Re: Crashes in linux-next on powerpc with CONFIG_PPC_KUAP and CONFIG_JUMP_LABEL_FEATURE_CHECK_DEBUG

From: Petr Mladek
Date: Thu May 09 2019 - 05:30:56 EST

On Wed 2019-05-08 00:54:51, Michael Ellerman wrote:
> Hi folks,
> Just an FYI in case anyone else is seeing crashes very early in boot in
> linux-next with the above config options.
> The problem is the combination of some new code called via printk(),
> check_pointer() which calls probe_kernel_read(). That then calls
> allow_user_access() (PPC_KUAP) and that uses mmu_has_feature() too early
> (before we've patched features). With the JUMP_LABEL debug enabled that
> causes us to call printk() & dump_stack() and we end up recursing and
> overflowing the stack.

Sigh, the check_pointer() stuff is in Linus's tree now, see
the commit 3e5903eb9cff707301712 ("vsprintf: Prevent crash when
dereferencing invalid pointers").

> Because it happens so early you don't get any output, just an apparently
> dead system.
> The stack trace (which you don't see) is something like:
> ...
> dump_stack+0xdc
> probe_kernel_read+0x1a4
> check_pointer+0x58
> string+0x3c
> vsnprintf+0x1bc
> vscnprintf+0x20
> printk_safe_log_store+0x7c
> printk+0x40
> dump_stack_print_info+0xbc
> dump_stack+0x8
> probe_kernel_read+0x1a4
> probe_kernel_read+0x19c
> check_pointer+0x58
> string+0x3c
> vsnprintf+0x1bc
> vscnprintf+0x20
> vprintk_store+0x6c
> vprintk_emit+0xec
> vprintk_func+0xd4
> printk+0x40
> cpufeatures_process_feature+0xc8
> scan_cpufeatures_subnodes+0x380
> of_scan_flat_dt_subnodes+0xb4
> dt_cpu_ftrs_scan_callback+0x158
> of_scan_flat_dt+0xf0
> dt_cpu_ftrs_scan+0x3c
> early_init_devtree+0x360
> early_setup+0x9c
> The simple fix is to use early_mmu_has_feature() in allow_user_access(),
> but we'd rather not do that because it penalises all
> copy_to/from_users() for the life of the system with the cost of the
> runtime check vs the jump label. The irony is probe_kernel_read()
> shouldn't be allowing user access at all, because we're reading the
> kernel not userspace.

I have tried to find a lightweight way for a safe reading of unknown
kernel pointer. But I have not succeeded so far. I see only variants
with user access. The user access is handled in arch-specific code
and I do not see any variant without it.

I am not sure on which level it should get fixed.

Could you please send it to lkml to get a wider audience?

Best Regards,

> For now if you're hitting it just turn off
> cheers