[PATCH 2/6] x86/ia32: Fix ia32_restore_sigcontext AC leak

Peter Zijlstra
2019-02-25T12:53:28Z

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@infradead.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();