[v3,1/3] x86: relax GDT check in arch_set_info_guest()

Commit Message

Jan Beulich May 20, 2020, 7:53 a.m. UTC

It is wrong for us to check frames beyond the guest specified limit
(in the compat case another loop bound is already correct).

Signed-off-by: Jan Beulich <[e-mail]>
---
v3: Move nr_gdt_frames range check earlier. Avoid |= where not really
    needed.

Comments

Roger Pau Monne May 20, 2020, 10:20 a.m. UTC | #1

On Wed, May 20, 2020 at 09:53:50AM +0200, Jan Beulich wrote:
> It is wrong for us to check frames beyond the guest specified limit
> (in the compat case another loop bound is already correct).
> 
> Signed-off-by: Jan Beulich <[e-mail]>

Reviewed-by: Roger Pau Monné <[e-mail]>

Thanks, Roger.

Andrew Cooper May 22, 2020, 1:27 p.m. UTC | #2

On 20/05/2020 08:53, Jan Beulich wrote:
> It is wrong for us to check frames beyond the guest specified limit
> (in the compat case another loop bound is already correct).
>
> Signed-off-by: Jan Beulich <[e-mail]>

I'm still not overly convinced this is a good idea, because all it will
allow people to do is write lazy code which breaks on older Xen.

However, if you still insist, Acked-by: Andrew Cooper
<[e-mail]>

Jan Beulich May 22, 2020, 2:14 p.m. UTC | #3

On 22.05.2020 15:27, Andrew Cooper wrote:
> On 20/05/2020 08:53, Jan Beulich wrote:
>> It is wrong for us to check frames beyond the guest specified limit
>> (in the compat case another loop bound is already correct).
>>
>> Signed-off-by: Jan Beulich <[e-mail]>
> 
> I'm still not overly convinced this is a good idea, because all it will
> allow people to do is write lazy code which breaks on older Xen.

Sounds a little like keeping bugs for the sake of keeping things
broken. The range of misbehaving versions could be shrunk by
backporting this change; I didn't intend to though, so far.

> However, if you still insist, Acked-by: Andrew Cooper
> <[e-mail]>

Thanks!

Jan

Patch

--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -840,6 +840,7 @@  int arch_set_info_guest(
 #ifdef CONFIG_PV
     mfn_t cr3_mfn;
     struct page_info *cr3_page = NULL;
+    unsigned int nr_gdt_frames;
     int rc = 0;
 #endif
 
@@ -957,6 +958,10 @@  int arch_set_info_guest(
     /* Ensure real hardware interrupts are enabled. */
     v->arch.user_regs.eflags |= X86_EFLAGS_IF;
 
+    nr_gdt_frames = DIV_ROUND_UP(c(gdt_ents), 512);
+    if ( nr_gdt_frames > ARRAY_SIZE(v->arch.pv.gdt_frames) )
+        return -EINVAL;
+
     if ( !v->is_initialised )
     {
         if ( !compat && !(flags & VGCF_in_kernel) && !c.nat->ctrlreg[1] )
@@ -988,9 +993,9 @@  int arch_set_info_guest(
             fail = compat_pfn_to_cr3(pfn) != c.cmp->ctrlreg[3];
         }
 
-        for ( i = 0; i < ARRAY_SIZE(v->arch.pv.gdt_frames); ++i )
-            fail |= v->arch.pv.gdt_frames[i] != c(gdt_frames[i]);
         fail |= v->arch.pv.gdt_ents != c(gdt_ents);
+        for ( i = 0; !fail && i < nr_gdt_frames; ++i )
+            fail = v->arch.pv.gdt_frames[i] != c(gdt_frames[i]);
 
         fail |= v->arch.pv.ldt_base != c(ldt_base);
         fail |= v->arch.pv.ldt_ents != c(ldt_ents);
@@ -1095,12 +1100,8 @@  int arch_set_info_guest(
     else
     {
         unsigned long gdt_frames[ARRAY_SIZE(v->arch.pv.gdt_frames)];
-        unsigned int nr_frames = DIV_ROUND_UP(c.cmp->gdt_ents, 512);
-
-        if ( nr_frames > ARRAY_SIZE(v->arch.pv.gdt_frames) )
-            return -EINVAL;
 
-        for ( i = 0; i < nr_frames; ++i )
+        for ( i = 0; i < nr_gdt_frames; ++i )
             gdt_frames[i] = c.cmp->gdt_frames[i];
 
         rc = (int)pv_set_gdt(v, gdt_frames, c.cmp->gdt_ents);