linux-kernel - [PATCH v4 29/34] x86/ia32: do not modify the DPL bits for a null selector

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Date:   Wed,  1 Mar 2023 21:25:06 -0800
From:   Xin Li <xin3.li@...el.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, peterz@...radead.org,
        andrew.cooper3@...rix.com, seanjc@...gle.com, pbonzini@...hat.com,
        ravi.v.shankar@...el.com
Subject: [PATCH v4 29/34] x86/ia32: do not modify the DPL bits for a null selector

When a null selector is to be loaded into a segment register,
reload_segments() sets its DPL bits to 3. Later when the IRET
instruction loads it, it zeros the segment register. The two
operations offset each other to actually effect a nop.

Unlike IRET, ERETU does not make any of DS, ES, FS, or GS null
if it is found to have DPL < 3. It is expected that a FRED-enabled
operating system will return to ring 3 (in compatibility mode)
only when those segments all have DPL = 3.

Thus when FRED is enabled, we end up with having 3 in a segment
register even when it is initially set to 0.

Fix it by not modifying the DPL bits for a null selector.

Tested-by: Shan Kang <shan.kang@...el.com>
Signed-off-by: Xin Li <xin3.li@...el.com>
---
 arch/x86/kernel/signal_32.c | 21 +++++++++++++--------
 1 file changed, 13 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/signal_32.c b/arch/x86/kernel/signal_32.c
index 9027fc088f97..7796cf84fca2 100644
--- a/arch/x86/kernel/signal_32.c
+++ b/arch/x86/kernel/signal_32.c
@@ -36,22 +36,27 @@
 #ifdef CONFIG_IA32_EMULATION
 #include <asm/ia32_unistd.h>
 
+static inline u16 usrseg(u16 sel)
+{
+	return sel <= 3 ? sel : sel | 3;
+}
+
 static inline void reload_segments(struct sigcontext_32 *sc)
 {
 	unsigned int cur;
 
 	savesegment(gs, cur);
-	if ((sc->gs | 0x03) != cur)
-		load_gs_index(sc->gs | 0x03);
+	if (usrseg(sc->gs) != cur)
+		load_gs_index(usrseg(sc->gs));
 	savesegment(fs, cur);
-	if ((sc->fs | 0x03) != cur)
-		loadsegment(fs, sc->fs | 0x03);
+	if (usrseg(sc->fs) != cur)
+		loadsegment(fs, usrseg(sc->fs));
 	savesegment(ds, cur);
-	if ((sc->ds | 0x03) != cur)
-		loadsegment(ds, sc->ds | 0x03);
+	if (usrseg(sc->ds) != cur)
+		loadsegment(ds, usrseg(sc->ds));
 	savesegment(es, cur);
-	if ((sc->es | 0x03) != cur)
-		loadsegment(es, sc->es | 0x03);
+	if (usrseg(sc->es) != cur)
+		loadsegment(es, usrseg(sc->es));
 }
 
 #define sigset32_t			compat_sigset_t
-- 
2.34.1