xen/pv: Fix a boot up hang revealed by int3 self test [Linux 4.14.137]

This Linux kernel change "xen/pv: Fix a boot up hang revealed by int3 self test" is included in the Linux 4.14.137 release. This change is authored by Zhenzhong Duan <zhenzhong.duan [at] oracle.com> on Sun Jul 14 17:15:32 2019 +0800. The commit for this change in Linux stable tree is 7cb3e16 (patch) which is from upstream commit b23e584. The same Linux upstream change may have been applied to various maintained Linux releases and you can find all Linux releases containing changes from upstream b23e584.

xen/pv: Fix a boot up hang revealed by int3 self test

[ Upstream commit b23e5844dfe78a80ba672793187d3f52e4b528d7 ]

Commit 7457c0da024b ("x86/alternatives: Add int3_emulate_call()
selftest") is used to ensure there is a gap setup in int3 exception stack
which could be used for inserting call return address.

This gap is missed in XEN PV int3 exception entry path, then below panic

[    0.772876] general protection fault: 0000 [#1] SMP NOPTI
[    0.772886] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.2.0+ #11
[    0.772893] RIP: e030:int3_magic+0x0/0x7
[    0.772905] RSP: 3507:ffffffff82203e98 EFLAGS: 00000246
[    0.773334] Call Trace:
[    0.773334]  alternative_instructions+0x3d/0x12e
[    0.773334]  check_bugs+0x7c9/0x887
[    0.773334]  ? __get_locked_pte+0x178/0x1f0
[    0.773334]  start_kernel+0x4ff/0x535
[    0.773334]  ? set_init_arg+0x55/0x55
[    0.773334]  xen_start_kernel+0x571/0x57a

For 64bit PV guests, Xen's ABI enters the kernel with using SYSRET, with
%rcx/%r11 on the stack. To convert back to "normal" looking exceptions,
the xen thunks do 'xen_*: pop %rcx; pop %r11; jmp *'.

E.g. Extracting 'xen_pv_trap xenint3' we have:
 pop %rcx;
 pop %r11;
 jmp xenint3

As xenint3 and int3 entry code are same except xenint3 doesn't generate
a gap, we can fix it by using int3 and drop useless xenint3.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

There are 6 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 arch/x86/entry/entry_64.S    | 1 -
 arch/x86/include/asm/traps.h | 2 +-
 arch/x86/xen/enlighten_pv.c  | 2 +-
 arch/x86/xen/xen-asm_64.S    | 1 -
 4 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index e09ba4b..b2524d3 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1113,7 +1113,6 @@ idtentry stack_segment        do_stack_segment    has_error_code=1
 #ifdef CONFIG_XEN
 idtentry xennmi            do_nmi          has_error_code=0
 idtentry xendebug      do_debug        has_error_code=0
-idtentry xenint3       do_int3         has_error_code=0

 idtentry general_protection    do_general_protection   has_error_code=1
diff --git a/arch/x86/include/asm/traps.h b/arch/x86/include/asm/traps.h
index afbc872..b771bb3 100644
--- a/arch/x86/include/asm/traps.h
+++ b/arch/x86/include/asm/traps.h
@@ -40,7 +40,7 @@
 asmlinkage void xen_divide_error(void);
 asmlinkage void xen_xennmi(void);
 asmlinkage void xen_xendebug(void);
-asmlinkage void xen_xenint3(void);
+asmlinkage void xen_int3(void);
 asmlinkage void xen_overflow(void);
 asmlinkage void xen_bounds(void);
 asmlinkage void xen_invalid_op(void);
diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index 481d792..f79a0cd 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -598,12 +598,12 @@ struct trap_array_entry {

 static struct trap_array_entry trap_array[] = {
    { debug,                       xen_xendebug,                    true },
-   { int3,                        xen_xenint3,                     true },
    { double_fault,                xen_double_fault,                true },
 #ifdef CONFIG_X86_MCE
    { machine_check,               xen_machine_check,               true },
    { nmi,                         xen_xennmi,                      true },
+   { int3,                        xen_int3,                        false },
    { overflow,                    xen_overflow,                    false },
    { entry_INT80_compat,          xen_entry_INT80_compat,          false },
diff --git a/arch/x86/xen/xen-asm_64.S b/arch/x86/xen/xen-asm_64.S
index 417b339..3a6feed 100644
--- a/arch/x86/xen/xen-asm_64.S
+++ b/arch/x86/xen/xen-asm_64.S
@@ -30,7 +30,6 @@ xen_pv_trap divide_error
 xen_pv_trap debug
 xen_pv_trap xendebug
 xen_pv_trap int3
-xen_pv_trap xenint3
 xen_pv_trap xennmi
 xen_pv_trap overflow
 xen_pv_trap bounds

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