On Tue, Jun 11, 2024 at 01:26:09PM -0700, Gatlin Newhouse wrote:
> On Mon, Jun 03, 2024 at 06:13:53PM UTC, Thomas Gleixner wrote:
> > On Sat, Jun 01 2024 at 03:10, Gatlin Newhouse wrote:
> >
> > > Bring x86 to parity with arm64, similar to commit 25b84002afb9
> > > ("arm64: Support Clang UBSAN trap codes for better reporting").
> > > Enable the output of UBSAN type information on x86 architectures
> > > compiled with clang when CONFIG_UBSAN_TRAP=y. Currently ARM
> > > architectures output which specific sanitizer caused the trap,
> > > via the encoded data in the trap instruction. Clang on x86
> > > currently encodes the same data in ud1 instructions but the x86
> > > handle_bug() and is_valid_bugaddr() functions currently only look
> > > at ud2s.
> >
> > Please structure your change log properly instead of one paragraph of
> > unstructured word salad. See:
> >
> >
> > https://www.kernel.org/doc/html/latest/process/maintainer-tip.html#changelog
> >
> > > +/*
> > > + * Check for UD1, UD2, with or without Address Size Override Prefixes
> > > instructions.
> > > + */
> > > __always_inline int is_valid_bugaddr(unsigned long addr)
> > > {
> > > if (addr < TASK_SIZE_MAX)
> > > @@ -88,7 +92,13 @@ __always_inline int is_valid_bugaddr(unsigned long
> > > addr)
> > > * We got #UD, if the text isn't readable we'd have gotten
> > > * a different exception.
> > > */
> > > - return *(unsigned short *)addr == INSN_UD2;
> > > + if (*(u16 *)addr == INSN_UD2)
> > > + return INSN_UD2;
> > > + if (*(u16 *)addr == INSN_UD1)
> > > + return INSN_UD1;
> > > + if (*(u8 *)addr == INSN_ASOP && *(u16 *)(addr + 1) == INSN_UD1)
> >
> > s/1/LEN_ASOP/ ?
> >
> > > + return INSN_ASOP;
> > > + return 0;
> >
> > I'm not really a fan of the reuse of the INSN defines here. Especially
> > not about INSN_ASOP. Also 0 is just lame.
> >
> > Neither does the function name make sense anymore. is_valid_bugaddr() is
> > clearly telling that it's a boolean check (despite the return value
> > being int for hysterical raisins). But now you turn it into a
> > non-boolean integer which returns a instruction encoding. That's
> > hideous. Programming should result in obvious code and that should be
> > pretty obvious to people who create tools to validate code.
> >
> > Also all UBSAN cares about is the actual failure type and not the
> > instruction itself:
> >
> > #define INSN_UD_MASK 0xFFFF
> > #define INSN_ASOP_MASK 0x00FF
> >
> > #define BUG_UD_NONE 0xFFFF
> > #define BUG_UD2 0xFFFE
> >
> > __always_inline u16 get_ud_type(unsigned long addr)
> > {
> > u16 insn;
> >
> > if (addr < TASK_SIZE_MAX)
> > return BUD_UD_NONE;
> >
> > insn = *(u16 *)addr;
> > if ((insn & INSN_UD_MASK) == INSN_UD2)
> > return BUG_UD2;
> >
> > if ((insn & INSN_ASOP_MASK) == INSN_ASOP)
> > insn = *(u16 *)(++addr);
> >
> > // UBSAN encodes the failure type in the two bytes after UD1
> > if ((insn & INSN_UD_MASK) == INSN_UD1)
> > return *(u16 *)(addr + LEN_UD1);
> >
> > return BUG_UD_NONE;
> > }
> >
> > No?
>
> Thanks for the feedback.
>
> It seems that is_valid_bugaddr() needs to be implemented on all architectures
> and the function get_ud_type() replaces it here. So how should the patch
> handle
> is_valid_bugaddr()? Should the function remain as-is in traps.c despite no
> longer being used?
Yeah, this is why I'd suggested to Gatlin in early designs to reuse
is_valid_bugaddr()'s int value. It's a required function, so it seemed
sensible to just repurpose it from yes/no to no/type1/type2/type3/etc.
-Kees
--
Kees Cook
