Messages in this thread Patch in this message | | | Date | Thu, 23 Apr 2020 18:12:24 +0200 | From | Borislav Petkov <> | Subject | [PATCH] x86: Fix early boot crash on gcc-10, next try |
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Ok,
I have tried to summarize our odyssey so far and here's what I came up with. Just built latest gcc from the git repo and it seems to work.
Next I need to come up with a slick way of testing the compiler...
Thx.
--- From: Borislav Petkov <bp@suse.de>
... 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@gentoo.org> Signed-off-by: Borislav Petkov <bp@suse.de> Link: https://lkml.kernel.org/r/20200314164451.346497-1-slyfox@gentoo.org --- arch/x86/kernel/smpboot.c | 8 ++++++++ 1 file changed, 8 insertions(+)
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c index 3b9bf8c7e29d..e9f44727fccd 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 functions up, in + * boot_init_stack_canary() and must not be checked before tail calling + * another function. + */ + asm (""); } /** -- 2.21.0
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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