Re: [PATCH v2 2/3] x86/traps: Print non-canonical address on #GP

[Dmitry Vyukov <dvyukov@google.com>]

On Mon, Nov 18, 2019 at 3:21 PM Borislav Petkov <bp@alien8.de> wrote:
>
> On Fri, Nov 15, 2019 at 08:17:27PM +0100, Jann Horn wrote:
> >  dotraplinkage void
> >  do_general_protection(struct pt_regs *regs, long error_code)
> >  {
> > @@ -547,8 +581,15 @@ do_general_protection(struct pt_regs *regs, long error_code)
> >                       return;
> >
> >               if (notify_die(DIE_GPF, desc, regs, error_code,
> > -                            X86_TRAP_GP, SIGSEGV) != NOTIFY_STOP)
> > -                     die(desc, regs, error_code);
> > +                            X86_TRAP_GP, SIGSEGV) == NOTIFY_STOP)
> > +                     return;
> > +
> > +             if (error_code)
> > +                     pr_alert("GPF is segment-related (see error code)\n");
> > +             else
> > +                     print_kernel_gp_address(regs);
> > +
> > +             die(desc, regs, error_code);
>
> Right, this way, those messages appear before the main "general
> protection ..." message:
>
> [    2.434372] traps: probably dereferencing non-canonical address 0xdfff000000000001
> [    2.442492] general protection fault: 0000 [#1] PREEMPT SMP
>
> Can we glue/merge them together? Or is this going to confuse tools too much:
>
> [    2.542218] general protection fault while derefing a non-canonical address 0xdfff000000000001: 0000 [#1] PREEMPT SMP
>
> (and that sentence could be shorter too:
>
>         "general protection fault for non-canonical address 0xdfff000000000001"
>
> looks ok to me too.)

This exact form will confuse syzkaller crash parsing for Linux kernel:
https://github.com/google/syzkaller/blob/1daed50ac33511e1a107228a9c3b80e5c4aebb5c/pkg/report/linux.go#L1347
It expects a "general protection fault:" line for these crashes.

A graceful way to update kernel crash messages would be to add more
tests with the new format here:
https://github.com/google/syzkaller/tree/1daed50ac33511e1a107228a9c3b80e5c4aebb5c/pkg/report/testdata/linux/report
Update parsing code. Roll out new version. Update all other testing
systems that detect and parse kernel crashes. Then commit kernel
changes.

An unfortunate consequence of offloading testing to third-party systems...



> Here's a dirty diff together with a reproducer ontop of yours:
>
> ---
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index bf796f8c9998..dab702ba28a6 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -515,7 +515,7 @@ dotraplinkage void do_bounds(struct pt_regs *regs, long error_code)
>   * On 64-bit, if an uncaught #GP occurs while dereferencing a non-canonical
>   * address, print that address.
>   */
> -static void print_kernel_gp_address(struct pt_regs *regs)
> +static unsigned long get_kernel_gp_address(struct pt_regs *regs)
>  {
>  #ifdef CONFIG_X86_64
>         u8 insn_bytes[MAX_INSN_SIZE];
> @@ -523,7 +523,7 @@ static void print_kernel_gp_address(struct pt_regs *regs)
>         unsigned long addr_ref;
>
>         if (probe_kernel_read(insn_bytes, (void *)regs->ip, MAX_INSN_SIZE))
> -               return;
> +               return 0;
>
>         kernel_insn_init(&insn, insn_bytes, MAX_INSN_SIZE);
>         insn_get_modrm(&insn);
> @@ -532,22 +532,22 @@ static void print_kernel_gp_address(struct pt_regs *regs)
>
>         /* Bail out if insn_get_addr_ref() failed or we got a kernel address. */
>         if (addr_ref >= ~__VIRTUAL_MASK)
> -               return;
> +               return 0;
>
>         /* Bail out if the entire operand is in the canonical user half. */
>         if (addr_ref + insn.opnd_bytes - 1 <= __VIRTUAL_MASK)
> -               return;
> +               return 0;
>
> -       pr_alert("probably dereferencing non-canonical address 0x%016lx\n",
> -                addr_ref);
> +       return addr_ref;
>  #endif
>  }
>
> +#define GPFSTR "general protection fault"
>  dotraplinkage void
>  do_general_protection(struct pt_regs *regs, long error_code)
>  {
> -       const char *desc = "general protection fault";
>         struct task_struct *tsk;
> +       char desc[90];
>
>         RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
>         cond_local_irq_enable(regs);
> @@ -584,12 +584,18 @@ do_general_protection(struct pt_regs *regs, long error_code)
>                                X86_TRAP_GP, SIGSEGV) == NOTIFY_STOP)
>                         return;
>
> -               if (error_code)
> -                       pr_alert("GPF is segment-related (see error code)\n");
> -               else
> -                       print_kernel_gp_address(regs);
> +               if (error_code) {
> +                       snprintf(desc, 90, "segment-related " GPFSTR);
> +               } else {
> +                       unsigned long addr_ref = get_kernel_gp_address(regs);
> +
> +                       if (addr_ref)
> +                               snprintf(desc, 90, GPFSTR " while derefing a non-canonical address 0x%lx", addr_ref);
> +                       else
> +                               snprintf(desc, 90, GPFSTR);
> +               }
>
> -               die(desc, regs, error_code);
> +               die((const char *)desc, regs, error_code);
>                 return;
>         }
>
> diff --git a/init/main.c b/init/main.c
> index 91f6ebb30ef0..7acc7e660be9 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1124,6 +1124,9 @@ static int __ref kernel_init(void *unused)
>
>         rcu_end_inkernel_boot();
>
> +       asm volatile("mov $0xdfff000000000001, %rax\n\t"
> +                    "jmpq *%rax\n\t");
> +
>         if (ramdisk_execute_command) {
>                 ret = run_init_process(ramdisk_execute_command);
>                 if (!ret)
>
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette
>
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