Messages in this thread | | | From | Jann Horn <> | Date | Wed, 27 Nov 2019 21:27:20 +0100 | Subject | Re: [PATCH v2 2/3] x86/traps: Print non-canonical address on #GP |
| |
On Sun, Nov 24, 2019 at 12:08 AM Andy Lutomirski <luto@kernel.org> wrote: > On Fri, Nov 15, 2019 at 11:17 AM 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. [...] > > +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); [...] > > +} > > 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 :)
Mmmh... the instruction decoder doesn't currently give us a reliable access size though. (I know, I'm using it here regardless, but it doesn't really matter here if the decoded size is too big from time to time... whereas I imagine that that'd matter quite a bit for #AC handling.) IIRC e.g. a MOVZX that loads 1 byte into a 4-byte register is decoded as having .opnd_bytes==4; and if you look through arch/x86/lib/insn.c, there isn't even anything that would ever set ->opnd_bytes to 1. You'd have to add some plumbing to get reliable access sizes. I don't want to add a helper for this before the underlying infrastructure actually works properly.
| |