From: Andy Lutomirski <[email protected]> If we overflow the stack into a guard page and then try to unwind it with ORC, it should work well: by construction, there can't be any meaningful data in the guard page because no writes to the guard page will have succeeded.
This patch fixes a bug that unwinding from working correctly: if the starting register state has RSP pointing into a stack guard page, the ORC unwinder bails out immediately. This patch fixes that: the ORC unwinder will start the unwind. I tested this by intentionally overflowing the task stack. The result is an accurate call trace instead of a trace consisting purely of '?' entries. There are a few other bugs that are triggered if the unwinder encounters a stack overflow after the first step, and Josh has WIP patches to fix those as well. Signed-off-by: Andy Lutomirski <[email protected]> Cc: Borislav Petkov <[email protected]> Cc: Brian Gerst <[email protected]> Cc: Dave Hansen <[email protected]> Cc: Josh Poimboeuf <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Thomas Gleixner <[email protected]> Link: http://lkml.kernel.org/r/927042950d7f1a7007dd0f58538966a593508f8b.1511715954.git.l...@kernel.org Signed-off-by: Ingo Molnar <[email protected]> --- arch/x86/kernel/unwind_orc.c | 14 ++++++++++++-- 1 file changed, 12 insertions(+), 2 deletions(-) diff --git a/arch/x86/kernel/unwind_orc.c b/arch/x86/kernel/unwind_orc.c index a3f973b2c97a..7f6e3935666b 100644 --- a/arch/x86/kernel/unwind_orc.c +++ b/arch/x86/kernel/unwind_orc.c @@ -553,8 +553,18 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task, } if (get_stack_info((unsigned long *)state->sp, state->task, - &state->stack_info, &state->stack_mask)) - return; + &state->stack_info, &state->stack_mask)) { + /* + * We weren't on a valid stack. It's possible that + * we overflowed a valid stack into a guard page. + * See if the next page up is valid so that we can + * generate some kind of backtrace if this happens. + */ + void *next_page = (void *)PAGE_ALIGN((unsigned long)regs->sp); + if (get_stack_info(next_page, state->task, &state->stack_info, + &state->stack_mask)) + return; + } /* * The caller can provide the address of the first frame directly -- 2.14.1
