[xen stable-4.18] x86/entry: don't clear DF when raising #UD for lack of syscall handler

[patchbot@xxxxxxx]

commit 7e636b8a16412d4f0d94b2b24d7ebcd2c749afff
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Thu Jul 4 14:14:49 2024 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu Jul 4 14:14:49 2024 +0200

    x86/entry: don't clear DF when raising #UD for lack of syscall handler
    
    While doing so is intentional when invoking the actual callback, to
    mimic a hard-coded SYCALL_MASK / FMASK MSR, the same should not be done
    when no handler is available and hence #UD is raised.
    
    Fixes: ca6fcf4321b3 ("x86/pv: Inject #UD for missing SYSCALL callbacks")
    Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: d2fe9ab3048d503869ec81bc49db07e55a4a2386
    master date: 2024-07-02 12:01:21 +0200
---
 xen/arch/x86/x86_64/entry.S | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 054fcb225f..d3def49ea3 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -38,6 +38,14 @@ switch_to_kernel:
         setc  %cl
         leal  (,%rcx,TBF_INTERRUPT),%ecx
 
+        /*
+         * The PV ABI hardcodes the (guest-inaccessible and virtual)
+         * SYSCALL_MASK MSR such that DF (and nothing else) would be cleared.
+         * Note that the equivalent of IF (VGCF_syscall_disables_events) is
+         * dealt with separately above.
+         */
+        mov   $~X86_EFLAGS_DF, %esi
+
         test  %rax, %rax
 UNLIKELY_START(z, syscall_no_callback) /* TB_eip == 0 => #UD */
         mov   VCPU_trap_ctxt(%rbx), %rdi
@@ -47,12 +55,14 @@ UNLIKELY_START(z, syscall_no_callback) /* TB_eip == 0 => 
#UD */
         testb $4, X86_EXC_UD * TRAPINFO_sizeof + TRAPINFO_flags(%rdi)
         setnz %cl
         lea   TBF_EXCEPTION(, %rcx, TBF_INTERRUPT), %ecx
+        or    $~0, %esi                 /* Don't clear DF */
 UNLIKELY_END(syscall_no_callback)
 
         movq  %rax,TRAPBOUNCE_eip(%rdx)
         movb  %cl,TRAPBOUNCE_flags(%rdx)
         call  create_bounce_frame
-        andl  $~X86_EFLAGS_DF,UREGS_eflags(%rsp)
+        /* Conditionally clear DF */
+        and   %esi, UREGS_eflags(%rsp)
 /* %rbx: struct vcpu */
 test_all_events:
         ASSERT_NOT_IN_ATOMIC
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.18