Subject: [RFC PATCH v3 2/3] arm64/kernel: don't ban ADRP to work around Cortex-A53 erratum #843419

From: [email-masked] (Will Deacon)
Date: Mon, 5 Mar 2018 17:34:28 +0000

On Mon, Mar 05, 2018 at 05:26:35PM +0000, Ard Biesheuvel wrote:
> On 5 March 2018 at 17:18, Will Deacon <[email-masked]> wrote:
> > On Wed, Feb 14, 2018 at 11:36:44AM +0000, Ard Biesheuvel wrote:
> >> Working around Cortex-A53 erratum #843419 involves special handling of
> >> ADRP instructions that end up in the last two instruction slots of a
> >> 4k page, or whose output register gets overwritten without having been
> >> read. (Note that the latter instruction sequence is never emitted by
> >> a properly functioning compiler)
> >
> > Does the workaround currently implemented in the linker also make this
> > same assumption? If not, I'm a little wary that we're making an assumption
> > about compiler behaviour with no way to detect whether its been violated or
> > not.
> >
> 
> I can check, but I don't see how a compiler would ever choose 'adrp'
> when it emits what amounts to a nop instruction.

Agreed, but it wouldn't be the first time we've seen compilers do weird
things.

> >> diff --git a/arch/arm64/kernel/module-plts.c b/arch/arm64/kernel/module-plts.c
> >> index ea640f92fe5a..93b808056cb4 100644
> >> --- a/arch/arm64/kernel/module-plts.c
> >> +++ b/arch/arm64/kernel/module-plts.c
> >> @@ -41,6 +41,46 @@ u64 module_emit_plt_entry(struct module *mod, void *loc, const Elf64_Rela *rela,
> >>       return (u64)&plt[i];
> >>  }
> >>
> >> +#ifdef CONFIG_ARM64_ERRATUM_843419
> >> +u64 module_emit_adrp_veneer(struct module *mod, void *loc, u64 val)
> >> +{
> >> +     struct mod_plt_sec *pltsec = !in_init(mod, loc) ? &mod->arch.core :
> >> +                                                       &mod->arch.init;
> >> +     struct plt_entry *plt = (struct plt_entry *)pltsec->plt->sh_addr;
> >> +     int i = pltsec->plt_num_entries++;
> >> +     u32 mov0, mov1, mov2, br;
> >> +     int rd;
> >> +
> >> +     BUG_ON(pltsec->plt_num_entries > pltsec->plt_max_entries);
> >
> > I'd prefer just to fail loading the module, but I see we already have
> > a BUG_ON for the existing PLT code. Oh well.
> >
> 
> I can fix that in a followup patch

Thanks.

> >> +
> >> +     /* get the destination register of the ADRP instruction */
> >> +     rd = aarch64_insn_decode_register(AARCH64_INSN_REGTYPE_RD,
> >> +                                       le32_to_cpup((__le32 *)loc));
> >> +
> >> +     /* generate the veneer instructions */
> >> +     mov0 = aarch64_insn_gen_movewide(rd, (u16)~val, 0,
> >> +                                      AARCH64_INSN_VARIANT_64BIT,
> >> +                                      AARCH64_INSN_MOVEWIDE_INVERSE);
> >> +     mov1 = aarch64_insn_gen_movewide(rd, (u16)(val >> 16), 16,
> >> +                                      AARCH64_INSN_VARIANT_64BIT,
> >> +                                      AARCH64_INSN_MOVEWIDE_KEEP);
> >> +     mov2 = aarch64_insn_gen_movewide(rd, (u16)(val >> 32), 32,
> >> +                                      AARCH64_INSN_VARIANT_64BIT,
> >> +                                      AARCH64_INSN_MOVEWIDE_KEEP);
> >> +     br = aarch64_insn_gen_branch_imm((u64)&plt[i].br, (u64)loc + 4,
> >> +                                      AARCH64_INSN_BRANCH_NOLINK);
> >> +
> >> +     plt[i] = (struct plt_entry){
> >> +                     cpu_to_le32(mov0),
> >> +                     cpu_to_le32(mov1),
> >> +                     cpu_to_le32(mov2),
> >> +                     cpu_to_le32(br)
> >> +             };
> >> +
> >> +     return (u64)&plt[i];
> >> +}
> >> +#endif
> >> +
> >>  #define cmp_3way(a,b)        ((a) < (b) ? -1 : (a) > (b))
> >>
> >>  static int cmp_rela(const void *a, const void *b)
> >> @@ -68,16 +108,21 @@ static bool duplicate_rel(const Elf64_Rela *rela, int num)
> >>  }
> >>
> >>  static unsigned int count_plts(Elf64_Sym *syms, Elf64_Rela *rela, int num,
> >> -                            Elf64_Word dstidx)
> >> +                            Elf64_Word dstidx, Elf_Shdr *dstsec)
> >>  {
> >>       unsigned int ret = 0;
> >>       Elf64_Sym *s;
> >>       int i;
> >>
> >>       for (i = 0; i < num; i++) {
> >> +             u64 min_align;
> >> +
> >>               switch (ELF64_R_TYPE(rela[i].r_info)) {
> >>               case R_AARCH64_JUMP26:
> >>               case R_AARCH64_CALL26:
> >> +                     if (!IS_ENABLED(CONFIG_RANDOMIZE_BASE))
> >> +                             break;
> >> +
> >>                       /*
> >>                        * We only have to consider branch targets that resolve
> >>                        * to symbols that are defined in a different section.
> >> @@ -109,6 +154,31 @@ static unsigned int count_plts(Elf64_Sym *syms, Elf64_Rela *rela, int num,
> >>                       if (rela[i].r_addend != 0 || !duplicate_rel(rela, i))
> >>                               ret++;
> >>                       break;
> >> +             case R_AARCH64_ADR_PREL_PG_HI21_NC:
> >> +             case R_AARCH64_ADR_PREL_PG_HI21:
> >> +                     if (!IS_ENABLED(CONFIG_ARM64_ERRATUM_843419))
> >> +                             break;
> >> +
> >> +                     /*
> >> +                      * Determine the minimal safe alignment for this ADRP
> >> +                      * instruction: the section alignment at which it is
> >> +                      * guaranteed not to appear at a vulnerable offset.
> >> +                      */
> >> +                     min_align = 2 << ffz(rela[i].r_offset | 0x7);
> >
> > I'm struggling to decipher this, can you give me a hint please? Why 0x7? Is
> > the "2" because of the two vulnerable offsets?
> >
> 
> What it basically does is preserve 0 bits in the address of the instruction.
> 
> Given that ADRP instructions at addresses ending in fff8 or fffc may
> be vulnerable, if any of the bits [11:3] are zero, we can increase the
> alignment of this section to ensure it remains zero, guaranteeing that
> the resulting address won't end in fff8 or fffc
> 
> Bits 0 .. 2 don't count, hence the 0x7. If bit 3 is zero, we can align
> to '1 << (3 + 1)' == '2 << 3', and bit 3 will remain zero.

Please put this in a comment ;)

Will