[Xen-devel] [PATCH] x86-64: clear DF for kernel when forwarding syscall

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

While this is not really matching native behavior, no guest seems to assume
EFLAGS.DF being set or reflecting application state. Thus clear it for now,
the syscall/sysenter patch that I'll hopefully be able to get to work will
then introduce a more consistent solution. In any case, without this any
app can easily force kernel data corruption.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>

Index: 2007-06-18/xen/arch/x86/x86_64/entry.S
===================================================================
--- 2007-06-18.orig/xen/arch/x86/x86_64/entry.S 2007-06-22 16:35:55.000000000 
+0200
+++ 2007-06-18/xen/arch/x86/x86_64/entry.S      2007-06-22 16:36:59.000000000 
+0200
@@ -34,6 +34,7 @@ switch_to_kernel:
         jnc   1f
         movb  $TBF_INTERRUPT,TRAPBOUNCE_flags(%rdx)
 1:      call  create_bounce_frame
+        andl  $~X86_EFLAGS_DF,UREGS_eflags(%rsp)
         jmp   test_all_events
 
 /* %rbx: struct vcpu, interrupts disabled */




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