[xen stable-4.15] x86/entry: Track the IST-ness of an entry for the exit paths

[patchbot@xxxxxxx]

commit d39e5cf76f2a15120491b3b020d973d04c181388
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Wed Sep 13 12:20:12 2023 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Sep 19 19:38:35 2023 +0100

    x86/entry: Track the IST-ness of an entry for the exit paths
    
    Use %r12 to hold an ist_exit boolean.  This register is zero elsewhere in 
the
    entry/exit asm, so it only needs setting in the IST path.
    
    As this is subtle and fragile, add check_ist_exit() to be used in debugging
    builds to cross-check that the ist_exit boolean matches the entry vector.
    
    Write check_ist_exit() it in C, because it's debug only and the logic more
    complicated than I care to maintain in asm.
    
    For now, we only need to use this signal in the exit-to-Xen path, but some
    exit-to-guest paths happen in IST context too.  Check the correctness in all
    exit paths to avoid the logic bit-rotting.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    (cherry picked from commit 21bdc25b05a0f8ab6bc73520a9ca01327360732c)
    
    x86/entry: Partially revert IST-exit checks
    
    The patch adding check_ist_exit() didn't account for the fact that
    reset_stack_and_jump() is not an ABI-preserving boundary.  The IST-ness in
    %r12 doesn't survive into the next context, and is a stale value C.
    
    This shows up in Gitlab CI for the Clang build:
    
      https://gitlab.com/xen-project/people/andyhhp/xen/-/jobs/5112783827
    
    and in OSSTest for GCC 8:
    
      
http://logs.test-lab.xenproject.org/osstest/logs/183045/test-amd64-amd64-xl-qemuu-debianhvm-amd64/serial-pinot0.log
    
    There's no straightforward way to reconstruct the IST-exit-ness on the
    exit-to-guest path after a context switch.  For now, we only need IST-exit 
on
    the return-to-Xen path.
    
    Fixes: 21bdc25b05a0 ("x86/entry: Track the IST-ness of an entry for the 
exit paths")
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    (cherry picked from commit 9b57c800b79b96769ea3dcd6468578fa664d19f9)
---
 xen/arch/x86/traps.c        | 13 +++++++++++++
 xen/arch/x86/x86_64/entry.S | 13 ++++++++++++-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index a4f12ad3ee..71dae00428 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -2236,6 +2236,19 @@ void asm_domain_crash_synchronous(unsigned long addr)
         do_softirq();
 }
 
+#ifdef CONFIG_DEBUG
+void check_ist_exit(const struct cpu_user_regs *regs, bool ist_exit)
+{
+    const unsigned int ist_mask =
+        (1U << X86_EXC_NMI) | (1U << X86_EXC_DB) |
+        (1U << X86_EXC_DF)  | (1U << X86_EXC_MC);
+    uint8_t ev = regs->entry_vector;
+    bool is_ist = (ev < TRAP_nr) && ((1U << ev) & ist_mask);
+
+    ASSERT(is_ist == ist_exit);
+}
+#endif
+
 /*
  * Local variables:
  * mode: C
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 3cec072f81..fd6836daf0 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -593,8 +593,15 @@ ret_from_intr:
         .section .text.entry, "ax", @progbits
 
         ALIGN
-/* No special register assumptions. */
+/* %r12=ist_exit */
 restore_all_xen:
+
+#ifdef CONFIG_DEBUG
+        mov   %rsp, %rdi
+        mov   %r12, %rsi
+        call  check_ist_exit
+#endif
+
         /*
          * Check whether we need to switch to the per-CPU page tables, in
          * case we return to late PV exit code (from an NMI or #MC).
@@ -973,6 +980,10 @@ handle_ist_exception:
         INDIRECT_CALL %rdx
         mov   %r15, STACK_CPUINFO_FIELD(xen_cr3)(%r14)
         mov   %bl, STACK_CPUINFO_FIELD(use_pv_cr3)(%r14)
+
+        /* This is an IST exit */
+        mov   $1, %r12d
+
         cmpb  $TRAP_nmi,UREGS_entry_vector(%rsp)
         jne   ret_from_intr
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.15