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

1 files changed, 6 insertions, 0 deletions

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 034b0a644efc..448c91bf543b 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 */