[xen staging] x86/pv: Don't clobber NT on return-to-guest

[patchbot@xxxxxxx]

commit 5bcac985498ed83d89666959175ca9c9ed561ae1
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Jun 29 11:32:37 2020 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Thu Sep 24 21:02:35 2020 +0100

    x86/pv: Don't clobber NT on return-to-guest
    
    A 64bit IRET can restore NT - the faulting case is when NT is set in the 
live
    flags.  This change had an unintended consequence of causing the NT flag to
    spontaneously disappear from guest context whenever a interrupt/exception
    occurred.
    
    In combination with a SYSENTER which sets both TF and NT, Xen's handling of
    the #DB exceptions clears NT before it is even recorded suitably in the 
guest
    kernel's view of what userspace was doing.
    
    Reported-by: Andy Lutomirski <luto@xxxxxxxxxx>
    Fixes: 0e47f92b0 ("x86: force EFLAGS.IF on when exiting to PV guests")
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/x86_64/compat/entry.S | 2 +-
 xen/arch/x86/x86_64/entry.S        | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/x86_64/compat/entry.S 
b/xen/arch/x86/x86_64/compat/entry.S
index 73619f57ca..3b2136b272 100644
--- a/xen/arch/x86/x86_64/compat/entry.S
+++ b/xen/arch/x86/x86_64/compat/entry.S
@@ -119,7 +119,7 @@ compat_process_trap:
 /* %rbx: struct vcpu, interrupts disabled */
 ENTRY(compat_restore_all_guest)
         ASSERT_INTERRUPTS_DISABLED
-        mov   $~(X86_EFLAGS_IOPL|X86_EFLAGS_NT|X86_EFLAGS_VM),%r11d
+        mov   $~(X86_EFLAGS_IOPL | X86_EFLAGS_VM), %r11d
         and   UREGS_eflags(%rsp),%r11d
 
 .macro alt_cr4_pv32
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 44a110b9c8..000eb9722b 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -182,7 +182,7 @@ restore_all_guest:
         jz    iret_exit_to_guest
 
         movq  24(%rsp),%r11           # RFLAGS
-        andq  $~(X86_EFLAGS_IOPL|X86_EFLAGS_NT|X86_EFLAGS_VM),%r11
+        andq  $~(X86_EFLAGS_IOPL | X86_EFLAGS_VM), %r11
         orq   $X86_EFLAGS_IF,%r11
 
         /* Don't use SYSRET path if the return address is not canonical. */
@@ -213,7 +213,7 @@ restore_all_guest:
         movq  8(%rsp), %rcx           # RIP
 /* No special register assumptions. */
 iret_exit_to_guest:
-        andl  $~(X86_EFLAGS_IOPL|X86_EFLAGS_NT|X86_EFLAGS_VM),24(%rsp)
+        andl  $~(X86_EFLAGS_IOPL | X86_EFLAGS_VM), 24(%rsp)
         orl   $X86_EFLAGS_IF,24(%rsp)
         addq  $8,%rsp
 .Lft0:  iretq
--
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