The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: a9a3ed1eff3601b63aea4fb462d8b3b92c7c1e7e Gitweb: https://git.kernel.org/tip/a9a3ed1eff3601b63aea4fb462d8b3b92c7c1e7e Author: Borislav Petkov bp@suse.de AuthorDate: Wed, 22 Apr 2020 18:11:30 +02:00 Committer: Borislav Petkov bp@suse.de CommitterDate: Fri, 15 May 2020 11:48:01 +02:00

x86: Fix early boot crash on gcc-10, third 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, 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 --- 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(-)

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 8c89e4d..2f24c33 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..448c91b 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() mb() + #endif /* __LINUX_COMPILER_H */ diff --git a/init/main.c b/init/main.c index 1a5da2c..ad3812b 100644 --- a/init/main.c +++ b/init/main.c @@ -1036,6 +1036,8 @@ asmlinkage __visible void __init start_kernel(void)

/* 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. */