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

[Andy Lutomirski]

On Fri, Nov 15, 2019 at 11:17 AM Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> A frequent cause of #GP exceptions are memory accesses to non-canonical
> addresses. Unlike #PF, #GP doesn't come with a fault address in CR2, so
> the kernel doesn't currently print the fault address for #GP.
> Luckily, we already have the necessary infrastructure for decoding X86
> instructions and computing the memory address that is being accessed;
> hook it up to the #GP handler so that we can figure out whether the #GP
> looks like it was caused by a non-canonical address, and if so, print
> that address.
>
> While it is already possible to compute the faulting address manually by
> disassembling the opcode dump and evaluating the instruction against the
> register dump, this should make it slightly easier to identify crashes
> at a glance.
>
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> ---
>
> Notes:
>     v2:
>      - print different message for segment-related GP (Borislav)
>      - rewrite check for non-canonical address (Sean)
>      - make it clear we don't know for sure why the GP happened (Andy)
>
>  arch/x86/kernel/traps.c | 45 +++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 43 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index c90312146da0..12d42697a18e 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -56,6 +56,8 @@
>  #include <asm/mpx.h>
>  #include <asm/vm86.h>
>  #include <asm/umip.h>
> +#include <asm/insn.h>
> +#include <asm/insn-eval.h>
>
>  #ifdef CONFIG_X86_64
>  #include <asm/x86_init.h>
> @@ -509,6 +511,38 @@ dotraplinkage void do_bounds(struct pt_regs *regs, long error_code)
>         do_trap(X86_TRAP_BR, SIGSEGV, "bounds", regs, error_code, 0, NULL);
>  }
>
> +/*
> + * 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)
> +{
> +#ifdef CONFIG_X86_64
> +       u8 insn_bytes[MAX_INSN_SIZE];
> +       struct insn insn;
> +       unsigned long addr_ref;
> +
> +       if (probe_kernel_read(insn_bytes, (void *)regs->ip, MAX_INSN_SIZE))
> +               return;
> +
> +       kernel_insn_init(&insn, insn_bytes, MAX_INSN_SIZE);
> +       insn_get_modrm(&insn);
> +       insn_get_sib(&insn);
> +       addr_ref = (unsigned long)insn_get_addr_ref(&insn, regs);
> +
> +       /* Bail out if insn_get_addr_ref() failed or we got a kernel address. */
> +       if (addr_ref >= ~__VIRTUAL_MASK)
> +               return;
> +
> +       /* Bail out if the entire operand is in the canonical user half. */
> +       if (addr_ref + insn.opnd_bytes - 1 <= __VIRTUAL_MASK)
> +               return;
> +
> +       pr_alert("probably dereferencing non-canonical address 0x%016lx\n",
> +                addr_ref);
> +#endif
> +}

Could you refactor this a little bit so that we end up with a helper
that does the computation?  Something like:

int probe_insn_get_memory_ref(void **addr, size_t *len, void *insn_addr);

returns 1 if there was a memory operand and fills in addr and len,
returns 0 if there was no memory operand, and returns a negative error
on error.

I think we're going to want this for #AC handling, too :)