linux-kernel - [PATCH 2/6] x86/ia32: Fix ia32_restore

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Message-Id: <20190225125231.936952143@infradead.org>
Date:   Mon, 25 Feb 2019 13:43:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     torvalds@...ux-foundation.org, tglx@...utronix.de, hpa@...or.com,
        julien.thierry@....com, will.deacon@....com, luto@...capital.net,
        mingo@...nel.org, catalin.marinas@....com, james.morse@....com,
        valentin.schneider@....com, brgerst@...il.com, jpoimboe@...hat.com,
        luto@...nel.org, bp@...en8.de, dvlasenk@...hat.com
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org
Subject: [PATCH 2/6] x86/ia32: Fix ia32_restore_sigcontext AC leak

Don't call load_gs_index() with AC set; delay the segment setting
until after the AC section.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/ia32/ia32_signal.c |   29 +++++++++++++++++++----------
 1 file changed, 19 insertions(+), 10 deletions(-)

--- a/arch/x86/ia32/ia32_signal.c
+++ b/arch/x86/ia32/ia32_signal.c
@@ -60,17 +60,21 @@
 	regs->seg = GET_SEG(seg) | 3;			\
 } while (0)
 
-#define RELOAD_SEG(seg)		{		\
-	unsigned int pre = GET_SEG(seg);	\
-	unsigned int cur = get_user_seg(seg);	\
-	pre |= 3;				\
-	if (pre != cur)				\
-		set_user_seg(seg, pre);		\
+#define LOAD_SEG(seg)		{			\
+	pre_##seg = 3 | GET_SEG(seg);			\
+	cur_##seg = get_user_seg(seg);			\
+}
+
+#define RELOAD_SEG(seg)		{			\
+	if (pre_##seg != cur_##seg)			\
+		set_user_seg(seg, pre_##seg);		\
 }
 
 static int ia32_restore_sigcontext(struct pt_regs *regs,
 				   struct sigcontext_32 __user *sc)
 {
+	u16 pre_gs, pre_fs, pre_ds, pre_es;
+	u16 cur_gs, cur_fs, cur_ds, cur_es;
 	unsigned int tmpflags, err = 0;
 	void __user *buf;
 	u32 tmp;
@@ -85,10 +89,10 @@ static int ia32_restore_sigcontext(struc
 		 * the handler, but does not clobber them at least in the
 		 * normal case.
 		 */
-		RELOAD_SEG(gs);
-		RELOAD_SEG(fs);
-		RELOAD_SEG(ds);
-		RELOAD_SEG(es);
+		LOAD_SEG(gs);
+		LOAD_SEG(fs);
+		LOAD_SEG(ds);
+		LOAD_SEG(es);
 
 		COPY(di); COPY(si); COPY(bp); COPY(sp); COPY(bx);
 		COPY(dx); COPY(cx); COPY(ip); COPY(ax);
@@ -106,6 +110,11 @@ static int ia32_restore_sigcontext(struc
 		buf = compat_ptr(tmp);
 	} get_user_catch(err);
 
+	RELOAD_SEG(gs);
+	RELOAD_SEG(fs);
+	RELOAD_SEG(ds);
+	RELOAD_SEG(es);
+
 	err |= fpu__restore_sig(buf, 1);
 
 	force_iret();