The following commit has been merged into the x86/build branch of tip: Commit-ID: f670269a42bfdd2c83a1118cc3d1b475547eac22 Gitweb: https://git.kernel.org/tip/f670269a42bfdd2c83a1118cc3d1b475547eac22 Author: Borislav Petkov <bp@xxxxxxx> AuthorDate: Wed, 22 Apr 2020 18:11:30 +02:00 Committer: Borislav Petkov <bp@xxxxxxx> CommitterDate: Mon, 27 Apr 2020 13:32:04 +02:00 x86: Fix early boot crash on gcc-10, next try ... 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@xxxxxxxxxx> Signed-off-by: Borislav Petkov <bp@xxxxxxx> Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx> Link: https://lkml.kernel.org/r/20200314164451.346497-1-slyfox@xxxxxxxxxx --- 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 91e29b6..9804a79 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 fe3ab96..4f275ac 100644 --- a/arch/x86/kernel/smpboot.c +++ b/arch/x86/kernel/smpboot.c @@ -266,6 +266,14 @@ 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 8fb8a50..f2adb63 100644 --- a/arch/x86/xen/smp_pv.c +++ b/arch/x86/xen/smp_pv.c @@ -93,6 +93,7 @@ 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 034b0a6..732754d 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 */
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