lguest: fix guest crash on non-linear addresses in gdt pvops

Fixes guest crash 'lguest: bad read address 0x4800000 len 256'

The new per-cpu allocator ends up handing a non-linear address to
write_gdt_entry.  We do __pa() on it, and hand it to the host, which
kills us.

I've long wanted to make the hypercall "LOAD_GDT_ENTRY" to match the IDT
code, but had no pressing reason until now.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: lguest@ozlabs.org

1 files changed, 7 insertions, 6 deletions

diff --git a/drivers/lguest/segments.c b/drivers/lguest/segments.c
index 4f15439b7f1..7ede64ffeef 100644
--- a/drivers/lguest/segments.c
+++ b/drivers/lguest/segments.c
@@ -144,18 +144,19 @@ void copy_gdt(const struct lg_cpu *cpu, struct desc_struct *gdt)
                         gdt[i] = cpu->arch.gdt[i];
 }
 
-/*H:620 This is where the Guest asks us to load a new GDT (LHCALL_LOAD_GDT).
- * We copy it from the Guest and tweak the entries. */
-void load_guest_gdt(struct lg_cpu *cpu, unsigned long table, u32 num)
+/*H:620 This is where the Guest asks us to load a new GDT entry
+ * (LHCALL_LOAD_GDT_ENTRY).  We tweak the entry and copy it in. */
+void load_guest_gdt_entry(struct lg_cpu *cpu, u32 num, u32 lo, u32 hi)
 {
         /* We assume the Guest has the same number of GDT entries as the
          * Host, otherwise we'd have to dynamically allocate the Guest GDT. */
         if (num > ARRAY_SIZE(cpu->arch.gdt))
                 kill_guest(cpu, "too many gdt entries %i", num);
 
-        /* We read the whole thing in, then fix it up. */
-        __lgread(cpu, cpu->arch.gdt, table, num * sizeof(cpu->arch.gdt[0]));
-        fixup_gdt_table(cpu, 0, ARRAY_SIZE(cpu->arch.gdt));
+        /* Set it up, then fix it. */
+        cpu->arch.gdt[num].a = lo;
+        cpu->arch.gdt[num].b = hi;
+        fixup_gdt_table(cpu, num, num+1);
         /* Mark that the GDT changed so the core knows it has to copy it again,
          * even if the Guest is run on the same CPU. */
         cpu->changed |= CHANGED_GDT;