Messages in this thread | | | Date | Wed, 20 Nov 2019 12:18:59 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH v3 2/4] x86/traps: Print non-canonical address on #GP |
| |
* 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
| |