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-rw-r--r--0005-x86-Fix-early-boot-crash-on-gcc-10-next-try.patch131
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diff --git a/0005-x86-Fix-early-boot-crash-on-gcc-10-next-try.patch b/0005-x86-Fix-early-boot-crash-on-gcc-10-next-try.patch
new file mode 100644
index 00000000000..ee7b469fb8d
--- /dev/null
+++ b/0005-x86-Fix-early-boot-crash-on-gcc-10-next-try.patch
@@ -0,0 +1,131 @@
+From 53e90d763b7fe8bec6a0c86b6813131cd8e25026 Mon Sep 17 00:00:00 2001
+From: Borislav Petkov <bp@suse.de>
+Date: Wed, 22 Apr 2020 18:11:30 +0200
+Subject: [PATCH 5/6] x86: Fix early boot crash on gcc-10, next try
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+... or the odyssey of trying to disable the stack protector for the
+function which generates the stack canary value.
+
+The whole story started with Sergei reporting a boot crash with a kernel
+built with gcc-10:
+
+ Kernel panic — not syncing: stack-protector: Kernel stack is corrupted in: start_secondary
+ CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.6.0-rc5—00235—gfffb08b37df9 #139
+ Hardware name: Gigabyte Technology Co., Ltd. To be filled by O.E.M./H77M—D3H, BIOS F12 11/14/2013
+ Call Trace:
+ dump_stack
+ panic
+ ? start_secondary
+ __stack_chk_fail
+ start_secondary
+ secondary_startup_64
+ -—-[ end Kernel panic — not syncing: stack—protector: Kernel stack is corrupted in: start_secondary
+
+This happens because gcc-10 tail-call optimizes the last function call
+in start_secondary() - cpu_startup_entry() - and thus emits a stack
+canary check which fails because the canary value changes after the
+boot_init_stack_canary() call.
+
+To fix that, the initial attempt was to mark the one function which
+generates the stack canary with:
+
+ __attribute__((optimize("-fno-stack-protector"))) ... start_secondary(void *unused)
+
+however, using the optimize attribute doesn't work cumulatively
+as the attribute does not add to but rather replaces previously
+supplied optimization options - roughly all -fxxx options.
+
+The key one among them being -fno-omit-frame-pointer and thus leading to
+not present frame pointer - frame pointer which the kernel needs.
+
+The next attempt to prevent compilers from tail-call optimizing
+the last function call cpu_startup_entry(), shy of carving out
+start_secondary() into a separate compilation unit and building it with
+-fno-stack-protector, is this one.
+
+The current solution is short and sweet, and reportedly, is supported by
+both compilers so let's see how far we'll get this time.
+
+Reported-by: Sergei Trofimovich <slyfox@gentoo.org>
+Signed-off-by: Borislav Petkov <bp@suse.de>
+Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
+Reviewed-by: Kees Cook <keescook@chromium.org>
+Link: https://lkml.kernel.org/r/20200314164451.346497-1-slyfox@gentoo.org
+---
+ arch/x86/include/asm/stackprotector.h | 7 ++++++-
+ arch/x86/kernel/smpboot.c | 8 ++++++++
+ arch/x86/xen/smp_pv.c | 1 +
+ include/linux/compiler.h | 6 ++++++
+ 4 files changed, 21 insertions(+), 1 deletion(-)
+
+diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
+index 91e29b6a86a5..9804a7957f4e 100644
+--- a/arch/x86/include/asm/stackprotector.h
++++ b/arch/x86/include/asm/stackprotector.h
+@@ -55,8 +55,13 @@
+ /*
+ * Initialize the stackprotector canary value.
+ *
+- * NOTE: this must only be called from functions that never return,
++ * NOTE: this must only be called from functions that never return
+ * and it must always be inlined.
++ *
++ * In addition, it should be called from a compilation unit for which
++ * stack protector is disabled. Alternatively, the caller should not end
++ * with a function call which gets tail-call optimized as that would
++ * lead to checking a modified canary value.
+ */
+ static __always_inline void boot_init_stack_canary(void)
+ {
+diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
+index 69881b2d446c..9674321ce3a3 100644
+--- a/arch/x86/kernel/smpboot.c
++++ b/arch/x86/kernel/smpboot.c
+@@ -262,6 +262,14 @@ static void notrace start_secondary(void *unused)
+
+ wmb();
+ cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
++
++ /*
++ * Prevent tail call to cpu_startup_entry() because the stack protector
++ * guard has been changed a couple of function calls up, in
++ * boot_init_stack_canary() and must not be checked before tail calling
++ * another function.
++ */
++ prevent_tail_call_optimization();
+ }
+
+ /**
+diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
+index 802ee5bba66c..0cebe5db691d 100644
+--- a/arch/x86/xen/smp_pv.c
++++ b/arch/x86/xen/smp_pv.c
+@@ -92,6 +92,7 @@ asmlinkage __visible void cpu_bringup_and_idle(void)
+ cpu_bringup();
+ boot_init_stack_canary();
+ cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
++ prevent_tail_call_optimization();
+ }
+
+ void xen_smp_intr_free_pv(unsigned int cpu)
+diff --git a/include/linux/compiler.h b/include/linux/compiler.h
+index 034b0a644efc..732754d96039 100644
+--- a/include/linux/compiler.h
++++ b/include/linux/compiler.h
+@@ -356,4 +356,10 @@ static inline void *offset_to_ptr(const int *off)
+ /* &a[0] degrades to a pointer: a different type from an array */
+ #define __must_be_array(a) BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
+
++/*
++ * This is needed in functions which generate the stack canary, see
++ * arch/x86/kernel/smpboot.c::start_secondary() for an example.
++ */
++#define prevent_tail_call_optimization() asm("")
++
+ #endif /* __LINUX_COMPILER_H */
+--
+2.26.2
+