| From a9a3ed1eff3601b63aea4fb462d8b3b92c7c1e7e Mon Sep 17 00:00:00 2001 |
| From: Borislav Petkov <bp@suse.de> |
| Date: Wed, 22 Apr 2020 18:11:30 +0200 |
| Subject: x86: Fix early boot crash on gcc-10, third try |
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| |
| From: Borislav Petkov <bp@suse.de> |
| |
| commit a9a3ed1eff3601b63aea4fb462d8b3b92c7c1e7e upstream. |
| |
| ... 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, was to add an empty asm(""). |
| |
| This current solution was short and sweet, and reportedly, is supported |
| by both compilers but we didn't get very far this time: future (LTO?) |
| optimization passes could potentially eliminate this, which leads us |
| to the third attempt: having an actual memory barrier there which the |
| compiler cannot ignore or move around etc. |
| |
| That should hold for a long time, but hey we said that about the other |
| two solutions too so... |
| |
| Reported-by: Sergei Trofimovich <slyfox@gentoo.org> |
| Signed-off-by: Borislav Petkov <bp@suse.de> |
| Tested-by: Kalle Valo <kvalo@codeaurora.org> |
| Cc: <stable@vger.kernel.org> |
| Link: https://lkml.kernel.org/r/20200314164451.346497-1-slyfox@gentoo.org |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.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 ++++++ |
| init/main.c | 2 ++ |
| 5 files changed, 23 insertions(+), 1 deletion(-) |
| |
| --- 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) |
| { |
| --- a/arch/x86/kernel/smpboot.c |
| +++ b/arch/x86/kernel/smpboot.c |
| @@ -262,6 +262,14 @@ static void notrace start_secondary(void |
| |
| 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(); |
| } |
| |
| /** |
| --- a/arch/x86/xen/smp_pv.c |
| +++ b/arch/x86/xen/smp_pv.c |
| @@ -92,6 +92,7 @@ asmlinkage __visible void cpu_bringup_an |
| 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) |
| --- a/include/linux/compiler.h |
| +++ b/include/linux/compiler.h |
| @@ -356,4 +356,10 @@ static inline void *offset_to_ptr(const |
| /* &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() mb() |
| + |
| #endif /* __LINUX_COMPILER_H */ |
| --- a/init/main.c |
| +++ b/init/main.c |
| @@ -1032,6 +1032,8 @@ asmlinkage __visible void __init start_k |
| |
| /* Do the rest non-__init'ed, we're now alive */ |
| arch_call_rest_init(); |
| + |
| + prevent_tail_call_optimization(); |
| } |
| |
| /* Call all constructor functions linked into the kernel. */ |