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From: Andy Lutomirski <luto@kernel.org>
To: x86@kernel.org
Cc: LKML <linux-kernel@vger.kernel.org>,
	Yu-cheng Yu <yu-cheng.yu@intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>
Subject: [PATCH 02/13] x86/fault: Check user_mode(regs) when validating a stack extension
Date: Mon, 19 Nov 2018 14:45:26 -0800	[thread overview]
Message-ID: <e2e8e92d67d64700ba88aec73a4fc2b7986c0f87.1542667307.git.luto@kernel.org> (raw)
In-Reply-To: <cover.1542667307.git.luto@kernel.org>
In-Reply-To: <cover.1542667307.git.luto@kernel.org>

The fault handling code tries to validate that a page fault from
user mode that would extend the stack is within a certain range of
the user SP.  regs->sp is only equal to the user SP if
user_mode(regs).  In the extremely unlikely event that that
sw_error_code had the USER bit set but the faulting instruction was
in the kernel (i.e. the faulting instruction was WRUSS), then the
*kernel* stack pointer would have been checked, which would be an
info leak.

Note to -stable maintainers: don't backport this unless you backport
CET.  The bug it fixes is unobservable in current kernels.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/mm/fault.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 91d4d2722f2e..eae7ee3ce89b 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1377,7 +1377,7 @@ void do_user_addr_fault(struct pt_regs *regs,
 		bad_area(regs, sw_error_code, address);
 		return;
 	}
-	if (sw_error_code & X86_PF_USER) {
+	if (user_mode(regs)) {
 		/*
 		 * Accessing the stack below %sp is always a bug.
 		 * The large cushion allows instructions like enter
-- 
2.17.2


  parent reply	other threads:[~2018-11-19 22:45 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-19 22:45 [PATCH 00/13] x86/fault: #PF improvements, mostly related to USER bit Andy Lutomirski
2018-11-19 22:45 ` [PATCH 01/13] x86/fault: Check user_mode(regs) when avoiding an mmap_sem deadlock Andy Lutomirski
2018-11-20  8:14   ` [tip:x86/mm] " tip-bot for Andy Lutomirski
2018-11-20  8:15   ` [PATCH 01/13] " Peter Zijlstra
2018-11-19 22:45 ` Andy Lutomirski [this message]
2018-11-20  7:39   ` [PATCH 02/13] x86/fault: Check user_mode(regs) when validating a stack extension Ingo Molnar
2018-11-20  8:13     ` Ingo Molnar
2018-11-19 22:45 ` [PATCH 03/13] x86/cpufeatures, x86/fault: Mark SMAP as disabled when configured out Andy Lutomirski
2018-11-20  8:15   ` [tip:x86/mm] " tip-bot for Andy Lutomirski
2018-11-19 22:45 ` [PATCH 04/13] x86/fault: Fold smap_violation() into do_user_addr_fault() Andy Lutomirski
2018-11-20  8:15   ` [tip:x86/mm] " tip-bot for Andy Lutomirski
2018-11-19 22:45 ` [PATCH 05/13] x86/fault: Fix SMAP #PF handling buglet for implicit supervisor accesses Andy Lutomirski
2018-11-20  8:16   ` [tip:x86/mm] " tip-bot for Andy Lutomirski
2018-11-19 22:45 ` [PATCH 06/13] x86/fault: Improve the condition for signalling vs OOPSing Andy Lutomirski
2018-11-20  8:16   ` [tip:x86/mm] " tip-bot for Andy Lutomirski
2018-11-19 22:45 ` [PATCH 07/13] x86/fault: Make error_code sanitization more robust Andy Lutomirski
2018-11-20  8:17   ` [tip:x86/mm] " tip-bot for Andy Lutomirski
2018-11-19 22:45 ` [PATCH 08/13] x86/fault: Don't set thread.cr2, etc before OOPSing Andy Lutomirski
2018-11-20  8:17   ` [tip:x86/mm] " tip-bot for Andy Lutomirski
2018-11-19 22:45 ` [PATCH 09/13] x86/fault: Remove sw_error_code Andy Lutomirski
2018-11-19 22:45 ` [PATCH 10/13] x86/fault: Don't try to recover from an implicit supervisor access Andy Lutomirski
2018-11-19 22:45 ` [PATCH 11/13] x86/oops: Show the correct CS value in show_regs() Andy Lutomirski
2018-11-19 22:45 ` [PATCH 12/13] x86/fault: Decode page fault OOPSes better Andy Lutomirski
2018-11-27 14:46   ` Sean Christopherson
2018-11-19 22:45 ` [PATCH 13/13] x86/vsyscall/64: Use X86_PF constants in the simulated #PF error code Andy Lutomirski

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