[v2,1/3] x86: relax GDT check in arch_set_info_guest()
diff mbox series

Message ID 3f78d1dc-720d-6bf3-0911-c19da1a2ddbb@suse.com
State New
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Series
  • x86: XSA-298 follow-up
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Commit Message

Jan Beulich Dec. 20, 2019, 1:49 p.m. UTC
It is wrong for us to check frames beyond the guest specified limit
(in the native case, other than in the compat one).

Signed-off-by: Jan Beulich <jbeulich@suse.com>

Comments

Andrew Cooper Dec. 27, 2019, 3:09 p.m. UTC | #1
On 20/12/2019 13:49, Jan Beulich wrote:
> It is wrong for us to check frames beyond the guest specified limit
> (in the native case, other than in the compat one).
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Just like the restriction on sharing L2's, no guest is ever going to be
able to not zero all of this to operate on older hypervisors.

I agree that it is not ideal that this got into the ABI to begin with,
but as I said before, all you are doing is complicating
arch_set_info_guest() for a relaxation which no guest can use.

~Andrew
Jan Beulich Jan. 3, 2020, 10:24 a.m. UTC | #2
On 27.12.2019 16:09, Andrew Cooper wrote:
> On 20/12/2019 13:49, Jan Beulich wrote:
>> It is wrong for us to check frames beyond the guest specified limit
>> (in the native case, other than in the compat one).
>>
>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> 
> Just like the restriction on sharing L2's, no guest is ever going to be
> able to not zero all of this to operate on older hypervisors.
> 
> I agree that it is not ideal that this got into the ABI to begin with,
> but as I said before, all you are doing is complicating
> arch_set_info_guest() for a relaxation which no guest can use.

As asked before - would you mind clarifying where I'm complicating
things? I think I'm rather simplifying matters, by
- pulling out a calculation, storing the result into a now common
  (between native and compat cases) local variable,
- as a result making native and compat cases behave consistently,
  eliminating the need for reader to (try to) figure out why there
  is a difference in behavior.

Jan

Patch
diff mbox series

--- 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
 
@@ -951,6 +952,8 @@  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 ( !v->is_initialised )
     {
         if ( !compat && !(flags & VGCF_in_kernel) && !c.nat->ctrlreg[1] )
@@ -982,9 +985,9 @@  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);
@@ -1089,12 +1092,11 @@  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) )
+        if ( nr_gdt_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);