The access did not originate in userspace.  Note: either the
-	 *    hardware or earlier page fault code may set X86_PF_USER
-	 *    in sw_error_code.
+	 * 2. The access did not originate in userspace.
 	 */
 	if (unlikely(!down_read_trylock(&mm->mmap_sem))) {
-		if (!(sw_error_code & X86_PF_USER) &&
-		    !search_exception_tables(regs->ip)) {
+		if (!user_mode(regs) && !search_exception_tables(regs->ip)) {
 			/*
 			 * Fault from code in kernel from
 			 * which we do not expect faults.
-- 
2.17.2