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+From 2f18ace75890ccd9681f210efdb58662db0cb4c4 Mon Sep 17 00:00:00 2001
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+From: Borislav Petkov <bp@suse.de>
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+Date: Wed, 22 Apr 2020 18:11:30 +0200
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+Subject: [PATCH 3/3] x86: Fix early boot crash on gcc-10, next try
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+MIME-Version: 1.0
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+Content-Type: text/plain; charset=UTF-8
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+Content-Transfer-Encoding: 8bit
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+
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+... or the odyssey of trying to disable the stack protector for the
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+function which generates the stack canary value.
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+
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+The whole story started with Sergei reporting a boot crash with a kernel
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+built with gcc-10:
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+
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+ Kernel panic — not syncing: stack-protector: Kernel stack is corrupted in: start_secondary
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+ CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.6.0-rc5—00235—gfffb08b37df9 #139
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+ Hardware name: Gigabyte Technology Co., Ltd. To be filled by O.E.M./H77M—D3H, BIOS F12 11/14/2013
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+ Call Trace:
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+ dump_stack
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+ panic
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+ ? start_secondary
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+ __stack_chk_fail
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+ start_secondary
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+ secondary_startup_64
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+ -—-[ end Kernel panic — not syncing: stack—protector: Kernel stack is corrupted in: start_secondary
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+
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+This happens because gcc-10 tail-call optimizes the last function call
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+in start_secondary() - cpu_startup_entry() - and thus emits a stack
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+canary check which fails because the canary value changes after the
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+boot_init_stack_canary() call.
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+
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+To fix that, the initial attempt was to mark the one function which
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+generates the stack canary with:
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+
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+ __attribute__((optimize("-fno-stack-protector"))) ... start_secondary(void *unused)
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+
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+however, using the optimize attribute doesn't work cumulatively
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+as the attribute does not add to but rather replaces previously
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+supplied optimization options - roughly all -fxxx options.
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+
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+The key one among them being -fno-omit-frame-pointer and thus leading to
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+not present frame pointer - frame pointer which the kernel needs.
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+
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+The next attempt to prevent compilers from tail-call optimizing
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+the last function call cpu_startup_entry(), shy of carving out
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+start_secondary() into a separate compilation unit and building it with
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+-fno-stack-protector, is this one.
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+
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+The current solution is short and sweet, and reportedly, is supported by
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+both compilers so let's see how far we'll get this time.
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+
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+Reported-by: Sergei Trofimovich <slyfox@gentoo.org>
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+Signed-off-by: Borislav Petkov <bp@suse.de>
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+Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
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+Reviewed-by: Kees Cook <keescook@chromium.org>
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+Link: https://lkml.kernel.org/r/20200314164451.346497-1-slyfox@gentoo.org
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+---
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+ arch/x86/include/asm/stackprotector.h | 7 ++++++-
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+ arch/x86/kernel/smpboot.c | 8 ++++++++
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+ arch/x86/xen/smp_pv.c | 1 +
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+ include/linux/compiler.h | 6 ++++++
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+ 4 files changed, 21 insertions(+), 1 deletion(-)
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+
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+diff --git a/arch/x86/include/asm/stackprotector.h b/arch/x86/include/asm/stackprotector.h
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+index 91e29b6a86a5..9804a7957f4e 100644
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+--- a/arch/x86/include/asm/stackprotector.h
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++++ b/arch/x86/include/asm/stackprotector.h
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+@@ -55,8 +55,13 @@
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+ /*
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+ * Initialize the stackprotector canary value.
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+ *
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+- * NOTE: this must only be called from functions that never return,
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++ * NOTE: this must only be called from functions that never return
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+ * and it must always be inlined.
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++ *
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++ * In addition, it should be called from a compilation unit for which
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++ * stack protector is disabled. Alternatively, the caller should not end
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++ * with a function call which gets tail-call optimized as that would
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++ * lead to checking a modified canary value.
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+ */
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+ static __always_inline void boot_init_stack_canary(void)
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+ {
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+diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
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+index 69881b2d446c..9674321ce3a3 100644
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+--- a/arch/x86/kernel/smpboot.c
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++++ b/arch/x86/kernel/smpboot.c
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+@@ -262,6 +262,14 @@ static void notrace start_secondary(void *unused)
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+
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+ wmb();
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+ cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
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++
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++ /*
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++ * Prevent tail call to cpu_startup_entry() because the stack protector
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++ * guard has been changed a couple of function calls up, in
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++ * boot_init_stack_canary() and must not be checked before tail calling
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++ * another function.
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++ */
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++ prevent_tail_call_optimization();
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+ }
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+
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+ /**
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+diff --git a/arch/x86/xen/smp_pv.c b/arch/x86/xen/smp_pv.c
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+index 802ee5bba66c..0cebe5db691d 100644
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+--- a/arch/x86/xen/smp_pv.c
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++++ b/arch/x86/xen/smp_pv.c
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+@@ -92,6 +92,7 @@ asmlinkage __visible void cpu_bringup_and_idle(void)
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+ cpu_bringup();
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+ boot_init_stack_canary();
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+ cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
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++ prevent_tail_call_optimization();
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+ }
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+
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+ void xen_smp_intr_free_pv(unsigned int cpu)
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+diff --git a/include/linux/compiler.h b/include/linux/compiler.h
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+index 034b0a644efc..732754d96039 100644
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+--- a/include/linux/compiler.h
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++++ b/include/linux/compiler.h
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+@@ -356,4 +356,10 @@ static inline void *offset_to_ptr(const int *off)
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+ /* &a[0] degrades to a pointer: a different type from an array */
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+ #define __must_be_array(a) BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))
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+
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++/*
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++ * This is needed in functions which generate the stack canary, see
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++ * arch/x86/kernel/smpboot.c::start_secondary() for an example.
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++ */
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++#define prevent_tail_call_optimization() asm("")
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++
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+ #endif /* __LINUX_COMPILER_H */
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+--
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+2.26.2
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+
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Maximilian Luz