linux-kernel - [tip:x86/asm] x86/arch_prctl: Fix ARCH_GET_FS and ARCH_GET

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Date:	Wed, 13 Apr 2016 04:29:02 -0700
From:	tip-bot for Andy Lutomirski <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	peterz@...radead.org, mingo@...nel.org, luto@...capital.net,
	torvalds@...ux-foundation.org, r.marek@...embler.cz,
	dvlasenk@...hat.com, bp@...en8.de, linux-kernel@...r.kernel.org,
	bp@...e.de, brgerst@...il.com, luto@...nel.org, tglx@...utronix.de,
	hpa@...or.com
Subject: [tip:x86/asm] x86/arch_prctl: Fix ARCH_GET_FS and ARCH_GET_GS

Commit-ID:  d47b50e7a111bb7a56fb1c974728b56209d7f515
Gitweb:     http://git.kernel.org/tip/d47b50e7a111bb7a56fb1c974728b56209d7f515
Author:     Andy Lutomirski <luto@...nel.org>
AuthorDate: Thu, 7 Apr 2016 17:31:45 -0700
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 13 Apr 2016 10:20:41 +0200

x86/arch_prctl: Fix ARCH_GET_FS and ARCH_GET_GS

ARCH_GET_FS and ARCH_GET_GS attempted to figure out the fsbase and
gsbase respectively from saved thread state.  This was wrong: fsbase
and gsbase live in registers while a thread is running, not in
memory.

For reasons I can't fathom, the fsbase and gsbase code were
different.  Since neither was correct, I didn't try to figure out
what the point of the difference was.

Change it to simply read the MSRs.

The code for reading the base for a remote thread is also completely
wrong if the target thread uses its own descriptors (which is the case
for all 32-bit threaded programs), but fixing that is a different
story.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
Reviewed-by: Borislav Petkov <bp@...e.de>
Cc: Andy Lutomirski <luto@...capital.net>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Brian Gerst <brgerst@...il.com>
Cc: Denys Vlasenko <dvlasenk@...hat.com>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Rudolf Marek <r.marek@...embler.cz>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/c6e7b507c72ca3bdbf6c7a8a3ceaa0334e873bd9.1460075211.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 arch/x86/kernel/process_64.c | 19 +++++++------------
 1 file changed, 7 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 6cbab31..c671b9b 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -566,10 +566,10 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr)
 		break;
 	case ARCH_GET_FS: {
 		unsigned long base;
-		if (task->thread.fsindex == FS_TLS_SEL)
-			base = read_32bit_tls(task, FS_TLS);
-		else if (doit)
+		if (doit)
 			rdmsrl(MSR_FS_BASE, base);
+		else if (task->thread.fsindex == FS_TLS_SEL)
+			base = read_32bit_tls(task, FS_TLS);
 		else
 			base = task->thread.fs;
 		ret = put_user(base, (unsigned long __user *)addr);
@@ -577,16 +577,11 @@ long do_arch_prctl(struct task_struct *task, int code, unsigned long addr)
 	}
 	case ARCH_GET_GS: {
 		unsigned long base;
-		unsigned gsindex;
-		if (task->thread.gsindex == GS_TLS_SEL)
+		if (doit)
+			rdmsrl(MSR_KERNEL_GS_BASE, base);
+		else if (task->thread.gsindex == GS_TLS_SEL)
 			base = read_32bit_tls(task, GS_TLS);
-		else if (doit) {
-			savesegment(gs, gsindex);
-			if (gsindex)
-				rdmsrl(MSR_KERNEL_GS_BASE, base);
-			else
-				base = task->thread.gs;
-		} else
+		else
 			base = task->thread.gs;
 		ret = put_user(base, (unsigned long __user *)addr);
 		break;