lguest: Fix Malicious Guest GDT Host Crash

If a Guest makes hypercall which sets a GDT entry to not present, we
currently set any segment registers using that GDT entry to 0.
Unfortunately, this is not sufficient: there are other ways of
altering GDT entries which will cause a fault.

The correct solution to do what Linux does: let them set any GDT value
they want and handle the #GP when popping causes a fault.  This has
the added benefit of making our Switcher slightly more robust in the
case of any other bugs which cause it to fault.

We kill the Guest if it causes a fault in the Switcher: it's the
Guest's responsibility to make sure it's not using segments when it
changes them.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 5 insertions, 57 deletions

diff --git a/drivers/lguest/segments.c b/drivers/lguest/segments.c
index f675a41..9b81119 100644
--- a/drivers/lguest/segments.c
+++ b/drivers/lguest/segments.c
@@ -43,22 +43,6 @@
  * begin.
  */
 
-/* Is the descriptor the Guest wants us to put in OK?
- *
- * The flag which Intel says must be zero: must be zero.  The descriptor must
- * be present, (this is actually checked earlier but is here for thorougness),
- * and the descriptor type must be 1 (a memory segment).  */
-static int desc_ok(const struct desc_struct *gdt)
-{
-	return ((gdt->b & 0x00209000) == 0x00009000);
-}
-
-/* Is the segment present?  (Otherwise it can't be used by the Guest). */
-static int segment_present(const struct desc_struct *gdt)
-{
-	return gdt->b & 0x8000;
-}
-
 /* There are several entries we don't let the Guest set.  The TSS entry is the
  * "Task State Segment" which controls all kinds of delicate things.  The
  * LGUEST_CS and LGUEST_DS entries are reserved for the Switcher, and the
@@ -71,37 +55,11 @@ static int ignored_gdt(unsigned int num)
 		|| num == GDT_ENTRY_DOUBLEFAULT_TSS);
 }
 
-/* If the Guest asks us to remove an entry from the GDT, we have to be careful.
- * If one of the segment registers is pointing at that entry the Switcher will
- * crash when it tries to reload the segment registers for the Guest.
- *
- * It doesn't make much sense for the Guest to try to remove its own code, data
- * or stack segments while they're in use: assume that's a Guest bug.  If it's
- * one of the lesser segment registers using the removed entry, we simply set
- * that register to 0 (unusable). */
-static void check_segment_use(struct lguest *lg, unsigned int desc)
-{
-	/* GDT entries are 8 bytes long, so we divide to get the index and
-	 * ignore the bottom bits. */
-	if (lg->regs->gs / 8 == desc)
-		lg->regs->gs = 0;
-	if (lg->regs->fs / 8 == desc)
-		lg->regs->fs = 0;
-	if (lg->regs->es / 8 == desc)
-		lg->regs->es = 0;
-	if (lg->regs->ds / 8 == desc
-	    || lg->regs->cs / 8 == desc
-	    || lg->regs->ss / 8 == desc)
-		kill_guest(lg, "Removed live GDT entry %u", desc);
-}
-/*:*/
-/*M:009 We wouldn't need to check for removal of in-use segments if we handled
- * faults in the Switcher.  However, it's probably not a worthwhile
- * optimization. :*/
-
-/*H:610 Once the GDT has been changed, we look through the changed entries and
- * see if they're OK.  If not, we'll call kill_guest() and the Guest will never
- * get to use the invalid entries. */
+/*H:610 Once the GDT has been changed, we fix the new entries up a little.  We
+ * don't care if they're invalid: the worst that can happen is a General
+ * Protection Fault in the Switcher when it restores a Guest segment register
+ * which tries to use that entry.  Then we kill the Guest for causing such a
+ * mess: the message will be "unhandled trap 256". */
 static void fixup_gdt_table(struct lguest *lg, unsigned start, unsigned end)
 {
 	unsigned int i;
@@ -112,16 +70,6 @@ static void fixup_gdt_table(struct lguest *lg, unsigned start, unsigned end)
 		if (ignored_gdt(i))
 			continue;
 
-		/* We could fault in switch_to_guest if they are using
-		 * a removed segment. */
-		if (!segment_present(&lg->gdt[i])) {
-			check_segment_use(lg, i);
-			continue;
-		}
-
-		if (!desc_ok(&lg->gdt[i]))
-			kill_guest(lg, "Bad GDT descriptor %i", i);
-
 		/* Segment descriptors contain a privilege level: the Guest is
 		 * sometimes careless and leaves this as 0, even though it's
 		 * running at privilege level 1.  If so, we fix it here. */