Commit a9a3ed1e authored Apr 22, 2020 by Borislav Petkov

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

parent 2ef96a5b

arch/x86/include/asm/stackprotector.h

+6 −1

Original line number	Diff line number	Diff line
		@@ -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)
		{

arch/x86/kernel/smpboot.c

+8 −0

Original line number	Diff line number	Diff line
		@@ -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();
		}

		/**

arch/x86/xen/smp_pv.c

+1 −0

Original line number	Diff line number	Diff line
		@@ -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)

include/linux/compiler.h

+6 −0

Original line number	Diff line number	Diff line
		@@ -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 */

init/main.c

+2 −0

Original line number	Diff line number	Diff line
		@@ -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. */

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