On Fri, Mar 01, 2019 at 04:12:00AM +0100, Jann Horn wrote:
> When the frame unwinder is invoked for an oops caused by a call to NULL,
> it currently skips the parent function because BP still points to the
> parent's stack frame; the (nonexistent) current function only has the first
> half of a stack frame, and BP doesn't point to it yet.
>
> Add a special case for IP==0 that calculates a fake BP from SP, then uses
> the real BP for the next frame.
>
> Note that this handles first_frame specially: We return information about
> the parent function as long as the saved IP is >=first_frame, even if the
> fake BP points below it.
>
> With an artificially-added NULL call in prctl_set_seccomp(), before this
> patch, the trace is:
>
> Call Trace:
> ? prctl_set_seccomp+0x3a/0x50
> __x64_sys_prctl+0x457/0x6f0
> ? __ia32_sys_prctl+0x750/0x750
> do_syscall_64+0x72/0x160
> entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> After this patch, the trace is:
>
> Call Trace:
> prctl_set_seccomp+0x3a/0x50
> __x64_sys_prctl+0x457/0x6f0
> ? __ia32_sys_prctl+0x750/0x750
> do_syscall_64+0x72/0x160
> entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> Signed-off-by: Jann Horn <ja...@google.com>
> ---
> arch/x86/include/asm/unwind.h | 6 ++++++
> arch/x86/kernel/unwind_frame.c | 25 ++++++++++++++++++++++---
> 2 files changed, 28 insertions(+), 3 deletions(-)
>
> diff --git a/arch/x86/include/asm/unwind.h b/arch/x86/include/asm/unwind.h
> index 1f86e1b0a5cd..499578f7e6d7 100644
> --- a/arch/x86/include/asm/unwind.h
> +++ b/arch/x86/include/asm/unwind.h
> @@ -23,6 +23,12 @@ struct unwind_state {
> #elif defined(CONFIG_UNWINDER_FRAME_POINTER)
> bool got_irq;
> unsigned long *bp, *orig_sp, ip;
> + /*
> + * If non-NULL: The current frame is incomplete and doesn't contain a
> + * valid BP. When looking for the next frame, use this instead of the
> + * non-existent saved BP.
> + */
> + unsigned long *next_bp;
> struct pt_regs *regs;
> #else
> unsigned long *sp;
> diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
> index 3dc26f95d46e..9b9fd4826e7a 100644
> --- a/arch/x86/kernel/unwind_frame.c
> +++ b/arch/x86/kernel/unwind_frame.c
> @@ -320,10 +320,14 @@ bool unwind_next_frame(struct unwind_state *state)
> }
>
> /* Get the next frame pointer: */
> - if (state->regs)
> + if (state->next_bp) {
> + next_bp = state->next_bp;
> + state->next_bp = NULL;
> + } else if (state->regs) {
> next_bp = (unsigned long *)state->regs->bp;
> - else
> + } else {
> next_bp = (unsigned long *)READ_ONCE_TASK_STACK(state->task,
> *state->bp);
> + }
>
> /* Move to the next frame if it's safe: */
> if (!update_stack_state(state, next_bp))
> @@ -398,6 +402,21 @@ void __unwind_start(struct unwind_state *state, struct
> task_struct *task,
>
> bp = get_frame_pointer(task, regs);
>
> + /*
> + * If we crash with IP==0, the last successfully executed instruction
> + * was probably an indirect function call with a NULL function pointer.
> + * That means that SP points into the middle of an incomplete frame:
> + * *SP is a return pointer, and *(SP-sizeof(unsigned long)) is where we
> + * would have written a frame pointer if we hadn't crashed.
> + * Pretend that the frame is complete and that BP points to it, but save
> + * the real BP so that we can use it when looking for the next frame.
> + */
> + if (regs && regs->ip == 0 &&
Would it make sense to do 'regs->ip < PAGE_SIZE', a la show_fault_oops()?
E.g. to handle bugs where a function pointer gets loaded with NULL+offset.
> + (unsigned long *)kernel_stack_pointer(regs) >= first_frame) {
> + state->next_bp = bp;
> + bp = ((unsigned long *)kernel_stack_pointer(regs)) - 1;
> + }
> +
> /* Initialize stack info and make sure the frame data is accessible: */
> get_stack_info(bp, state->task, &state->stack_info,
> &state->stack_mask);
> @@ -410,7 +429,7 @@ void __unwind_start(struct unwind_state *state, struct
> task_struct *task,
> */
> while (!unwind_done(state) &&
> (!on_stack(&state->stack_info, first_frame, sizeof(long)) ||
> - state->bp < first_frame))
> + (state->next_bp == NULL && state->bp < first_frame)))
> unwind_next_frame(state);
> }
> EXPORT_SYMBOL_GPL(__unwind_start);
> --
> 2.21.0.352.gf09ad66450-goog
>
