The following commit has been merged into the x86/urgent branch of tip:
Commit-ID: cc801833a171163edb6385425349ba8903bd1b20
Gitweb: https://git.kernel.org/tip/cc801833a171163edb6385425349ba8903bd1b20
Author: Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Fri, 03 Jul 2020 10:02:57 -07:00
Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Sat, 04 Jul 2020 19:47:26 +02:00
x86/ldt: Disable 16-bit segments on Xen PV
Xen PV doesn't implement ESPFIX64, so they don't work right. Disable
them. Also print a warning the first time anyone tries to use a
16-bit segment on a Xen PV guest that would otherwise allow it
to help people diagnose this change in behavior.
This gets us closer to having all x86 selftests pass on Xen PV.
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/92b2975459dfe5929ecf34c3896ad920bd9e3f2d.1593795633.git.luto@xxxxxxxxxx
---
arch/x86/kernel/ldt.c | 35 ++++++++++++++++++++++++++++++++++-
1 file changed, 34 insertions(+), 1 deletion(-)
diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
index 8748321..34e918a 100644
--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -29,6 +29,8 @@
#include <asm/mmu_context.h>
#include <asm/pgtable_areas.h>
+#include <xen/xen.h>
+
/* This is a multiple of PAGE_SIZE. */
#define LDT_SLOT_STRIDE (LDT_ENTRIES * LDT_ENTRY_SIZE)
@@ -543,6 +545,37 @@ static int read_default_ldt(void __user *ptr, unsigned long bytecount)
return bytecount;
}
+static bool allow_16bit_segments(void)
+{
+ if (!IS_ENABLED(CONFIG_X86_16BIT))
+ return false;
+
+#ifdef CONFIG_XEN_PV
+ /*
+ * Xen PV does not implement ESPFIX64, which means that 16-bit
+ * segments will not work correctly. Until either Xen PV implements
+ * ESPFIX64 and can signal this fact to the guest or unless someone
+ * provides compelling evidence that allowing broken 16-bit segments
+ * is worthwhile, disallow 16-bit segments under Xen PV.
+ */
+ if (xen_pv_domain()) {
+ static DEFINE_MUTEX(xen_warning);
+ static bool warned;
+
+ mutex_lock(&xen_warning);
+ if (!warned) {
+ pr_info("Warning: 16-bit segments do not work correctly in a Xen PV guest\n");
+ warned = true;
+ }
+ mutex_unlock(&xen_warning);
+
+ return false;
+ }
+#endif
+
+ return true;
+}
+
static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
{
struct mm_struct *mm = current->mm;
@@ -574,7 +607,7 @@ static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
/* The user wants to clear the entry. */
memset(&ldt, 0, sizeof(ldt));
} else {
- if (!IS_ENABLED(CONFIG_X86_16BIT) && !ldt_info.seg_32bit) {
+ if (!ldt_info.seg_32bit && !allow_16bit_segments()) {
error = -EINVAL;
goto out;
}
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