Re: [PATCH v3 2/4] x86/traps: Print non-canonical address on #GP

[Ingo Molnar <mingo@kernel.org>]

* Jann Horn <jannh@google.com> 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@google.com>
> ---
> 
> 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)
>     v3:
>      - change message format to one line (Borislav)
>     
>     I have already sent a patch to syzkaller that relaxes their parsing of GPF
>     messages (https://github.com/google/syzkaller/commit/432c7650) such that
>     changes like the one in this patch don't break it.
>     That patch has already made its way into syzbot's syzkaller instances
>     according to <https://syzkaller.appspot.com/upstream>.
> 
>  arch/x86/kernel/traps.c | 56 ++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 53 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index c90312146da0..19afedcd6f4e 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>
> @@ -509,11 +511,45 @@ 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, return that address.
> + */
> +static unsigned long get_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 0;
> +
> +	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);

I had to look twice to realize that the 'insn_bytes' isn't an integer 
that shows the number of bytes in the instruction, but the instruction 
buffer itself.

Could we please do s/insn_bytes/insn_buf or such?

> +
> +	/* Bail out if insn_get_addr_ref() failed or we got a kernel address. */
> +	if (addr_ref >= ~__VIRTUAL_MASK)
> +		return 0;
> +
> +	/* Bail out if the entire operand is in the canonical user half. */
> +	if (addr_ref + insn.opnd_bytes - 1 <= __VIRTUAL_MASK)
> +		return 0;

BTW., it would be nice to split this logic in two: return the faulting 
address to do_general_protection(), and print it out both for 
non-canonical and canonical addresses as well -and use the canonical 
check to *additionally* print out a short note when the operand is 
non-canonical?

> +#define GPFSTR "general protection fault"
>  dotraplinkage void

Please separate macro and function definitions by an additional newline.

>  do_general_protection(struct pt_regs *regs, long error_code)
>  {
> -	const char *desc = "general protection fault";
>  	struct task_struct *tsk;
> +	char desc[90] = GPFSTR;


How was this maximum string length of '90' derived? In what way will that 
have to change if someone changes the message?

Thanks,

	Ingo